Title: General Science
Author: Clark, Bertha M.
Language: English
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                            GENERAL SCIENCE



                                  BY

                        BERTHA M. CLARK, PH.D.


                    HEAD OF THE SCIENCE DEPARTMENT

           WILLIAM PENN HIGH SCHOOL FOR GIRLS, PHILADELPHIA



                      NEW YORK - CINCINNATI - CHICAGO

                         AMERICAN BOOK COMPANY

                                 1912



PREFACE


This book is not intended to prepare for college entrance
examinations; it will not, in fact, prepare for any of the present-day
stock examinations in physics, chemistry, or hygiene, but it should
prepare the thoughtful reader to meet wisely and actively some of
life's important problems, and should enable him to pass muster on the
principles and theories underlying scientific, and therefore economic,
management, whether in the shop or in the home.

We hear a great deal about the conservation of our natural resources,
such as forests and waterways; it is hoped that this book will show
the vital importance of the conservation of human strength and health,
and the irreparable loss to society of energy uselessly dissipated,
either in idle worry or in aimless activity. Most of us would reproach
ourselves for lack of shrewdness if we spent for any article more than
it was worth, yet few of us consider that we daily expend on domestic
and business tasks an amount of energy far in excess of that actually
required. The farmer who flails his grain instead of threshing it
wastes time and energy; the housewife who washes with her hands alone
and does not aid herself by the use of washing machine and proper
bleaching agents dissipates energy sadly needed for other duties.

The Chapter on machines is intended not only as a stimulus to the
invention of further labor-saving devices, but also as an eye opener
to those who, in the future struggle for existence, must perforce go
to the wall unless they understand how to make use of contrivances
whereby man's limited physical strength is made effective for larger
tasks.

The Chapter on musical instruments is more detailed than seems
warranted at first sight; but interest in orchestral instruments is
real and general, and there is a persistent desire for intelligent
information relative to musical instruments. The child of the laborer
as well as the child of the merchant finds it possible to attend some
of the weekly orchestral concerts, with their tiers of cheap seats,
and nothing adds more to the enjoyment and instruction of such hours
than an intimate acquaintance with the leading instruments. Unless
this is given in the public schools, a large percentage of mankind is
deprived of it, and it is for this reason that so large a share of the
treatment of sound has been devoted to musical instruments.

The treatment of electricity is more theoretical than that used in
preceding Chapters, but the subject does not lend itself readily to
popular presentation; and, moreover, it is assumed that the
information and training acquired in the previous work will give the
pupil power to understand the more advanced thought and method.

The real value of a book depends not so much upon the information
given as upon the permanent interest stimulated and the initiative
aroused. The youthful mind, and indeed the average adult mind as
well, is singularly non-logical and incapable of continued
concentration, and loses interest under too consecutive thought and
sustained style. For this reason the author has sacrificed at times
detail to general effect, logical development to present-day interest
and facts, and has made use of a popular, light style of writing as
well as of the more formal and logical style common to books of
science.

No claim is made to originality in subject matter. The actual facts,
theories, and principles used are such as have been presented in
previous textbooks of science, but the manner and sequence of
presentation are new and, so far as I know, untried elsewhere. These
are such as in my experience have aroused the greatest interest and
initiative, and such as have at the same time given the maximum
benefit from the informational standpoint. In no case, however, is
mental training sacrificed to information; but mental development is
sought through the student's willing and interested participation in
the actual daily happenings of the home and the shop and the field,
rather than through formal recitations and laboratory experiments.

Practical laboratory work in connection with the study of this book is
provided for in my _Laboratory Manual in General Science_, which
contains directions for a series of experiments designed to make the
pupil familiar with the facts and theories discussed in the textbook.

I have sought and have gained help from many of the standard
textbooks, new and old. The following firms have kindly placed cuts
at my disposal, and have thus materially aided in the preparation of
the illustrations: American Radiator Company; Commercial Museum,
Philadelphia; General Electric Company; Hershey Chocolate Company;
_Scientific American_; The Goulds Manufacturing Company; Victor
Talking Machine Company. Acknowledgment is also due to Professor Alvin
Davison for figures 19, 23, 29, 142, and 161.

Mr. W.D. Lewis, Principal of the William Penn High School, has read
the manuscript and has given me the benefit of his experience and
interest. Miss. Helen Hill, librarian of the same school, has been of
invaluable service as regards suggestions and proof reading. Miss.
Droege, of the Baldwin School, Bryn Mawr, has also been of very great
service. Practically all of my assistants have given of their time and
skill to the preparation of the work, but the list is too long for
individual mention.

BERTHA M. CLARK.

WILLIAM PENN HIGH SCHOOL.



CONTENTS


    CHAPTER

      I. HEAT

      II. TEMPERATURE AND HEAT

      III. OTHER FACTS ABOUT HEAT

      IV. BURNING OR OXIDATION

      V. FOOD

      VI. WATER

      VII. AIR

      VIII. GENERAL PROPERTIES OF GASES

      IX. INVISIBLE OBJECTS

      X. LIGHT

      XI. REFRACTION

      XII. PHOTOGRAPHY

      XIII. COLOR

      XIV. HEAT AND LIGHT AS COMPANIONS

      XV. ARTIFICIAL LIGHTING

      XVI. MAN'S WAY OF HELPING HIMSELF

      XVII. THE POWER BEHIND THE ENGINE

      XVIII. PUMPS AND THEIR VALUE TO MAN

      XIX. THE WATER PROBLEM OF A LARGE CITY

      XX. MAN'S CONQUEST OF SUBSTANCES

      XXI. FERMENTATION

      XXII. BLEACHING

      XXIII. DYEING

      XXIV. CHEMICALS AS DISINFECTANTS AND PRESERVATIVES

      XXV. DRUGS AND PATENT MEDICINES

      XXVI. NITROGEN AND ITS RELATION TO PLANTS

      XXVII. SOUND

      XXVIII. MUSICAL INSTRUMENTS

      XXIX. SPEAKING AND HEARING

      XXX. ELECTRICITY

      XXXI. SOME USES OF ELECTRICITY

      XXXII. MODERN ELECTRICAL INVENTIONS

      XXXIII. MAGNETS AND CURRENTS

      XXXIV. HOW ELECTRICITY MAY BE MEASURED

      XXXV. HOW ELECTRICITY IS OBTAINED ON A LARGE SCALE


      INDEX


GENERAL SCIENCE



CHAPTER I

HEAT


I. Value of Fire. Every day, uncontrolled fire wipes out human
lives and destroys vast amounts of property; every day, fire,
controlled and regulated in stove and furnace, cooks our food and
warms our houses. Fire melts ore and allows of the forging of iron, as
in the blacksmith's shop, and of the fashioning of innumerable objects
serviceable to man. Heated boilers change water into the steam which
drives our engines on land and sea. Heat causes rain and wind, fog and
cloud; heat enables vegetation to grow and thus indirectly provides
our food. Whether heat comes directly from the sun or from artificial
sources such as coal, wood, oil, or electricity, it is vitally
connected with our daily life, and for this reason the facts and
theories relative to it are among the most important that can be
studied. Heat, if properly regulated and controlled, would never be
injurious to man; hence in the following paragraphs heat will be
considered merely in its helpful capacity.

2. General Effect of Heat. _Expansion and Contraction_. One of the
best-known effects of heat is the change which it causes in the size
of a substance. Every housewife knows that if a kettle is filled with
cold water to begin with, there will be an overflow as soon as the
water becomes heated. Heat causes not only water, but all other
liquids, to occupy more space, or to expand, and in some cases the
expansion, or increase in size, is surprisingly large. For example, if
100 pints of ice water is heated in a kettle, the 100 pints will
steadily expand until, at the boiling point, it will occupy as much
space as 104 pints of ice water.

The expansion of water can be easily shown by heating a flask (Fig. I)
filled with water and closed by a cork through which a narrow tube
passes. As the water is heated, it expands and forces its way up the
narrow tube. If the heat is removed, the liquid cools, contracts, and
slowly falls in the tube, resuming in time its original size or
volume. A similar observation can be made with alcohol, mercury, or
any other convenient liquid.

[Illustration: FIG. 1.--As the water becomes warmer it expands and
rise in the narrow tube.]

Not only liquids are affected by heat and cold, but solids also are
subject to similar changes. A metal ball which when cool will just
slip through a ring (Fig. 2) will, when heated, be too large to slip
through the ring. Telegraph and telephone wires which in winter are
stretched taut from pole to pole, sag in hot weather and are much too
long. In summer they are exposed to the fierce rays of the sun, become
strongly heated, and expand sufficiently to sag. If the wires were
stretched taut in the summer, there would not be sufficient leeway for
the contraction which accompanies cold weather, and in winter they
would snap.

[Illustration: FIG. 2--When the ball is heated, it become too large to
slip through the ring.]

Air expands greatly when heated (Fig. 3), but since air is practically
invisible, we are not ordinarily conscious of any change in it. The
expansion of air can be readily shown by putting a drop of ink in a
thin glass tube, inserting the tube in the cork of a flask, and
applying heat to the flask (Fig. 4). The ink is forced up the tube by
the expanding air. Even the warmth of the hand is generally sufficient
to cause the drop to rise steadily in the tube. The rise of the drop
of ink shows that the air in the flask occupies more space than
formerly, and since the quantity of air has not changed, each cubic
inch of space must hold less warm air than| it held of cold air; that
is, one cubic inch of warm air weighs less than one cubic inch of cold
air, or warm air is less dense than cold air. All gases, if not
confined, expand when heated and contract as they cool. Heat, in
general, causes substances to expand or become less dense.

[Illustration: FIG. 3--As the air in _A_ is heated, it expands and
escapes in the form of bubbles.]

3. Amount of Expansion and Contraction. While most substances expand
when heated and contract when cooled, they are not all affected
equally by the same changes in temperature. Alcohol expands more than
water, and water more than mercury. Steel wire which measures 1/4 mile
on a snowy day will gain 25 inches in length on a warm summer day, and
an aluminum wire under the same conditions would gain 50 inches in
length.

[Illustration: FIG. 4.--As the air in _A_ is heated, it expands and
forces the drop of ink up the tube.]

4. Advantages and Disadvantages of Expansion and Contraction. We owe
the snug fit of metal tires and bands to the expansion and contraction
resulting from heating and cooling. The tire of a wagon wheel is made
slightly smaller than the wheel which it is to protect; it is then
put into a very hot fire and heated until it has expanded sufficiently
to slip on the wheel. As the tire cools it contracts and fits the
wheel closely.

In a railroad, spaces are usually left between consecutive rails in
order to allow for expansion during the summer.

The unsightly cracks and humps in cement floors are sometimes due to
the expansion resulting from heat (Fig. 5). Cracking from this cause
can frequently be avoided by cutting the soft cement into squares, the
spaces between them giving opportunity for expansion just as do the
spaces between the rails of railroads.

[Illustration: FIG. 5: A cement walk broken by expansion due to sun
heat.]

In the construction of long wire fences provision must be made for
tightening the wire in summer, otherwise great sagging would occur.

Heat plays an important part in the splitting of rocks and in the
formation of débris. Rocks in exposed places are greatly affected by
changes in temperature, and in regions where the changes in
temperature are sudden, severe, and frequent, the rocks are not able
to withstand the strain of expansion and contraction, and as a result
crack and split. In the Sahara Desert much crumbling of the rock into
sand has been caused by the intense heat of the day followed by the
sharp frost of night. The heat of the day causes the rocks to expand,
and the cold of night causes them to contract, and these two forces
constantly at work loosen the grains of the rock and force them out of
place, thus producing crumbling.

[Illustration: FIG. 6.--Splitting and crumbling of rock caused by
alternating heat and cold.]

The surface of the rock is the most exposed part, and during the day
the surface, heated by the sun's rays, expands and becomes too large
for the interior, and crumbling and splitting result from the strain.
With the sudden fall of temperature in the late afternoon and night,
the surface of the rock becomes greatly chilled and colder than the
rock beneath; the surface rock therefore contracts and shrinks more
than the underlying rock, and again crumbling results (Fig. 6).

[Illustration: FIG. 7.--Debris formed from crumbled rock.]

On bare mountains, the heating and cooling effects of the sun are very
striking(Fig. 7); the surface of many a mountain peak is covered with
cracked rock so insecure that a touch or step will dislodge the
fragments and start them down the mountain slope. The lower levels of
mountains are frequently buried several feet under débris which has
been formed in this way from higher peaks, and which has slowly
accumulated at the lower levels.

5. Temperature. When an object feels hot to the touch, we say that
it has a high temperature; when it feels cold to the touch, that it
has a low temperature; but we are not accurate judges of heat. Ice
water seems comparatively warm after eating ice cream, and yet we know
that ice water is by no means warm. A room may seem warm to a person
who has been walking in the cold air, while it may feel decidedly cold
to some one who has come from a warmer room. If the hand is cold,
lukewarm water feels hot, but if the hand has been in very hot water
and is then transferred to lukewarm water, the latter will seem cold.
We see that the sensation or feeling of warmth is not an accurate
guide to the temperature of a substance; and yet until 1592, one
hundred years after the discovery of America, people relied solely
upon their sensations for the measurement of temperature. Very hot
substances cannot be touched without injury, and hence inconvenience
as well as the necessity for accuracy led to the invention of the
thermometer, an instrument whose operation depends upon the fact that
most substances expand when heated and contract when cooled.

[Illustration: FIG. 8.--Making a thermometer.]

6. The Thermometer. The modern thermometer consists of a glass tube
at the lower end of which is a bulb filled with mercury or colored
alcohol (Fig. 8). After the bulb has been filled with the mercury, it
is placed in a beaker of water and the water is heated by a Bunsen
burner. As the water becomes warmer and warmer the level of the
mercury in the tube steadily rises until the water boils, when the
level remains stationary (Fig. 9). A scratch is made on the tube to
indicate the point to which the mercury rises when the bulb is placed
in boiling water, and this point is marked 212°. The tube is then
removed from the boiling water, and after cooling for a few minutes,
it is placed in a vessel containing finely chopped ice (Fig. 10). The
mercury column falls rapidly, but finally remains stationary, and at
this level another scratch is made on the tube and the point is marked
32°. The space between these two points, which represent the
temperatures of boiling water and of melting ice, is divided into 180
equal parts called degrees. The thermometer in use in the United
States is marked in this way and is called the Fahrenheit thermometer
after its designer. Before the degrees are etched on the thermometer
the open end of the tube is sealed.

[Illustration: FIG. 9.--Determining one of the fixed points of a
thermometer.]

The Centigrade thermometer, in use in foreign countries and in all
scientific work, is similar to the Fahrenheit except that the fixed
points are marked 100° and 0°, and the interval between the points is
divided into 100 equal parts instead of into 180.

_The boiling point of water is 212° F. or 100° C_.

_The melting point of ice is 32° F. or 0° C_.

Glass thermometers of the above type are the ones most generally used,
but there are many different types for special purposes.

[Illustration: FIG. 10.--Determining the lower fixed point of a
thermometer.]

7. Some Uses of a Thermometer. One of the chief values of a
thermometer is the service it has rendered to medicine. If a
thermometer is held for a few minutes under the tongue of a normal,
healthy person, the mercury will rise to about 98.4° F. If the
temperature of the body registers several degrees above or below this
point, a physician should be consulted immediately. The temperature of
the body is a trustworthy indicator of general physical condition;
hence in all hospitals the temperature of patients is carefully taken
at stated intervals.

Commercially, temperature readings are extremely important. In sugar
refineries the temperature of the heated liquids is observed most
carefully, since a difference in temperature, however slight, affects
not only the general appearance of sugars and sirups, but the quality
as well. The many varieties of steel likewise show the influence which
heat may have on the nature of a substance. By observation and tedious
experimentation it has been found that if hardened steel is heated to
about 450° F. and quickly cooled, it gives the fine cutting edge of
razors; if it is heated to about 500° F. and then cooled, the metal is
much coarser and is suitable for shears and farm implements; while if
it is heated but 50° F. higher, that is, to 550° F., it gives the fine
elastic steel of watch springs.

[Illustration: FIG. 11.--A well-made commercial thermometer.]

A thermometer could be put to good use in every kitchen; the
inexperienced housekeeper who cannot judge of the "heat" of the oven
would be saved bad bread, etc., if the thermometer were a part of her
equipment. The thermometer can also be used in detecting adulterants.
Butter should melt at 94° F.; if it does not, you may be sure that it
is adulterated with suet or other cheap fat. Olive oil should be a
clear liquid above 75° F.; if, above this temperature, it looks
cloudy, you may be sure that it too is adulterated with fat.

8. Methods of Heating Buildings. _Open Fireplaces and Stoves._
Before the time of stoves and furnaces, man heated his modest dwelling
by open fires alone. The burning logs gave warmth to the cabin and
served as a primitive cooking agent; and the smoke which usually
accompanies burning bodies was carried away by means of the chimney.
But in an open fireplace much heat escapes with the smoke and is lost,
and only a small portion streams into the room and gives warmth.

When fuel is placed in an open fireplace (Fig. 12) and lighted, the
air immediately surrounding the fire becomes warmer and, because of
expansion, becomes lighter than the cold air above. The cold air,
being heavier, falls and forces the warmer air upward, and along with
the warm air goes the disagreeable smoke. The fall of the colder and
heavier air, and the rise of the warmer and hence lighter air, is
similar to the exchange which takes place when water is poured on oil;
the water, being heavier than oil, sinks to the bottom and forces the
oil to the surface. The warmer air which escapes up the chimney
carries with it the disagreeable smoke, and when all the smoke is got
rid of in this way, the chimney is said to draw well.

[Illustration: FIG. 12.--The open fireplace as an early method of
heating.]

As the air is heated by the fire it expands, and is pushed up the
chimney by the cold air which is constantly entering through loose
windows and doors. Open fireplaces are very healthful because the air
which is driven out is impure, while the air which rushes in is fresh
and brings oxygen to the human being.

But open fireplaces, while pleasant to look at, are not efficient for
either heating or cooking. The possibilities for the latter are
especially limited, and the invention of stoves was a great advance in
efficiency, economy, and comfort. A stove is a receptacle for fire,
provided with a definite inlet for air and a definite outlet for
smoke, and able to radiate into the room most of the heat produced
from the fire which burns within. The inlet, or draft, admits enough
air to cause the fire to burn brightly or slowly as the case may be.
If we wish a hot fire, the draft is opened wide and enough air enters
to produce a strong glow. If we wish a low fire, the inlet is only
partially opened, and just enough air enters to keep the fuel
smoldering.

When the fire is started, the damper should be opened wide in order to
allow the escape of smoke; but after the fire is well started there is
less smoke, and the damper may be partly closed. If the damper is kept
open, coal is rapidly consumed, and the additional heat passes out
through the chimney, and is lost to use.

9. Furnaces. _Hot Air_. The labor involved in the care of numerous
stoves is considerable, and hence the advent of a central heating
stove, or furnace, was a great saving in strength and fuel. A furnace
is a stove arranged as in Figure 13. The stove _S_, like all other
stoves, has an inlet for air and an outlet _C_ for smoke; but in
addition, it has built around it a chamber in which air circulates and
is warmed. The air warmed by the stove is forced upward by cold air
which enters from outside. For example, cold air constantly entering
at _E_ drives the air heated by _S_ through pipes and ducts to the
rooms to be heated.

The metal pipes which convey the heated air from the furnace to the
ducts are sometimes covered with felt, asbestos, or other
non-conducting material in order that heat may not be lost during
transmission. The ducts which receive the heated air from the pipes
are built in the non-conducting walls of the house, and hence lose
practically no heat. The air which reaches halls and rooms is
therefore warm, in spite of its long journey from the cellar.

[Illustration: FIG. 13.--A furnace. Pipes conduct hot air to the
rooms.]

Not only houses are warmed by a central heating stove, but whole
communities sometimes depend upon a central heating plant. In the
latter case, pipes closely wrapped with a non-conducting material
carry steam long distances underground to heat remote buildings.
Overbrook and Radnor, Pa., are towns in which such a system is used.

10. Hot-water Heating. The heated air which rises from furnaces is
seldom hot enough to warm large buildings well; hence furnace heating
is being largely supplanted by hot-water heating.

The principle of hot-water heating is shown by the following simple
experiment. Two flasks and two tubes are arranged as in Figure 15, the
upper flask containing a colored liquid and the lower flask clear
water. If heat is applied to _B_, one can see at the end of a few
seconds the downward circulation of the colored liquid and the upward
circulation of the clear water. If we represent a boiler by _B_, a
radiator by the coiled tube, and a safety tank by _C_, we shall have a
very fair illustration of the principle of a hot-water heating system.
The hot water in the radiators cools and, in cooling, gives up its
heat to the rooms and thus warms them.

[Illustration: FIG. 14.--Hot-water heating.]

In hot-water heating systems, fresh air is not brought to the rooms,
for the radiators are closed pipes containing hot water. It is largely
for this reason that thoughtful people are careful to raise windows at
intervals. Some systems of hot-water heating secure ventilation by
confining the radiators to the basement, to which cold air from
outside is constantly admitted in such a way that it circulates over
the radiators and becomes strongly heated. This warm fresh air then
passes through ordinary flues to the rooms above.

[Illustration: FIG. 15.--The principle of hot-water heating.]

In Figure 16, a radiator is shown in a boxlike structure in the
cellar. Fresh air from outside enters a flue at the right, passes the
radiator, where it is warmed, and then makes its way to the room
through a flue at the left. The warm air which thus enters the room is
thoroughly fresh. The actual labor involved in furnace heating and in
hot-water heating is practically the same, since coal must be fed to
the fire, and ashes must be removed; but the hot-water system has the
advantage of economy and cleanliness.

[Illustration: FIG. 16.--Fresh air from outside circulates over the
radiators and then rises into the rooms to be heated.]

11. Fresh Air. Fresh air is essential to normal healthy living, and
2000 cubic feet of air per hour is desirable for each individual. If a
gentle breeze is blowing, a barely perceptible opening of a window
will give the needed amount, even if there are no additional drafts of
fresh air into the room through cracks. Most houses are so loosely
constructed that fresh air enters imperceptibly in many ways, and
whether we will or no, we receive some fresh air. The supply is,
however, never sufficient in itself and should not be depended upon
alone. At night, or at any other time when gas lights are required,
the need for ventilation increases, because every gas light in a room
uses up the same amount of air as four people.

[Illustration: FIG. 17.--The air which goes to the schoolrooms is
warmed by passage over the radiators.]

In the preceding Section, we learned that many houses heated by hot
water are supplied with fresh-air pipes which admit fresh air into
separate rooms or into suites of rooms. In some cases the amount which
enters is so great that the air in a room is changed three or four
times an hour. The constant inflow of cold air and exit of warm air
necessitates larger radiators and more hot water and hence more coal
to heat the larger quantity of water, but the additional expense is
more than compensated by the gain in health.

12. Winds and Currents. The gentlest summer breezes and the fiercest
blasts of winter are produced by the unequal heating of air. We have
seen that the air nearest to a stove or hot object becomes hotter than
the adjacent air, that it tends to expand and is replaced and pushed
upward and outward by colder, heavier air falling downward. We have
learned also that the moving liquid or gas carries with it heat which
it gradually gives out to surrounding bodies.

When a liquid or a gas moves away from a hot object, carrying heat
with it, the process is called _convection_.

Convection is responsible for winds and ocean currents, for land and
sea breezes, and other daily phenomena.

The Gulf Stream illustrates the transference of heat by convection. A
large body of water is strongly heated at the equator, and then moves
away, carrying heat with it to distant regions, such as England and
Norway.

Owing to the shape of the earth and its position with respect to the
sun, different portions of the earth are unequally heated. In those
portions where the earth is greatly heated, the air likewise will be
heated; there will be a tendency for the air to rise, and for the cold
air from surrounding regions to rush in to fill its place. In this way
winds are produced. There are many circumstances which modify winds
and currents, and it is not always easy to explain their direction
and velocity, but one very definite cause is the unequal heating of
the surface of the earth.

13. Conduction. A poker used in stirring a fire becomes hot and
heats the hand grasping the poker, although only the opposite end of
the poker has actually been in the fire. Heat from the fire passed
into the poker, traveled along it, and warmed it. When heat flows in
this way from a warm part of a body to a colder part, the process is
called _conduction_. A flatiron is heated by conduction, the heat from
the warm stove passing into the cold flatiron and gradually heating
it.

In convection, air and water circulate freely, carrying heat with
them; in conduction, heat flows from a warm region toward a cold
region, but there is no apparent motion of any kind.

Heat travels more readily through some substances than through others.
All metals conduct heat well; irons placed on the fire become heated
throughout and cannot be grasped with the bare hand; iron utensils are
frequently made with wooden handles, because wood is a poor conductor
and does not allow heat from the iron to pass through it to the hand.
For the same reason a burning match may be held without discomfort
until the flame almost reaches the hand.

Stoves and radiators are made of metal, because metals conduct heat
readily, and as fast as heat is generated within the stove by the
burning of fuel, or introduced into the radiator by the hot water, the
heat is conducted through the metal and escapes into the room.

Hot-water pipes and steam pipes are usually wrapped with a
non-conducting substance, or insulator, such as asbestos, in order
that the heat may not escape, but shall be retained within the pipes
until it reaches the radiators within the rooms.

The invention of the "Fireless Cooker" depended in part upon the
principle of non-conduction. Two vessels, one inside the other, are
separated by sawdust, asbestos, or other poor conducting material
(Fig. 18). Foods are heated in the usual way to the boiling point or
to a high temperature, and are then placed in the inner vessel. The
heat of the food cannot escape through the non-conducting material
which surrounds it, and hence remains in the food and slowly cooks it.

[Illustration: FIG. 18.--A fireless cooker.]

A very interesting experiment for the testing of the efficacy of
non-conductors may be easily performed. Place hot water in a metal
vessel, and note by means of a thermometer the _rapidity_ with which
the water cools; then place water of the same temperature in a second
metal vessel similar to the first, but surrounded by asbestos or other
non-conducting material, and note the _slowness_ with which the
temperature falls.

Chemical Change, an Effect of Heat. This effect of heat has a vital
influence on our lives, because the changes which take place when food
is cooked are due to it. The doughy mass which goes into the oven,
comes out a light spongy loaf; the small indigestible rice grain comes
out the swollen, fluffy, digestible grain. Were it not for the
chemical changes brought about by heat, many of our present foods
would be useless to man. Hundreds of common materials like glass,
rubber, iron, aluminum, etc., are manufactured by processes which
involve chemical action caused by heat.



CHAPTER II

TEMPERATURE AND HEAT


14. Temperature not a Measure of the Amount of Heat Present. If two
similar basins containing unequal quantities of water are placed in
the sunshine on a summer day, the smaller quantity of water will
become quite warm in a short period of time, while the larger quantity
will become only lukewarm. Both vessels receive the same amount of
heat from the sun, but in one case the heat is utilized in heating to
a high temperature a small quantity of water, while in the second case
the heat is utilized in warming to a lower degree a larger quantity of
water. Equal amounts of heat do not necessarily produce equivalent
temperatures, and equal temperatures do not necessarily indicate equal
amounts of heat. It takes more heat to raise a gallon of water to the
boiling point than it does to raise a pint of water to the boiling
point, but a thermometer would register the same temperature in the
two cases. The temperature of boiling water is 100° C. whether there
is a pint of it or a gallon. Temperature is independent of the
quantity of matter present; but the amount of heat contained in a
substance at any temperature is not independent of quantity, being
greater in the larger quantity.

15. The Unit of Heat. It is necessary to have a unit of heat just as
we have a unit of length, or a unit of mass, or a unit of time. One
unit of heat is called a _calorie_, and is the amount of heat which
will change the temperature of 1 gram of water 1° C. It is the amount
of heat given out by 1 gram of water when its temperature falls 1° C.,
or the amount of heat absorbed by 1 gram of water when its temperature
rises 1° C. If 400 grams of water are heated from 0° to 5° C., the
amount of heat which has entered the water is equivalent to 5 × 400 or
2000 calories; if 200 grams of water cool from 25° to 20° C., the heat
given out by the water is equivalent to 5 × 200 or 1000 calories.

16. Some Substances Heat more readily than Others. If two equal
quantities of water at the same temperature are exposed to the sun for
the same length of time, their final temperatures will be the same.
If, however, equal quantities of different substances are exposed, the
temperatures resulting from the heating will not necessarily be the
same. If a basin containing 1 lb. of mercury is put on the fire, side
by side with a basin containing an equal quantity of water, the
temperatures of the two substances will vary greatly at the end of a
short time. The mercury will have a far higher temperature than the
water, in spite of the fact that the amount of mercury is as great as
the amount of water and that the heat received from the fire has been
the same in each case. Mercury is not so difficult to heat as water;
less heat being required to raise its temperature 1° than is required
to raise the temperature of an equal quantity of water 1°. In fact,
mercury is 30 times as easy to heat as water, and it requires only one
thirtieth as much fire to heat a given quantity of mercury 1° as to
heat the same quantity of water 1°.

17. Specific Heat. We know that different substances are differently
affected by heat. Some substances, like water, change their
temperature slowly when heated; others, like mercury, change their
temperature very rapidly when heated. The number of calories needed by
1 gram of a substance in order that its temperature may be increased
1° C. is called the _specific heat_ of a substance; or, specific heat
is the number of calories given out by 1 gram of a substance when its
temperature falls 1° C. For experiments on the determination of
specific heat, see Laboratory Manual.

Water has the highest specific heat of any known substance except
hydrogen; that is, it requires more heat to raise the temperature of
water a definite number of degrees than it does to raise the
temperature of an equal amount of any other substance the same number
of degrees. Practically this same thing can be stated in another way:
Water in cooling gives out more heat than any other substance in
cooling through the same number of degrees. For this reason water is
used in foot warmers and in hot-water bags. If a copper lid were used
as a foot warmer, it would give the feet only .095 as much heat as an
equal weight of water; a lead weight only .031 as much heat as water.
Flatirons are made of iron because of the relatively high specific
heat of iron. The flatiron heats slowly and cools slowly, and, because
of its high specific heat, not only supplies the laundress with
considerable heat, but eliminates for her the frequent changing of the
flatiron.

18. Water and Weather. About four times as much heat is required to
heat a given quantity of water one degree as to heat an equal quantity
of earth. In summer, when the rocks and the sand along the shore are
burning hot, the ocean and lakes are pleasantly cool, although the
amount of heat present in the water is as great as that present in the
earth. In winter, long after the rocks and sand have given out their
heat and have become cold, the water continues to give out the vast
store of heat accumulated during the summer. This explains why lands
situated on or near large bodies of water usually have less variation
in temperature than inland regions. In the summer the water cools the
region; in the winter, on the contrary, the water heats the region,
and hence extremes of temperature are practically unknown.

19. Sources of Heat. Most of the heat which we enjoy and use we owe
to the sun. The wood which blazes on the hearth, the coal which glows
in the furnace, and the oil which burns in the stove owe their
existence to the sun.

Without the warmth of the sun seeds could not sprout and develop into
the mighty trees which yield firewood. Even coal, which lies buried
thousands of feet below the earth's surface, owes its existence in
part to the sun. Coal is simply buried vegetation,--vegetation which
sprouted and grew under the influence of the sun's warm rays. Ages ago
trees and bushes grew "thick and fast," and the ground was always
covered with a deep layer of decaying vegetable matter. In time some
of this vast supply sank into the moist soil and became covered with
mud. Then rock formed, and the rock pressed down upon the sunken
vegetation. The constant pressure, the moisture in the ground, and
heat affected the underground vegetable mass, and slowly changed it
into coal.

The buried forest and thickets were not all changed into coal. Some
were changed into oil and gas. Decaying animal matter was often mixed
with the vegetable mass. When the mingled animal and vegetable matter
sank into moist earth and came under the influence of pressure, it was
slowly changed into oil and gas.

The heat of our bodies comes from the foods which we eat. Fruits,
grain, etc., could not grow without the warmth and the light of the
sun. The animals which supply our meats likewise depend upon the sun
for light and warmth.

The sun, therefore, is the great source of heat; whether it is the
heat which comes directly from the sun and warms the atmosphere, or
the heat which comes from burning coal, wood, and oil.



CHAPTER III

OTHER FACTS ABOUT HEAT


20. Boiling. _Heat absorbed in Boiling_. If a kettle of water is
placed above a flame, the temperature of the water gradually
increases, and soon small bubbles form at the bottom of the kettle and
begin to rise through the water. At first the bubbles do not get far
in their ascent, but disappear before they reach the surface; later,
as the water gets hotter and hotter, the bubbles become larger and
more numerous, rise higher and higher, and finally reach the surface
and pass from the water into the air; steam comes from the vessel, and
the water is said to _boil_. The temperature at which a liquid boils
is called the boiling point.

While the water is heating, the temperature steadily rises, but as
soon as the water begins to boil the thermometer reading becomes
stationary and does not change, no matter how hard the water boils and
in spite of the fact that heat from the flame is constantly passing
into the water.

If the flame is removed from the boiling water for but a second, the
boiling ceases; if the flame is replaced, the boiling begins again
immediately. Unless heat is constantly supplied, water at the boiling
point cannot be transformed into steam.

_The number of calories which must be supplied to 1 gram of water at
the boiling point in order to change it into steam at the same
temperature is called the heat of vaporization_; it is the heat
necessary to change 1 gram of water at the boiling point into steam of
the same temperature.

21. The Amount of Heat Absorbed. The amount of heat which must be
constantly supplied to water at the boiling point in order to change
it into steam is far greater than we realize. If we put a beaker of
ice water (water at 0° C.) over a steady flame, and note (1) the time
which elapses before the water begins to boil, and (2) the time which
elapses before the boiling water completely boils away, we shall see
that it takes about 5-1/4 times as long to change water into steam as
it does to change its temperature from 0° C. to 100° C. Since, with a
steady flame, it takes 5-1/4 times as long to change water into steam
as it does to change its temperature from 0° C. to the boiling point,
we conclude that it takes 5-1/4 times as much heat to convert water at
the boiling point into steam as it does to raise it from the
temperature of ice water to that of boiling water.

The amount of heat necessary to raise the temperature of 1 gram of
water 1° C. is equal to 1 calorie, and the amount necessary to raise
the temperature 100° C. is equal to 100 calories; hence the amount of
heat necessary to convert 1 gram of water at the boiling point into
steam at that same temperature is equal to approximately 525 calories.
Very careful experiments show the exact heat of vaporization to be
536.1 calories. (See Laboratory Manual.)

22. General Truths. Statements similar to the above hold for other
liquids and for solutions. If milk is placed upon a stove, the
temperature rises steadily until the boiling point is reached; further
heating produces, not a change in temperature, but a change of the
water of the milk into steam. As soon as the milk, or any other liquid
food, comes to a boil, the gas flame should be lowered until only an
occasional bubble forms, because so long as any bubbles form the
temperature is that of the boiling point, and further heat merely
results in waste of fuel.

We find by experiment that every liquid has its own specific boiling
point; for example, alcohol boils at 78° C. and brine at 103° C. Both
specific heat and the heat of vaporization vary with the liquid used.

23. Condensation. If one holds a cold lid in the steam of boiling
water, drops of water gather on the lid; the steam is cooled by
contact with the cold lid and _condenses_ into water. Bottles of water
brought from a cold cellar into a warm room become covered with a mist
of fine drops of water, because the moisture in the air, chilled by
contact with the cold bottles, immediately condenses into drops of
water. Glasses filled with ice water show a similar mist.

In Section 21, we saw that 536 calories are required to change 1 gram
of water into steam; if, now, the steam in turn condenses into water,
it is natural to expect a release of the heat used in transforming
water into steam. Experiment shows not only that vapor gives out heat
during condensation, but that the amount of heat thus set free is
exactly equal to the amount absorbed during vaporization. (See
Laboratory Manual.)

We learn that the heat of vaporization is the same whether it is
considered as the heat absorbed by 1 gram of water in its change to
steam, or as the heat given out by 1 gram of steam during its
condensation into water.

24. Practical Application. We understand now the value of steam as a
heating agent. Water is heated in a boiler in the cellar, and the
steam passes through pipes which run to the various rooms; there the
steam condenses into water in the radiators, each gram of steam
setting free 536 calories of heat. When we consider the size of the
radiators and the large number of grams of steam which they contain,
and consider further that each gram in condensing sets free 536
calories, we understand the ease with which buildings are heated by
steam.

Most of us have at times profited by the heat of condensation. In cold
weather, when there is a roaring fire in the range, the water
frequently becomes so hot that it "steams" out of open faucets. If, at
such times, the hot water is turned on in a small cold bathroom, and
is allowed to run until the tub is well filled, vapor condenses on
windows, mirrors, and walls, and the cold room becomes perceptibly
warmer. The heat given out by the condensing steam passes into the
surrounding air and warms the room.

There is, however, another reason for the rise in temperature. If a
large pail of hot soup is placed in a larger pail of cold water, the
soup will gradually cool and the cold water will gradually become
warmer. A red-hot iron placed on a stand gradually cools, but warms
the stand. A hot body loses heat so long as a cooler body is near it;
the cold object is heated at the expense of the warmer object, and one
loses heat and the other gains heat until the temperature of both is
the same. Now the hot water in the tub gradually loses heat and the
cold air of the room gradually gains heat by convection, but the
amount given the room by convection is relatively small compared with
the large amount set free by the condensing steam.

25. Distillation. If impure, muddy water is boiled, drops of water
will collect on a cold plate held in the path of the steam, but the
drops will be clear and pure. When impure water is boiled, the steam
from it does not contain any of the impurities because these are left
behind in the vessel. If all the water were allowed to boil away, a
layer of mud or of other impurities would be found at the bottom of
the vessel. Because of this fact, it is possible to purify water in a
very simple way. Place over a fire a large kettle closed except for a
spout which is long enough to reach across the stove and dip into a
bottle. As the liquid boils, steam escapes through the spout, and on
reaching the cold bottle condenses and drops into the bottle as pure
water. The impurities remain behind in the kettle. Water freed from
impurities in this way is called _distilled water_, and the process is
called _distillation_ (Fig. 19). By this method, the salt water of the
ocean may be separated into pure drinking water and salt, and many of
the large ocean liners distill from the briny deep all the drinking
water used on their ocean voyages.

[Illustration: FIG. 19.--In order that the steam which passes through
the coiled tube may be quickly cooled and condensed, cold water is
made to circulate around the coil. The condensed steam escapes at
_w_.]

Commercially, distillation is a very important process. Turpentine,
for example, is made by distilling the sap of pine trees. Incisions
are cut in the bark of the long-leaf pine trees, and these serve as
channels for the escape of crude resin. This crude liquid is collected
in barrels and taken to a distillery, where it is distilled into
turpentine and rosin. The turpentine is the product which passes off
as vapor, and the rosin is the mass left in the boiler after the
distillation of the turpentine.

26. Evaporation. If a stopper is left off a cologne bottle, the
contents of the bottle will slowly evaporate; if a dish of water is
placed out of doors on a hot day, evaporation occurs very rapidly. The
liquids which have disappeared from the bottle and the dish have
passed into the surrounding air in the form of vapor. In Section 20,
we saw that water could not pass into vapor without the addition of
heat; now the heat necessary for the evaporation of the cologne and
water was taken from the air, leaving it slightly cooler. If wet hands
are not dried with a towel, but are left to dry by evaporation, heat
is taken from the hand in the process, leaving a sensation of
coolness. Damp clothing should never be worn, because the moisture in
it tends to evaporate at the expense of the bodily heat, and this
undue loss of heat from the body produces chills. After a bath the
body should be well rubbed, otherwise evaporation occurs at the
expense of heat which the body cannot ordinarily afford to lose.

Evaporation is a slow process occurring at all times; it is hastened
during the summer, because of the large amount of heat present in the
atmosphere. Many large cities make use of the cooling effect of
evaporation to lower the temperature of the air in summer; streets are
sprinkled not only to lay the dust, but in order that the surrounding
air may be cooled by the evaporation of the water.

Some thrifty housewives economize by utilizing the cooling effects of
evaporation. Butter, cheese, and other foods sensitive to heat are
placed in porous vessels wrapped in wet cloths. Rapid evaporation of
the water from the wet cloths keeps the contents of the jars cool, and
that without expense other than the muscular energy needed for wetting
the cloths frequently.

27. Rain, Snow, Frost, Dew. The heat of the sun causes constant
evaporation of the waters of oceans, rivers, streams, and marshes, and
the water vapor set free by evaporation passes into the air, which
becomes charged with vapor or is said to be humid. Constant, unceasing
evaporation of our lakes, streams, and pools would mean a steady
decrease in the supply of water available for daily use, if the
escaped water were all retained by the atmosphere and lost to the
earth. But although the escaped vapor mingles with the atmosphere,
hovering near the earth's surface, or rising far above the level of
the mountains, it does not remain there permanently. When this vapor
meets a cold wind or is chilled in any way, condensation takes place,
and a mass of tiny drops of water or of small particles of snow is
formed. When these drops or particles become large enough, they fall
to the earth as rain or snow, and in this way the earth is compensated
for the great loss of moisture due to evaporation. Fog is formed when
vapor condenses near the surface of the earth, and when the drops are
so small that they do not fall but hover in the air, the fog is said
"not to lift" or "not to clear."

If ice water is poured into a glass, a mist will form on the outside
of the glass. This is because the water vapor in the air becomes
chilled by contact with the glass and condenses. Often leaves and
grass and sidewalks are so cold that the water vapor in the atmosphere
condenses on them, and we say a heavy dew has formed. If the
temperature of the air falls to the freezing point while the dew is
forming, the vapor is frozen and frost is seen instead of dew.

The daily evaporation of moisture into the atmosphere keeps the
atmosphere more or less full of water vapor; but the atmosphere can
hold only a definite amount of vapor at a given temperature, and as
soon as it contains the maximum amount for that temperature, further
evaporation ceases. If clothes are hung out on a damp, murky day they
do not dry, because the air contains all the moisture it can hold, and
the moisture in the clothes has no chance to evaporate. When the air
contains all the moisture it can hold, it is said to be saturated, and
if a slight fall in temperature occurs when the air is saturated,
condensation immediately begins in the form of rain, snow, or fog. If,
however, the air is not saturated, a fall in temperature may occur
without producing precipitation. The temperature at which air is
saturated and condensation begins is called the _dew point_.

28. How Chills are Caused. The discomfort we feel in an overcrowded
room is partly due to an excess of moisture in the air, resulting from
the breathing and perspiration of many persons. The air soon becomes
saturated with vapor and cannot take away the perspiration from our
bodies, and our clothing becomes moist and our skin tender. When we
leave the crowded "tea" or lecture and pass into the colder, drier,
outside air, clothes and skin give up their load of moisture through
sudden evaporation. But evaporation requires heat, and this heat is
taken from our bodies, and a chill results.

Proper ventilation would eliminate much of the physical danger of
social events; fresh, dry air should be constantly admitted to crowded
rooms in order to replace the air saturated by the breath and
perspiration of the occupants.

29. Weather Forecasts. When the air is near the saturation point,
the weather is oppressive and is said to be very humid. For comfort
and health, the air should be about two thirds saturated. The presence
of some water vapor in the air is absolutely necessary to animal and
plant life. In desert regions where vapor is scarce the air is so dry
that throat trouble accompanied by disagreeable tickling is prevalent;
fallen leaves become so dry that they crumble to dust; plants lose
their freshness and beauty.

The likelihood of rain or frost is often determined by temperature and
humidity. If the air is near saturation and the temperature is
falling, it is safe to predict bad weather, because the fall of
temperature will probably cause rapid condensation, and hence rain.
If, however, the air is not near the saturation point, a fall in
temperature will not necessarily produce bad weather.

The measurement of humidity is of far wider importance than the mere
forecasting of local weather conditions. The close relation between
humidity and health has led many institutions, such as hospitals,
schools, and factories, to regulate the humidity of the atmosphere as
carefully as they do the temperature. Too great humidity is
enervating, and not conducive to either mental or physical exertion;
on the other hand, too dry air is equally harmful. In summer the
humidity conditions cannot be well regulated, but in winter, when
houses are artificially heated, the humidity of a room can be
increased by placing pans of water near the registers or on radiators.

30. Heat Needed to Melt Substances. If a spoon is placed in a vessel
of hot water for a few seconds and then removed, it will be warmer
than before it was placed in the hot water. If a lump of melting ice
is placed in the vessel of hot water and then removed, the ice will
not be warmer than before, but there will be less of it. The heat of
the water has been used in melting the ice, not in changing its
temperature.

If, on a bitter cold day, a pail of snow is brought into a warm room
and a thermometer is placed in the snow, the temperature rises
gradually until 32° F. is reached, when it becomes stationary, and the
snow begins to melt. If the pail is put on the fire, the temperature
still remains 32°F., but the snow melts more rapidly. As soon as all
the snow is completely melted, however, the temperature begins to rise
and rises steadily until the water boils, when it again becomes
stationary and remains so during the passage of water into vapor.

We see that heat must be supplied to ice at 0° C. or 32° F. in order
to change it into water, and further, that the temperature of the
mixture does not rise so long as any ice is present, no matter how
much heat is supplied. The amount of heat necessary to melt 1 gram of
ice is easily calculated. (See Laboratory Manual.)

Heat must be supplied to ice to melt it. On the other hand, water, in
freezing, loses heat, and the amount of heat lost by freezing water is
exactly equal to the amount of heat absorbed by melting ice.

The number of units of heat required to melt a unit mass of ice is
called the _heat of fusion_ of water.

31. Climate. Water, in freezing, loses heat, even though its
temperature remains at 0° C. Because water loses heat when it freezes,
the presence of large streams of water greatly influences the climate
of a region. In winter the heat from the freezing water keeps the
temperature of the surrounding higher than it would naturally be, and
consequently the cold weather is less severe. In summer water
evaporates, heat is taken from the air, and consequently the warm
weather is less intense.

32. Molding of Glass and Forging of Iron. The fire which is hot
enough to melt a lump of ice may not be hot enough to melt an iron
poker; on the other hand, it may be sufficiently hot to melt a tin
spoon. Different substances melt, or liquefy, at different
temperatures; for example, ice melts at 0° C., and tin at 233° C.,
while iron requires the relatively high temperature of 1200° C. Most
substances have a definite melting or freezing point which never
changes so long as the surrounding conditions remain the same.

But while most substances have a definite melting point, some
substances do not. If a glass rod is held in a Bunsen burner, it will
gradually grow softer and softer, and finally a drop of molten glass
will fall from the end of the rod into the fire. The glass did not
suddenly become a liquid at a definite temperature; instead it
softened gradually, and then melted. While glass is in the soft,
yielding, pliable state, it is molded into dishes, bottles, and other
useful objects, such as lamp shades, globes, etc. (Fig. 20). If glass
melted at a definite temperature, it could not be molded in this way.
Iron acts in a similar manner, and because of this property the
blacksmith can shape his horseshoes, and the workman can make his
engines and other articles of daily service to man.

[Illustration: FIG. 20.--Molten glass being rolled into a form
suitable for window panes.]

33. Strange Behavior of Water. One has but to remember that bottles
of water burst when they freeze, and that ice floats on water like
wood, to know that water expands on freezing or on solidifying. A
quantity of water which occupies 100 cubic feet of space will, on
becoming ice, need 109 cubic feet of space. On a cold winter night the
water sometimes freezes in the water pipes, and the pipes burst. Water
is very peculiar in expanding on solidification, because most
substances contract on solidifying; gelatin and jelly, for example,
contract so much that they shrink from the sides of the dish which
contains them.

If water contracted in freezing, ice would be heavier than water and
would sink in ponds and lakes as fast as it formed, and our streams
and ponds would become masses of solid ice, killing all animal and
plant life. But the ice is lighter than water and floats on top, and
animals in the water beneath are as free to live and swim as they were
in the warm sunny days of summer. The most severe winter cannot freeze
a deep lake solid, and in the coldest weather a hole made in the ice
will show water beneath the surface. Our ice boats cut and break the
ice of the river, and through the water beneath our boats daily ply
their way to and fro, independent of winter and its blighting blasts.

While most of us are familiar with the bursting of water pipes on a
cold night, few of us realize the influence which freezing water
exerts on the character of the land around us.

Water sinks into the ground and, on the approach of winter, freezes,
expanding about one tenth of its volume; the expanding ice pushes the
earth aside, the force in some cases being sufficient to dislodge even
huge rocks. In the early days in New England it was said by the
farmers that "rocks grew," because fields cleared of stones in the
fall became rock covered with the approach of spring; the rocks and
stones hidden underground and unseen in the fall were forced to the
surface by the winter's expansion. We have all seen fence posts and
bricks pushed out of place because of the heaving of the soil beneath
them. Often householders must relay their pavements and walks because
of the damage done by freezing water.

The most conspicuous effect of the expansive power of freezing water
is seen in rocky or mountainous regions (Fig. 21). Water easily finds
entrance into the cracks and crevices of the rocks, where it lodges
until frozen; then it expands and acts like a wedge, widening cracks,
chiseling off edges, and even breaking rocks asunder. In regions where
frequent frosts occur, the destructive action of water works constant
changes in the appearance of the land; small cracks and crevices are
enlarged, massive rocks are pried up out of position, huge slabs are
split off, and particles large and small are forced from the parent
rock. The greater part of the debris and rubbish brought down from the
mountain slopes by the spring rains owes its origin to the fact that
water expands when it freezes.

[Illustration: FIG. 21.--The destruction caused by freezing water.]

34. Heat Necessary to Dissolve a Substance. It requires heat to
dissolve any substance, just as it requires heat to change ice to
water. If a handful of common salt is placed in a small cup of water
and stirred with a thermometer, the temperature of the mixture falls
several degrees. This is just what one would expect, because the heat
needed to liquefy the salt must come from somewhere, and naturally it
comes from the water, thereby lowering the temperature of the water.
We know very well that potatoes cease boiling if a pinch of salt is
put in the water; this is because the temperature of the water has
been lowered by the amount of heat necessary to dissolve the salt.

Let some snow or chopped ice be placed in a vessel and mixed with one
third its weight of coarse salt; if then a small tube of cold water is
placed in this mixture, the water in the test tube will soon freeze
solid. As soon as the snow and salt are mixed they melt. The heat
necessary for this comes in part from the air and in part from the
water in the test tube, and the water in the tube becomes in
consequence cold enough to freeze. But the salt mixture does not
freeze because its freezing point is far below that of pure water. The
use of salt and ice in ice-cream freezers is a practical application
of this principle. The heat necessary for melting the mixture of salt
and ice is taken from the cream which thus becomes cold enough to
freeze.



CHAPTER IV

BURNING OR OXIDATION


35. Why Things Burn. The heat of our bodies comes from the food we
eat; the heat for cooking and for warming our houses comes from coal.
The production of heat through the burning of coal, or oil, or gas, or
wood, is called combustion. Combustion cannot occur without the
presence of a substance called oxygen, which exists rather abundantly
in the air; that is, one fifth of our atmosphere consists of this
substance which we call oxygen. We throw open our windows to allow
fresh air to enter, and we take walks in order to breathe the pure air
into our lungs. What we need for the energy and warmth of our bodies
is the oxygen in the air. Whether we burn gas or wood or coal, the
heat which is produced comes from the power which these various
substances possess to combine with oxygen. We open the draft of a
stove that it may "draw well": that it may secure oxygen for burning.
We throw a blanket over burning material to smother the fire: to keep
oxygen away from it. Burning, or oxidation, is combining with oxygen,
and the more oxygen you add to a fire, the hotter the fire will burn,
and the faster. The effect of oxygen on combustion may be clearly seen
by thrusting a smoldering splinter into a jar containing oxygen; the
smoldering splinter will instantly flare and blaze, while if it is
removed from the jar, it loses its flame and again burns quietly.
Oxygen for this experiment can be produced in the following way.

[Illustration: FIG. 22.--Preparing oxygen from potassium chlorate and
manganese dioxide.]

36. How to Prepare Oxygen. Mix a small quantity of potassium
chlorate with an equal amount of manganese dioxide and place the
mixture in a strong test tube. Close the mouth of the tube with a
one-hole rubber stopper in which is fitted a long, narrow tube, and
clamp the test tube to an iron support, as shown in Figure 22. Fill
the trough with water until the shelf is just covered and allow the
end of the delivery tube to rest just beneath the hole in the shelf.
Fill a medium-sized bottle with water, cover it with a glass plate,
invert the bottle in the trough, and then remove the glass plate. Heat
the test tube very gently, and when gas bubbles out of the tube, slip
the bottle over the opening in the shelf, so that the tube runs into
the bottle. The gas will force out the water and will finally fill the
bottle. When all the water has been forced out, slip the glass plate
under the mouth of the bottle and remove the bottle from the trough.
The gas in the bottle is oxygen.

Everywhere in a large city or in a small village, smoke is seen,
indicating the presence of fire; hence there must exist a large supply
of oxygen to keep all the fires alive. The supply of oxygen needed
for the fires of the world comes largely from the atmosphere.

37. Matches. The burning material is ordinarily set on fire by
matches, thin strips of wood tipped with sulphur or phosphorus, or
both. Phosphorus can unite with oxygen at a fairly low temperature,
and if phosphorus is rubbed against a rough surface, the friction
produced will raise the temperature of the phosphorus to a point where
it can combine with oxygen. The burning phosphorus kindles the wood of
the match, and from the burning match the fire is kindled. If you want
to convince yourself that friction produces heat, rub a cent
vigorously against your coat and note that the cent becomes warm.
Matches have been in use less than a hundred years. Primitive man
kindled his camp fire by rubbing pieces of dry wood together until
they took fire, and this method is said to be used among some isolated
distant tribes at the present time. A later and easier way was to
strike flint and steel together and to catch the spark thus produced
on tinder or dry fungus. Within the memory of some persons now living,
the tinder box was a valuable asset to the home, particularly in the
pioneer regions of the West.

38. Safety Matches. Ordinary phosphorus, while excellent as a
fire-producing material, is dangerously poisonous, and those to whom
the dipping of wooden strips into phosphorus is a daily occupation
suffer with a terrible disease which usually attacks the teeth and
bones of the jaw. The teeth rot and fall out, abscesses form, and
bones and flesh begin to decay; the only way to prevent the spread of
the disease is to remove the affected bone, and in some instances it
has been necessary to remove the entire jaw. Then, too, matches made
of yellow or white phosphorus ignite easily, and, when rubbed against
any rough surface, are apt to take fire. Many destructive fires have
been started by the accidental friction of such matches against rough
surfaces.

For these reasons the introduction of the so-called safety match was
an important event. When common phosphorus, in the dangerous and
easily ignited form, is heated in a closed vessel to about 250° C., it
gradually changes to a harmless red mass. The red phosphorus is not
only harmless, but it is difficult to ignite, and, in order to be
ignited by friction, must be rubbed on a surface rich in oxygen. The
head of a safety match is coated with a mixture of glue and
oxygen-containing compounds; the surface on which the match is to be
rubbed is coated with a mixture of red phosphorus and glue, to which
finely powdered glass is sometimes added in order to increase the
friction. Unless the head of the match is rubbed on the prepared
phosphorus coating, ignition does not occur, and accidental fires are
avoided.

Various kinds of safety matches have been manufactured in the last few
years, but they are somewhat more expensive than the ordinary form,
and hence manufacturers are reluctant to substitute them for the
cheaper matches. Some foreign countries, such as Switzerland, prohibit
the sale of the dangerous type, and it is hoped that the United States
will soon follow the lead of these countries in demanding the sale of
safety matches only.

39. Some Unfamiliar Forms of Burning. While most of us think of
burning as a process in which flames and smoke occur, there are in
reality many modes of burning accompanied by neither flame nor smoke.
Iron, for example, burns when it rusts, because it slowly combines
with the oxygen of the air and is transformed into new substances.
When the air is dry, iron does not unite with oxygen, but when
moisture is present in the air, the iron unites with the oxygen and
turns into iron rust. The burning is slow and unaccompanied by the
fire and smoke so familiar to us, but the process is none the less
burning, or combination with oxygen. Burning which is not accompanied
by any of the appearances of ordinary burning is known as oxidation.

The tendency of iron to rust lessens its efficiency and value, and
many devices have been introduced to prevent rusting. A coating of
paint or varnish is sometimes applied to iron in order to prevent
contact with air. The galvanizing of iron is another attempt to secure
the same result; in this process iron is dipped into molten zinc,
thereby acquiring a coating of zinc, and forming what is known as
galvanized iron. Zinc does not combine with oxygen under ordinary
circumstances, and hence galvanized iron is immune from rust.

Decay is a process of oxidation; the tree which rots slowly away is
undergoing oxidation, and the result of the slow burning is the
decomposed matter which we see and the invisible gases which pass into
the atmosphere. The log which blazes on our hearth gives out
sufficient heat to warm us; the log which decays in the forest gives
out an equivalent amount of heat, but the heat is evolved so slowly
that we are not conscious of it. Burning accompanied by a blaze and
intense heat is a rapid process; burning unaccompanied by fire and
appreciable heat is a slow, gradual process, requiring days, weeks,
and even long years for its completion.

Another form of oxidation occurs daily in the human body. In Section
35 we saw that the human body is an engine whose fuel is food; the
burning of that food in the body furnishes the heat necessary for
bodily warmth and the energy required for thought and action. Oxygen
is essential to burning, and the food fires within the body are kept
alive by the oxygen taken into the body at every breath by the lungs.
We see now one reason for an abundance of fresh air in daily life.

40. How to Breathe. Air, which is essential to life and health,
should enter the body through the nose and _not through the mouth_.
The peculiar nature and arrangement of the membranes of the nose
enable the nostrils to clean, and warm, and moisten the air which
passes through them to the lungs. Floating around in the atmosphere
are dust particles which ought not to get into the lungs. The nose is
provided with small hairs and a moist inner membrane which serve as
filters in removing solid particles from the air, and in thus
purifying it before its entrance into the lungs.

In the immediate neighborhood of three Philadelphia high schools,
having an approximate enrollment of over 8000 pupils, is a huge
manufacturing plant which day and night pours forth grimy smoke and
soot into the atmosphere which must supply oxygen to this vast group
of young lives. If the vital importance of nose breathing is impressed
upon these young people, the harmful effect of the foul air may be
greatly lessened, the smoke particles and germs being held back by the
nose filters and never reaching the lungs. If, however, this principle
of hygiene is not brought to their attention, the dangerous habit of
breathing through the open, or at least partially open, mouth will
continue, and objectionable matter will pass through the mouth and
find a lodging place in the lungs.

There is another very important reason why nose breathing is
preferable to mouth breathing. The temperature of the human body is
approximately 98° F., and the air which enters the lungs should not be
far below this temperature. If air reaches the lungs through the nose,
its journey is relatively long and slow, and there is opportunity for
it to be warmed before it reaches the lungs. If, on the other hand,
air passes to the lungs by way of the mouth, the warming process is
brief and insufficient, and the lungs suffer in consequence.
Naturally, the gravest danger is in winter.

41. Cause of Mouth Breathing. Some people find it difficult to
breathe through the nostrils on account of growths, called adenoids,
in the nose. If you have a tendency toward mouth breathing, let a
physician examine your nose and throat.

Adenoids not only obstruct breathing and weaken the whole system
through lack of adequate air, but they also press upon the blood
vessels and nerves of the head and interfere with normal brain
development. Moreover, they interfere in many cases with the hearing,
and in general hinder activity and growth. The removal of adenoids is
simple, and carries with it only temporary pain and no danger. Some
physicians claim that the growths disappear in later years, but even
if that is true, the physical and mental development of earlier years
is lost, and the person is backward in the struggle for life and
achievement.

[Illustration: FIG. 23.--Intelligent expression is often lacking in
children with adenoid growths.]

42. How to Build a Fire. Substances differ greatly as to the ease
with which they may be made to burn or, in technical terms, with which
they may be made to unite with oxygen. For this reason, we put light
materials, like shavings, chips, and paper, on the grate, twisting the
latter and arranging it so that air (oxygen in the air) can reach a
large surface; upon this we place small sticks of wood, piling them
across each other so as to allow entrance for the oxygen; and finally
upon this we place our hard wood or coal.

The coal and the large sticks cannot be kindled with a match, but the
paper and shavings can, and these in burning will heat the large
sticks until they take fire and in turn kindle the coal.

43. Spontaneous Combustion. We often hear of fires "starting
themselves," and sometimes the statement is true. If a pile of oily
rags is allowed to stand for a time, the oily matter will begin to
combine slowly with oxygen and as a result will give off heat. The
heat thus given off is at first insufficient to kindle a fire; but as
the heat is retained and accumulated, the temperature rises, and
finally the kindling point is reached and the whole mass bursts into
flames. For safety's sake, all oily cloths should be burned or kept in
metal vessels.

44. The Treatment of Burns. In spite of great caution, burns from
fires, steam, or hot water do sometimes occur, and it is well to know
how to relieve the suffering caused by them and how to treat the
injury in order to insure rapid healing.

Burns are dangerous because they destroy skin and thus open up an
entrance into the body for disease germs, and in addition because they
lay bare nerve tissue which thereby becomes irritated and causes a
shock to the entire system.

In mild burns, where the skin is not broken but is merely reddened, an
application of moist baking soda brings immediate relief. If this
substance is not available, flour paste, lard, sweet oil, or vaseline
may be used.

In more severe burns, where blisters are formed, the blisters should
be punctured with a sharp, sterilized needle and allowed to discharge
their watery contents before the above remedies are applied.

In burns severe enough to destroy the skin, disinfection of the open
wound with weak carbolic acid or hydrogen peroxide is very necessary.
After this has been done, a soft cloth soaked in a solution of linseed
oil and limewater should be applied and the whole bandaged. In such a
case, it is important not to use cotton batting, since this sticks to
the rough surface and causes pain when removed.

45. Carbon Dioxide. _A Product of Burning._ When any fuel, such as
coal, gas, oil, or wood, burns, it sends forth gases into the
surrounding atmosphere. These gases, like air, are invisible, and were
unknown to us for a long time. The chief gas formed by a burning
substance is called carbon dioxide (CO_2) because it is composed of
one part of carbon and two parts of oxygen. This gas has the
distinction of being the most widely distributed gaseous compound of
the entire world; it is found in the ocean depths and on the mountain
heights, in brilliantly lighted rooms, and most abundantly in
manufacturing towns where factory chimneys constantly pour forth hot
gases and smoke.

Wood and coal, and in fact all animal and vegetable matter, contain
carbon, and when these substances burn or decay, the carbon in them
unites with oxygen and forms carbon dioxide.

The food which we eat is either animal or vegetable, and it is made
ready for bodily use by a slow process of burning within the body;
carbon dioxide accompanies this bodily burning of food just as it
accompanies the fires with which we are more familiar. The carbon
dioxide thus produced within the body escapes into the atmosphere with
the breath.

We see that the source of carbon dioxide is practically inexhaustible,
coming as it does from every stove, furnace, and candle, and further
with every breath of a living organism.

46. Danger of Carbon Dioxide. When carbon dioxide occurs in large
quantities, it is dangerous to health, because it interferes with
normal breathing, lessening the escape of waste matter through the
breath and preventing the access to the lungs of the oxygen necessary
for life. Carbon dioxide is not poisonous, but it cuts off the supply
of oxygen, just as water cuts it off from a drowning man.

Since every man, woman, and child constantly breathes forth carbon
dioxide, the danger in overcrowded rooms is great, and proper
ventilation is of vital importance.

47. Ventilation. In estimating the quantity of air necessary to keep
a room well aired, we must take into account the number of lights
(electric lights do not count) to be used, and the number of people to
occupy the room. The average house should provide at the _minimum_ 600
cubic feet of space for each person, and in addition, arrangements for
allowing at least 300 cubic feet of fresh air per person to enter
every hour.

In houses which have not a ventilating system, the air should be kept
fresh by intelligent action in the opening of doors and windows; and
since relatively few houses are equipped with a satisfactory system,
the following suggestions relative to intelligent ventilation are
offered.

1. Avoid drafts in ventilation.

2. Ventilate on the sheltered side of the house. If the wind is
blowing from the north, open south windows.

48. What Becomes of the Carbon Dioxide. When we reflect that carbon
dioxide is constantly being supplied to the atmosphere and that it is
injurious to health, the question naturally arises as to how the air
remains free enough of the gas to support life. This is largely
because carbon dioxide is an essential food of plants. Through their
leaves plants absorb it from the atmosphere, and by a wonderful
process break it up into its component parts, oxygen and carbon. They
reject the oxygen, which passes back to the air, but they retain the
carbon, which becomes a part of the plant structure. Plants thus serve
to keep the atmosphere free from an excess of carbon dioxide and, in
addition, furnish oxygen to the atmosphere.

[Illustration: FIG. 24.--Making carbon dioxide from marble and
hydrochloric acid.]

49. How to Obtain Carbon Dioxide. There are several ways in which
carbon dioxide can be produced commercially, but for laboratory use
the simplest is to mix in a test tube powdered marble, or chalk, and
hydrochloric acid, and to collect the effervescing gas as shown in
Figure 24. The substance which remains in the test tube after the gas
has passed off is a solution of a salt and water. From a mixture of
hydrochloric acid (HCl) and marble are obtained a salt, water, and
carbon dioxide, the desired gas.

50. A Commercial Use of Carbon Dioxide. If a lighted splinter is
thrust into a test tube containing carbon dioxide, it is promptly
extinguished, because carbon dioxide cannot support combustion; if a
stream of carbon dioxide and water falls upon a fire, it acts like a
blanket, covering the flames and extinguishing them. The value of a
fire extinguisher depends upon the amount of carbon dioxide and water
which it can furnish. A fire extinguisher is a metal case containing a
solution of bicarbonate of soda, and a glass vessel full of strong
sulphuric acid. As long as the extinguisher is in an upright position,
these substances are kept separate, but when the extinguisher is
inverted, the acid escapes from the bottle, and mixes with the soda
solution. The mingling liquids interact and liberate carbon dioxide.
A part of the gas thus liberated dissolves in the water of the soda
solution and escapes from the tube with the outflowing liquid, while a
portion remains undissolved and escapes as a stream of gas. The fire
extinguisher is therefore the source of a liquid containing the
fire-extinguishing substance and further the source of a stream of
carbon dioxide gas.

[Illustration: FIG. 25.--Inside view of a fire extinguisher.]

51. Carbon. Although carbon dioxide is very injurious to health,
both of the substances of which it is composed are necessary to life.
We ourselves, our bones and flesh in particular, are partly carbon,
and every animal, no matter how small or insignificant, contains some
carbon; while the plants around us, the trees, the grass, the flowers,
contain a by no means meager quantity of carbon.

Carbon plays an important and varied role in our life, and, in some
one of its many forms, enters into the composition of most of the
substances which are of service and value to man. The food we eat, the
clothes we wear, the wood and coal we burn, the marble we employ in
building, the indispensable soap, and the ornamental diamond, all
contain carbon in some form.

52. Charcoal. One of the most valuable forms of carbon is charcoal;
valuable not in the sense that it costs hundreds of dollars, but in
the more vital sense, that its use adds to the cleanliness, comfort,
and health of man.

The foul, bad-smelling gases which arise from sewers can be prevented
from escaping and passing to streets and buildings by placing charcoal
filters at the sewer exits. Charcoal is porous and absorbs foul gases,
and thus keeps the region surrounding sewers sweet and clean and free
of odor. Good housekeepers drop small bits of charcoal into vases of
flowers to prevent discoloration of the water and the odor of decaying
stems.

If impure water filters through charcoal, it emerges pure, having left
its impurities in the pores of the charcoal. Practically all household
filters of drinking water are made of charcoal. But such a device may
be a source of disease instead of a prevention of disease, unless the
filter is regularly cleaned or renewed. This is because the pores soon
become clogged with the impurities, and unless they are cleaned, the
water which flows through the filter passes through a bed of
impurities and becomes contaminated rather than purified. Frequent
cleansing or renewal of the filter removes this difficulty.

Commercially, charcoal is used on a large scale in the refining of
sugars, sirups, and oils. Sugar, whether it comes from the maple tree,
or the sugar cane, or the beet, is dark colored. It is whitened by
passage through filters of finely pulverized charcoal. Cider and
vinegar are likewise cleared by passage through charcoal.

The value of carbon, in the form of charcoal, as a purifier is very
great, whether we consider it a deodorizer, as in the case of the
sewage, or a decolorizer, as in the case of the refineries, or whether
we consider the service it has rendered man in the elimination of
danger from drinking water.

53. How Charcoal is Made. Charcoal may be made by heating wood in an
oven to which air does not have free access. The absence of air
prevents ordinary combustion, nevertheless the intense heat affects
the wood and changes it into new substances, one of which is charcoal.

The wood which smolders on the hearth and in the stove is charcoal in
the making. Formerly wood was piled in heaps, covered with sod or sand
to prevent access of oxygen, and then was set fire to; the smoldering
wood, cut off from an adequate supply of air, was slowly transformed
into charcoal. Scattered over the country one still finds isolated
charcoal kilns, crude earthen receptacles, in which wood thus deprived
of air was allowed to smolder and form charcoal. To-day charcoal is
made commercially by piling wood on steel cars and then pushing the
cars into strong walled chambers. The chambers are closed to prevent
access of air, and heated to a high temperature. The intense heat
transforms the wood into charcoal in a few hours. A student can make
in the laboratory sufficient charcoal for art lessons by heating in an
earthen vessel wood buried in sand. The process will be slow, however,
because the heat furnished by a Bunsen burner is not great, and the
wood is transformed slowly.

A form of charcoal known as animal charcoal, or bone black, is
obtained from the charred remains of animals rather than plants, and
may be prepared by burning bones and animal refuse as in the case of
the wood.

Destructive Distillation. When wood is burned without sufficient
air, it is changed into soft brittle charcoal, which is very different
from wood. It weighs only one fourth as much as the original wood. It
is evident that much matter must leave the wood during the process of
charcoal making. We can prove this by putting some dry shavings in a
strong test tube fitted with a delivery tube. When the wood is heated
a gas passes off which we may collect and burn. Other substances also
come off in gaseous form, but they condense in the water. Among these
are wood alcohol, wood tar, and acetic acid. In the older method of
charcoal making all these products were lost. Can you give any uses of
these substances?

54. Matter and Energy. When wood is burned, a small pile of ashes is
left, and we think of the bulk of the wood as destroyed. It is true we
have less matter that is available for use or that is visible to
sight, but, nevertheless, no matter has been destroyed. The matter of
which the wood is composed has merely changed its character, some of
it is in the condition of ashes, and some in the condition of
invisible gases, such as carbon dioxide, but none of it has been
destroyed. It is a principle of science that matter can neither be
destroyed nor created; it can only be changed, or transformed, and it
is our business to see that we do not heedlessly transform it into
substances which are valueless to us and our descendants; as, for
example, when our magnificent forests are recklessly wasted. The
smoke, gases, and ashes left in the path of a raging forest fire are
no compensation to us for the valuable timber destroyed. The sum total
of matter has not been changed, but the amount of matter which man can
use has been greatly lessened.

The principle just stated embodies one of the fundamental laws of
science, called the law of the _conservation of matter_.

A similar law holds for energy as well. We can transform electric
energy into the motion of trolley cars, or we can make use of the
energy of streams to turn the wheels of our mills, but in all these
cases we are transforming, not creating, energy.

When a ball is fired from a rifle, most of the energy of the gunpowder
is utilized in motion, but some is dissipated in producing a flash and
a report, and in heat. The energy of the gunpowder has been scattered,
but the sum of the various forms of energy is equal to the energy
originally stored away in the powder. The better the gun is, the less
will be the energy dissipated in smoke and heat and noise.



CHAPTER V

FOOD


55. The Body as a Machine. Wholesome food and fresh air are
necessary for a healthy body. Many housewives, through ignorance,
supply to their hard-working husbands and their growing sons and
daughters food which satisfies the appetite, but which does not give
to the body the elements needed for daily work and growth. Some foods,
such as lettuce, cucumbers, and watermelons, make proper and
satisfactory changes in diet, but are not strength giving. Other
foods, like peas and beans, not only satisfy the appetite, but supply
to the body abundant nourishment. Many immigrants live cheaply and
well with beans and bread as their main diet.

It is of vital importance that the relative value of different foods
as heat producers be known definitely; and just as the yard measures
length and the pound measures weight the calorie is used to measure
the amount of heat which a food is capable of furnishing to the body.
Our bodies are human machines, and, like all other machines, require
fuel for their maintenance. The fuel supplied to an engine is not all
available for pulling the cars; a large portion of the fuel is lost in
smoke, and another portion is wasted as ashes. So it is with the fuel
that runs the body. The food we eat is not all available for
nourishment, much of it being as useless to us as are smoke and ashes
to an engine. The best foods are those which do the most for us with
the least possible waste.

56. Fuel Value. By fuel value is meant the capacity foods have for
yielding heat to the body. The fuel value of the foods we eat daily is
so important a factor in life that physicians, dietitians, nurses,
and those having the care of institutional cooking acquaint themselves
with the relative fuel values of practically all of the important food
substances. The life or death of a patient may be determined by the
patient's diet, and the working and earning capacity of a father
depends largely upon his prosaic three meals. An ounce of fat, whether
it is the fat of meat or the fat of olive oil or the fat of any other
food, produces in the body two and a quarter times as much heat as an
ounce of starch. Of the vegetables, beans provide the greatest
nourishment at the least cost, and to a large extent may be
substituted for meat. It is not uncommon to find an outdoor laborer
consuming one pound of beans per day, and taking meat only on "high
days and holidays."

[Illustration: FIG. 26.--The bomb calorimeter from which the fuel
value of food can be estimated.]

The fuel value of a food is determined by means of the _bomb
calorimeter_ (Fig. 26). The food substance is put into a chamber _A_
and ignited, and the heat of the burning substance raises the
temperature of the water in the surrounding vessel. If 1000 grams of
water are in the vessel, and the temperature of the water is raised 2°
C., the number of calories produced by the substance would be 2000,
and the fuel value would be 2000 calories.[A] From this the fuel value
of one quart or one pound of the substance can be determined, and the
food substance will be said to furnish the body with that number of
heat units, providing all of the pound of food were properly digested.

[Footnote A: As applied to food, the calorie is greater than that used
in the ordinary laboratory work, being the amount of heat necessary to
raise the temperature of 1000 grams of water 1° C., rather than 1 gram
1° C.]

    TABLE SHOWING THE NUMBER OF CALORIES FURNISHED BY
    ONE POUND OF VARIOUS FOODS
  ----------------------------------------------------
  |FOOD               |CALORIES|FOOD         |CALORIES|
  ----------------------------------------------------
  |Leg of lean mutton |     790|Carrots      |     210|
  ----------------------------------------------------
  |Rib of beef        |    1150|Lettuce      |      90|
  ----------------------------------------------------
  |Shad               |     380|Onion        |     225|
  ----------------------------------------------------
  |Chicken            |     505|Cucumber     |      80|
  ----------------------------------------------------
  |Apples             |     290|Almonds      |    3030|
  ----------------------------------------------------
  |Bananas            |     460|Walnuts      |    3306|
  ----------------------------------------------------
  |Prunes             |     370|Peanuts      |    2560|
  ----------------------------------------------------
  |Watermelons        |     140|Oatmeal      |    4673|
  ----------------------------------------------------
  |Lima beans         |     570|Rolled wheat |    4175|
  ----------------------------------------------------
  |Beets              |     215|Macaroni     |    1665|
  ----------------------------------------------------

57. Varied Diet. The human body is a much more varied and complex
machine than any ever devised by man; personal peculiarities, as well
as fuel values, influence very largely the diet of an individual.
Strawberries are excluded from some diets because of a rash which is
produced on the skin, pork is excluded from other diets for a like
reason; cauliflower is absolutely indigestible to some and is readily
digested by others. From practically every diet some foods must be
excluded, no matter what the fuel value of the substance may be.

Then, too, there are more uses for food than the production of heat.
Teeth and bones and nails need a constant supply of mineral matter,
and mineral matter is frequently found in greatest abundance in foods
of low fuel value, such as lettuce, watercress, etc., though
practically all foods yield at least a small mineral constituent. When
fuel values alone are considered, fruits have a low value, but because
of the flavor they impart to other foods, and because of the healthful
influence they exercise in digestion, they cannot be excluded from the
diet.

Care should be constantly exercised to provide substantial foods of
high fuel value. But the nutritive foods should be wisely supplemented
by such foods as fruits, whose real value is one of indirect rather
then direct service.

58. Our Bodies. Somewhat as a house is composed of a group of
bricks, or a sand heap of grains of sand, the human body is composed
of small divisions called cells. Ordinarily we cannot see these cells
because of their minuteness, but if we examine a piece of skin, or a
hair of the head, or a tiny sliver of bone under the microscope, we
see that each of these is composed of a group of different cells. A
merchant, watchful about the fineness of the wool which he is
purchasing, counts with his lens the number of threads to the inch; a
physician, when he wishes, can, with the aid of the microscope,
examine the cells in a muscle, or in a piece of fat, or in a nerve
fiber. Not only is the human body composed of cells, but so also are
the bodies of all animals from the tiny gnat which annoys us, and the
fly which buzzes around us, to the mammoth creatures of the tropics.
These cells do the work of the body, the bone cells build up the
skeleton, the nail cells form the finger and toe nails, the lung cells
take care of breathing, the muscle cells control motion, and the brain
cells are responsible for thought.

59. Why we eat so Much. The cells of the body are constantly, day by
day, minute by minute, breaking down and needing repair, are
constantly requiring replacement by new cells, and, in the case of the
child, are continually increasing in number. The repair of an ordinary
machine, an engine, for example, is made at the expense of money, but
the repair and replacement of our human cell machinery are
accomplished at the expense of food. More than one third of all the
food we eat goes to maintain the body cells, and to keep them in good
order. It is for this reason that we consume a large quantity of food.
If all the food we eat were utilized for energy, the housewife could
cook less, and the housefather could save money on grocer's and
butcher's bills. If you put a ton of coal in an engine, its available
energy is used to run the engine, but if the engine were like the
human body, one third of the ton would be used up by the engine in
keeping walls, shafts, wheels, belts, etc., in order, and only two
thirds would go towards running the engine. When an engine is not
working, fuel is not consumed, but the body requires food for mere
existence, regardless of whether it does active work or not. When we
work, the cells break down more quickly, and the repair is greater
than when we are at rest, and hence there is need of a larger amount
of food; but whether we work or not, food is necessary.

60. The Different Foods. The body is very exacting in its demands,
requiring certain definite foods for the formation and maintenance of
its cells, and other foods, equally definite, but of different
character, for heat; our diet therefore must contain foods of high
fuel value, and likewise foods of cell-forming power.

Although the foods which we eat are of widely different character,
such as fruits, vegetables, cereals, oils, meats, eggs, milk, cheese,
etc., they can be put into three great classes: the carbohydrates, the
fats, and the proteids.

61. The Carbohydrates. Corn, wheat, rye, in fact all cereals and
grains, potatoes, and most vegetables are rich in carbohydrates; as
are also sugar, molasses, honey, and maple sirup. The foods of the
first group are valuable because of the starch they contain; for
example, corn starch, wheat starch, potato starch. The substances of
the second group are valuable because of the sugar they contain; sugar
contains the maximum amount of carbohydrate. In the sirups there is a
considerable quantity of sugar, while in some fruits it is present in
more or less dilute form. Sweet peaches, apples, grapes, contain a
moderate amount of sugar; watermelons, pears, etc., contain less. Most
of our carbohydrates are of plant origin, being found in vegetables,
fruits, cereals, and sirups.

Carbohydrates, whether of the starch group or the sugar group, are
composed chiefly of three elements: carbon, hydrogen, and oxygen; they
are therefore combustible, and are great energy producers. On the
other hand, they are worthless for cell growth and repair, and if we
limited our diet to carbohydrates, we should be like a man who had
fuel but no engine capable of using it.

62. The Fats. The best-known fats are butter, lard, olive oil, and
the fats of meats, cheese, and chocolate. When we test fats for fuel
values by means of a calorimeter (Fig. 26), we find that they yield
twice as much heat as the carbohydrates, but that they burn out more
quickly. Dwellers in cold climates must constantly eat large
quantities of fatty foods if they are to keep their bodies warm and
survive the extreme cold. Cod liver oil is an excellent food medicine,
and if taken in winter serves to warm the body and to protect it
against the rigors of cold weather. The average person avoids fatty
foods in summer, knowing from experience that rich foods make him warm
and uncomfortable. The harder we work and the colder the weather, the
more food of that kind do we require; it is said that a lumberman
doing heavy out-of-door work in cold climates needs three times as
much food as a city clerk. Most of our fats, like lard and butter, are
of animal origin; some of them, however, like olive oil, peanut
butter, and coconut oil, are of plant origin.

[Illustration: FIG. 27.--_a_ is the amount of fat necessary to make
one calorie; _b_ is the amount of sugar or proteid necessary to make
one calorie.]

63. The Proteids. The proteids are the building foods, furnishing
muscle, bone, skin cells, etc., and supplying blood and other bodily
fluids. The best-known proteids are white of egg, curd of milk, and
lean of fish and meat; peas and beans have an abundant supply of this
substance, and nuts are rich in it. Most of our proteids are of animal
origin, but some protein material is also found in the vegetable
world. This class of foods contains carbon, oxygen, and hydrogen, and
in addition, two substances not found in carbohydrates or
fats--namely, sulphur and nitrogen. Proteids always contain nitrogen,
and hence they are frequently spoken of as nitrogenous foods. Since
the proteids contain all the elements found in the two other classes
of foods, they are able to contribute, if necessary, to the store of
bodily energy; but their main function is upbuilding, and the diet
should be chosen so that the proteids do not have a double task.

For an average man four ounces of dry proteid matter daily will
suffice to keep the body cells in normal condition.

It has been estimated that 300,000,000 blood cells alone need daily
repair or renewal. When we consider that the blood is but one part of
the body, and that all organs and fluids have corresponding
requirements, we realize how vast is the work to be done by the food
which we eat.

64. Mistakes in Buying. The body demands a daily ration of the three
classes of food stuffs, but it is for us to determine from what
meats, vegetables, fruits, cereals, etc., this supply shall be
obtained (Figs. 28 and 29).

[Illustration: FIG. 28.--Table of food values.]

[Illustration: FIG. 29.--Diagram showing the difference in the cost of
three foods which give about the same amount of nutrition each.]

Generally speaking, meats are the most expensive foods we can
purchase, and hence should be bought seldom and in small quantities.
Their place can be taken by beans, peas, potatoes, etc., and at less
than a quarter of the cost. The average American family eats meat
three times a day, while the average family of the more conservative
and older countries rarely eats meat more than once a day. The
following tables indicate the financial loss arising from an unwise
selection of foods:--

                     FOOD CONSUMED--ONE WEEK
|===========================|=======================================|
|         FAMILY No. 1      |       ||         FAMILY No. 2         |
|---------------------------|---------------------------------------|
|20 loaves of bread         | $1.00 ||15 lb. flour, bread           |
|10 to 12 lb. loin steak    |       ||   home made (skim milk used) | $.45
|   or meat of similar cost |  2.00 ||Yeast, shortening, and        |
|20 to 25 lb. rib roast     |       ||   skim milk                  |  .10
|   or similar meat         |  4.40 ||10 lb. steak (round, Hamburger|
|4 lb. high-priced cereal   |       ||   and some loin)             | 1.50
|      breakfast food, 20¢  |   .80 ||10 lb. other meats, boiling   |
|Cake and pastry purchased  |  3.00 ||   pieces, rump roast, etc.   | 1.00
|8 lb. butter, 30¢          |  2.40 || 5 lb. cheese, 16¢            |  .80
|Tea, coffee, spices, etc.  |   .75 || 5 lb. oatmeal (bulk)         |  .15
|Mushrooms                  |   .75 || 5 lb. beans                  |  .25
|Celery                     |  1.00 ||Home-made cake and pastry     | 1.00
|Oranges                    |  2.00 || 6 lb. butter, 30¢            | 1.80
|Potatoes                   |   .25 || 3 lb. home-made shortening   |  .25
|Miscellaneous canned goods |  2.00 ||Tea, coffee, and spices       |  .40
|Milk                       |   .50 ||Apples                        |  .50
|Miscellaneous foods        |  2.00 ||Prunes                        |  .25
|3 doz. eggs                |   .60 ||Potatoes                      |  .25
|                           |-------||Milk                          | 1.00
|                           |$23.45 ||Miscellaneous foods           | 1.00
|                           |       || 3 doz. eggs                  |  .60
|                           |       ||                             -|-----
|                           |       ||                             $|11.30
|    -----------------------|-----------------------------------------|---
|    -----------------------|-----------------------------------------|---

"The tables show that one family spends over twice as much in the
purchase of foods as the other family, and yet the one whose food
costs the less actually secures the larger amount of nutritive
material and is better fed than the family where more money is
expended."--From _Human Foods_, Snyder.

The Source of the Different Foods. All of our food comes from either
the plant world or the animal world. Broadly speaking, plants furnish
the carbohydrates, that is, starch and sugar; animals furnish the fats
and proteids. But although vegetable foods yield carbohydrates mainly,
some of them, like beans and peas, contain large quantities of protein
and can be substituted for meat without disadvantage to the body.
Other plant products, such as nuts, have fat as their most abundant
food constituent. The peanut, for example, contains 43% of fat, 30% of
proteids, and only 17% of carbohydrates; the Brazil nut has 65% of
fat, 17% of proteids, and only 9% of carbohydrates. Nuts make a good
meat substitute, and since they contain a fair amount of carbohydrates
besides the fats and proteins, they supply all of the essential food
constituents and form a well-balanced food.



CHAPTER VI

WATER


65. Destructive Action of Water. The action of water in stream and
sea, in springs and wells, is evident to all; but the activity of
ground water--that is, rain water which sinks into the soil and
remains there--is little known in general. The real activity of ground
water is due to its great solvent power; every time we put sugar into
tea or soap into water we are using water as a solvent. When rain
falls, it dissolves substances floating in the atmosphere, and when it
sinks into the ground and becomes ground water, it dissolves material
out of the rock which it encounters (Fig. 30). We know that water
contains some mineral matter, because kettles in which water is boiled
acquire in a short time a crust or coating on the inside. This crust
is due to the accumulation in the kettle of mineral matter which was
in solution in the water, but which was left behind when the water
evaporated. (See Section 25.)

[Illustration: FIG. 30.--Showing how caves and holes are formed by the
solvent action of water.]

The amount of dissolved mineral matter present in some wells and
springs is surprisingly great; the famous springs of Bath, England,
contain so much mineral matter in solution, that a column 9 feet in
diameter and 140 feet high could be built out of the mineral matter
contained in the water consumed yearly by the townspeople.

[Illustration: FIG. 31.--The work of water as a solvent.]

Rocks and minerals are not all equally soluble in water; some are so
little soluble that it is years before any change becomes apparent,
and the substances are said to be insoluble, yet in reality they are
slowly dissolving. Other rocks, like limestone, are so readily soluble
in water that from the small pores and cavities eaten out by the
water, there may develop in long centuries, caves and caverns (Fig.
30). Most rock, like granite, contains several substances, some of
which are readily soluble and others of which are not readily soluble;
in such rocks a peculiar appearance is presented, due to the rapid
disappearance of the soluble substance, and the persistence of the
more resistant substance (Fig. 31).

We see that the solvent power of water is constantly causing changes,
dissolving some mineral substances, and leaving others practically
untouched; eating out crevices of various shapes and sizes, and by
gradual solution through unnumbered years enlarging these crevices
into wonderful caves, such as the Mammoth Cave of Kentucky.

66. Constructive Action of Water. Water does not always act as a
destructive agent; what it breaks down in one place it builds up in
another. It does this by means of precipitation. Water dissolves salt,
and also dissolves lead nitrate, but if a salt solution is mixed with
a lead nitrate solution, a solid white substance is formed in the
water (Fig. 32). This formation of a solid substance from the mingling
of two liquids is called precipitation; such a process occurs daily in
the rocks beneath the surface of the earth. (See Laboratory Manual.)

[Illustration: FIG. 32.--From the mingling of two liquids a solid is
sometimes formed.]

Suppose water from different sources enters a crack in a rock,
bringing different substances in solution; then the mingling of the
waters may cause precipitation, and the solid thus formed will be
deposited in the crack and fill it up. Hence, while ground water tends
to make rock porous and weak by dissolving out of it large quantities
of mineral matter, it also tends under other conditions to make it
more compact because it deposits in cracks, crevices, and pores the
mineral matter precipitated from solution.

These two forces are constantly at work; in some places the
destructive action is more prominent, in other places the constructive
action; but always the result is to change the character of the
original substance. When the mineral matter precipitated from the
solutions is deposited in cracks, _veins_ are formed (Fig. 33), which
may consist of the ore of different metals, such as gold, silver,
copper, lead, etc. Man is almost entirely dependent upon these veins
for the supply of metal needed in the various industries, because in
the original condition of the rocks, the metallic substances are so
scattered that they cannot be profitably extracted.

[Illustration: FIG. 33.--Mineral matter precipitated from solution is
deposited in crevices and forms veins.]

Naturally, the veins themselves are not composed of one substance
alone, because several different precipitates may be formed. But there
is a decided grouping of valuable metals, and these can then be
readily separated by means of electricity.

67. Streams. Streams usually carry mud and sand along with them;
this is particularly well seen after a storm when rivers and brooks
are muddy. The puddles which collect at the foot of a hill after a
storm are muddy because of the particles of soil gathered by the water
as it runs down the hill. The particles are not dissolved in the
water, but are held there in suspension, as we call it technically.
The river made muddy after a storm by suspended particles usually
becomes clear and transparent after it has traveled onward for miles,
because, as it travels, the particles drop to the bottom and are
deposited there. Hence, materials suspended in the water are borne
along and deposited at various places (Fig. 34). The amount of
deposition by large rivers is so great that in some places channels
fill up and must be dredged annually, and vessels are sometimes caught
in the deposit and have to be towed away.

Running water in the form of streams and rivers, by carrying sand
particles, stones, and rocks from high slopes and depositing them at
lower levels, wears away land at one place and builds it up at
another, and never ceases in its work of changing the nature of the
earth's surface (Fig. 35).

[Illustration: FIG. 34.--Deposit left by running water.]

[Illustration: FIG. 35.--Water by its action constantly changes the
character of the land.]

68. Relation of Water to Human Life. Water is one of the most
essential of food materials, and whether we drink much or little
water, we nevertheless get a great deal of it. The larger part of many
of our foods is composed of water; more than half of the weight of the
meat we eat is made up of water; and vegetables are often more than
nine tenths water. (See Laboratory Manual.) Asparagus and tomatoes
have over 90 per cent. of water, and most fruits are more than three
fourths water; even bread, which contains as little water as any of
our common foods, is about one third water (Fig. 36).

[Illustration: FIG. 36.--Diagram of the composition of a loaf of bread
and of a potato: 1. ash; 2, food; 3, water.]

Without water, solid food material, although present in the body,
would not be in a condition suitable for bodily use. An abundant
supply of water enables the food to be dissolved or suspended in it,
and in solution the food material is easily distributed to all parts
of the body.

Further, water assists in the removal of the daily bodily wastes, and
thus rids the system of foul and poisonous substances.

The human body itself consists largely of water; indeed, about two
thirds of our own weight is water. The constant replenishing of this
large quantity is necessary to life, and a considerable amount of the
necessary supply is furnished by foods, particularly the fruits and
vegetables.

But while the supply furnished by the daily food is considerable, it
is by no means sufficient, and should be supplemented by good drinking
water.

69. Water and its Dangers. Our drinking water comes from far and
near, and as it moves from place to place, it carries with it in
solution or suspension anything which it can find, whether it be
animal, vegetable, or mineral matter. The power of water to gather up
matter is so great that the average drinking water contains 20 to 90
grains of solid matter per gallon; that is, if a gallon of ordinary
drinking water is left to evaporate, a residue of 20 to 90 grains will
be left. (See Laboratory Manual.) As water runs down a hill slope
(Fig. 37), it carries with it the filth gathered from acres of land;
carries with it the refuse of stable, barn, and kitchen; and too often
this impure surface water joins the streams which supply our cities.
Lakes and rivers which furnish drinking water should be carefully
protected from surface draining; that is, from water which has flowed
over the land and has thus accumulated the waste of pasture and
stable and, it may be, of dumping ground.

[Illustration: FIG. 37.--As water flows over the land, it gathers
filth and disease germs.]

It is not necessary that water should be absolutely free from all
foreign substances in order to be safe for daily use in drinking; a
limited amount of mineral matter is not injurious and may sometimes be
really beneficial. It is the presence of animal and vegetable matter
that causes real danger, and it is known that typhoid fever is due
largely to such impurities present in the drinking water.

70. Methods of Purification. Water is improved by any of the
following methods:--

(_a_) _Boiling_. The heat of boiling destroys animal and vegetable
germs. Hence water that has been boiled a few minutes is safe to use.
This is the most practical method of purification in the home, and is
very efficient. The boiled water should be kept in clean, corked
bottles; otherwise foreign substances from the atmosphere reënter the
water, and the advantage gained from boiling is lost.

(_b_) _Distillation_. By this method pure water is obtained, but this
method of purification cannot be used conveniently in the home
(Section 25).

(_c_) _Filtration_. In filtration, the water is forced through
porcelain or other porous substances which allow the passage of water,
but which hold back the minute foreign particles suspended in the
water. (See Laboratory Manual.) The filters used in ordinary dwellings
are of stone, asbestos, or charcoal. They are often valueless, because
they soon become choked and cannot be properly cleaned.

The filtration plants owned and operated by large cities are usually
safe; there is careful supervision of the filters, and frequent and
effective cleanings are made. In many cities the filtration system is
so good that private care of the water supply is unnecessary.

71. The Source of Water. In the beginning, the earth was stored with
water just as it was with metal, rock, etc. Some of the water
gradually took the form of rivers, lakes, streams, and wells, as now,
and it is this original supply of water which furnishes us all that we
have to-day. We quarry to obtain stone and marble for building, and we
fashion the earth's treasures into forms of our own, but we cannot
create these things. We bore into the ground and drill wells in order
to obtain water from hidden sources; we utilize rapidly flowing
streams to drive the wheels of commerce, but the total amount of water
remains practically unchanged.

The water which flows on the earth is constantly changing its form;
the heat of the sun causes it to evaporate, or to become vapor, and to
mingle with the atmosphere. In time, the vapor cools, condenses, and
falls as snow or rain; the water which is thus returned to the earth
feeds our rivers, lakes, springs, and wells, and these in turn supply
water to man. When water falls upon a field, it soaks into the ground,
or collects in puddles which slowly evaporate, or it runs off and
drains into small streams or into rivers. That which soaks into the
ground is the most valuable because it remains on the earth longest
and is the purest.

[Illustration: FIG. 38.--How springs are formed. _A_, porous layer;
_B_, non-porous layer; _C_, spring.]

Water which soaks into the ground moves slowly downward and after a
longer or shorter journey, meets with a non-porous layer of rock
through which it cannot pass, and which effectually hinders its
downward passage. In such regions, there is an accumulation of water,
and a well dug there would have an abundant supply of water. The
non-porous layer is rarely level, and hence the water whose vertical
path is obstructed does not "back up" on the soil, but flows down hill
parallel with the obstructing non-porous layer, and in some distant
region makes an outlet for itself, forming a spring (Fig. 38). The
streams originating in the springs flow through the land and
eventually join larger streams or rivers; from the surface of streams
and rivers evaporation occurs, the water once more becomes vapor and
passes into the atmosphere, where it is condensed and again falls to
the earth.

Water which has filtered through many feet of earth is far purer and
safer than that which fell directly into the rivers, or which ran off
from the land and joined the surface streams without passing through
the soil.

72. The Composition of Water. Water was long thought to be a simple
substance, but toward the end of the eighteenth century it was found
to consist of two quite different substances, oxygen (O) and hydrogen
(H.)

[Illustration: FIG. 39.--The decomposition of water.]

If we send an electric current through water (acidulated to make it a
good conductor), as shown in Figure 39, we see bubbles of gas rising
from the end of the wire by which the current enters the water, and
other bubbles of gas rising from the end of the wire by which the
current leaves the water. These gases have evidently come from the
water and are the substances of which it is composed, because the
water begins to disappear as the gases are formed. If we place over
each end of the wire an inverted jar filled with water, the gases are
easily collected. The first thing we notice is that there is always
twice as much of one gas as of the other; that is, water is composed
of two substances, one of which is always present in twice as large
quantities as the other.

73. The Composition of Water. On testing the gases into which water
is broken up by an electric current, we find them to be quite
different. One proves to be oxygen, a substance with which we are
already familiar. The other gas, hydrogen, is new to us and is
interesting as being the lightest known substance, being even "lighter
than a feather."

An important fact about hydrogen is that in burning it gives as much
heat as five times its weight of coal. Its flame is blue and almost
invisible by daylight, but intensely hot. If fine platinum wire is
placed in an ordinary gas flame, it does not melt, but if placed in a
flame of burning hydrogen, it melts very quickly.

74. How to prepare Hydrogen. There are many different methods of
preparing hydrogen, but the easiest laboratory method is to pour
sulphuric acid, or hydrochloric acid, on zinc shavings and to collect
in a bottle the gas which is given off. This gas proves to be
colorless, tasteless, and odorless. (See Laboratory Manual.)



CHAPTER VII

AIR


75. The Instability of the Air. We are usually not conscious of the
air around us, but sometimes we realize that the air is heavy, while
at other times we feel the bracing effect of the atmosphere. We live
in an ocean of air as truly as fish inhabit an ocean of water. If you
have ever been at the seashore you know that the ocean is never still
for a second; sometimes the waves surge back and forth in angry fury,
at other times the waves glide gently in to the shore and the surface
is as smooth as glass; but we know that there is perpetual motion of
the water even when the ocean is in its gentlest moods. Generally our
atmosphere is quiet, and we are utterly unconscious of it; at other
times we are painfully aware of it, because of its furious winds. Then
again we are oppressed by it because of the vast quantity of vapor
which it holds in the form of fog, or mist. The atmosphere around us
is as restless and varying as is the water of the sea. The air at the
top of a high tower is very different from the air at the base of the
tower. Not only does the atmosphere vary greatly at different
altitudes, but it varies at the same place from time to time, at one
period being heavy and raw, at another being fresh and invigorating.

Winds, temperature, and humidity all have a share in determining
atmospheric conditions, and no one of these plays a small part.

76. The Character of the Air. The atmosphere which envelops us at
all times extends more than fifty miles above us, its height being far
greater than the greatest depths of the sea. This atmosphere varies
from place to place; at the sea level it is heavy, on the mountain top
less heavy, and far above the earth it is so light that it does not
contain enough oxygen to permit man to live. Figure 40 illustrates by
a pile of pillows how the pressure of the air varies from level to
level.

[Illustration: FIG. 40.--To illustrate the decrease in pressure with
height.]

Sea level is a low portion of the earth's surface, hence at sea level
there is a high column of air, and a heavy air pressure. As one passes
from sea level to mountain top a gradual but steady decrease in the
height of the air column occurs, and hence a gradual but definite
lessening of the air pressure.

[Illustration: FIG. 41.--The water in the tube is at the same level as
that in the glass.]

77. Air Pressure. If an empty tube (Fig. 41) is placed upright in
water, the water will not rise in the tube, but if the tube is put in
water and the air is then drawn out of the tube by the mouth, the
water will rise in the tube (Fig. 42). This is what happens when we
take lemonade through a straw. When the air is withdrawn from the
straw by the mouth, the pressure within the straw is reduced, and the
liquid is forced up the straw by the air pressure on the surface of
the liquid in the glass. Even the ancient Greeks and Romans knew that
water would rise in a tube when the pressure within the tube was
reduced, and hence they tried to obtain water from wells in this
fashion, but the water could never be raised higher than 34 feet. Let
us see why water could rise 34 feet and no more. If an empty pipe is
placed in a cistern of water, the water in the pipe does not rise
above the level of the water in the cistern. If, however, the pressure
in the tube is removed, the water in the tube will rise to a height of
34 feet approximately. If now the air pressure in the tube is
restored, the water in the tube sinks again to the level of that in
the cistern. The air pressing on the liquid in the cistern tends to
push some liquid up the tube, but the air pressing on the water in the
tube pushes downwards, and tends to keep the liquid from rising, and
these two pressures balance each other. When, however, the pressure
within the tube is reduced, the liquid rises because of the unbalanced
pressure which acts on the water in the cistern.

[Illustration: FIG. 42.--Water rises in the tube when the air is
withdrawn.]

[Illustration: FIG. 43.--The air supports a column of mercury 30
inches high.]

The column of water which can be raised this way is approximately 34
feet, sometimes a trifle more, sometimes a trifle less. If water were
twice as heavy, just half as high a column could be supported by the
atmosphere. Mercury is about thirteen times as heavy as water and,
therefore, the column of mercury supported by the atmosphere is about
one thirteenth as high as the column of water supported by the
atmosphere. This can easily be demonstrated. Fill a glass tube about a
yard long with mercury, close the open end with a finger, and quickly
insert the end of the inverted tube in a dish of mercury (Fig. 43).
When the finger is removed, the mercury falls somewhat, leaving an
empty space in the top of the tube. If we measure the column in the
tube, we find its height is about one thirteenth of 34 feet or 30
inches, exactly what we should expect. Since there is no air pressure
within the tube, the atmospheric pressure on the mercury in the dish
is balanced solely by the mercury within the tube, that is, by a
column of mercury 30 inches high. The shortness of the mercury column
as compared with that of water makes the mercury more convenient for
both experimental and practical purposes. (See Laboratory Manual.)

78. The Barometer. Since the pressure of the air changes from time
to time, the height of the mercury will change from day to day, and
hour to hour. When the air pressure is heavy, the mercury will tend to
be high; when the air pressure is low, the mercury will show a shorter
column; and by reading the level of the mercury one can learn the
pressure of the atmosphere. If a glass tube and dish of mercury are
attached to a board and the dish of mercury is inclosed in a case for
protection from moisture and dirt, and further if a scale of inches or
centimeters is made on the upper portion of the board, we have a
mercurial barometer (Fig. 44).

[Illustration: FIG. 44.--A simple barometer.]

If the barometer is taken to the mountain top, the column of mercury
falls gradually during the ascent, showing that as one ascends, the
pressure decreases in agreement with the statement in Section 76.
Observations similar to these were made by Torricelli as early as the
sixteenth century. Taking a barometric reading consists in measuring
the height of the mercury column.

79. A Portable Barometer. The mercury barometer is large and
inconvenient to carry from place to place, and a more portable form
has been devised, known as the aneroid barometer (Fig. 45). This form
of barometer is extremely sensitive; indeed, it is so delicate that
it shows the slight difference between the pressure at the table top
and the pressure at the floor level, whereas the mercury barometer
would indicate only a much greater variation in atmospheric pressure.
The aneroid barometers are frequently made no larger than a watch and
can be carried conveniently in the pocket, but they get out of order
easily and must be frequently readjusted. The aneroid barometer is an
air-tight box whose top is made of a thin metallic disk which bends
inward or outward according to the pressure of the atmosphere. If the
atmospheric pressure increases, the thin disk is pushed slightly
inward; if, on the other hand, the atmospheric pressure decreases, the
pressure on the metallic disk decreases and the disk is not pressed so
far inward. The motion of the disk is small, and it would be
impossible to calculate changes in atmospheric pressure from the
motion of the disk, without some mechanical device to make the slight
changes in motion perceptible.

[Illustration: FIG. 45.--Aneroid barometer.]

In order to magnify the slight changes in the position of the disk,
the thin face is connected with a system of levers, or wheels, which
multiplies the changes in motion and communicates them to a pointer
which moves around a graduated circular face. In Figure 45 the real
barometer is scarcely visible, being securely inclosed in a metal case
for protection; the principle, however, can be understood by reference
to Figure 46.

[Illustration: FIG. 46.--Principle of the aneroid barometer.]

80. The Weight of the Air. We have seen that the pressure of the
atmosphere at any point is due to the weight of the air column which
stretches from that point far up into the sky above. This weight
varies slightly from time to time and from place to place, but it is
equal to about 15 pounds to the square inch as shown by actual
measurement. It comes to us as a surprise sometimes that air actually
has weight; for example, a mass of 12 cubic feet of air at average
pressure weighs 1 pound, and the air in a large assembly hall weighs
more than 1 ton.

We are practically never conscious of this really enormous pressure of
the atmosphere, which is exerted over every inch of our bodies,
because the pressure is exerted equally over the outside and the
inside of our bodies; the cells and tissues of our bodies containing
gases under atmospheric pressure. If, however, the finger is placed
over the open end of a tube and the air is sucked out of the tube by
the mouth, the flesh of the finger bulges into the tube because the
pressure within the finger is no longer equalized by the usual
atmospheric pressure (Fig. 47).

[Illustration: FIG. 47.--The flesh bulges out.]

Aëronauts have never ascended much higher than 7 miles; at that height
the barometer stands at 7 inches instead of at 30 inches, and the
internal pressure in cells and tissues is not balanced by an equal
external pressure. The unequalized internal pressure forces the blood
to the surface of the body and causes rupture of blood vessels and
other physical difficulties.

81. Use of the Barometer. Changes in air pressure are very closely
connected with changes in the weather. The barometer does not directly
foretell the weather, but a low or falling pressure, accompanied by a
simultaneous fall of the mercury, usually precedes foul weather, while
a rising pressure, accompanied by a simultaneous rise in the mercury,
usually precedes fair weather. The barometer is not an infallible
prophet, but it is of great assistance in predicting the general trend
of the weather. There are certain changes in the barometer which
follow no known laws, and which allow of no safe predictions, but on
the other hand, general future conditions for a few days ahead can be
fairly accurately determined. Figure 48 shows a barograph or
self-registering barometer which automatically registers air pressure.

[Illustration: FIG. 48.--Barograph.]

Seaport towns in particular, but all cities, large or small, and
villages too, are on request notified by the United States Weather
Bureau ten hours or more in advance, of probable weather conditions,
and in this way precautions are taken which annually save millions of
dollars and hundreds of lives.

I recollect a summer spent on a New Hampshire farm, and know that an
old farmer started his farm hands haying by moonlight at two o'clock
in the morning, because the Special Farmer's Weather Forecast of the
preceding evening had predicted rain for the following day. His
reliance on the weather report was not misplaced, since the storm came
with full force at noon. Sailing vessels, yachts, and fishing dories
remain within reach of port if the barometer foretells storms.

[Illustration: FIG. 49.--Isotherms.]

82. Isobaric and Isothermal Lines. If a line were drawn through all
points on the surface of the earth having an equal barometric pressure
at the same time, such a line would be called an isobar. For example,
if the height of barometers in different localities is observed at
exactly the same time, and if all the cities and towns which have the
same pressure are connected by a line, the curved lines will be called
isobars. By the aid of these lines the barometric conditions over a
large area can be studied. The Weather Bureau at Washington relies
greatly on these isobars for statements concerning local and distant
weather forecasts, any shift in isobaric lines showing change in
atmospheric pressure.

If a line is drawn through all points on the surface of the earth
having the same _temperature_ at the same instant, such a line is
called an isotherm (Fig. 49).

83. Weather Maps. Scattered over the United States are about 125
Government Weather Stations, at each of which three times a day, at
the same instant, accurate observations of the weather are made. These
observations, which consist of the reading of barometer and
thermometer, the determination of the velocity and direction of the
wind, the determination of the humidity and of the amount of rain or
snow, are telegraphed to the chief weather official at Washington.
From the reports of wind storms, excessive rainfall, hot waves,
clearing weather, etc., and their rate of travel, the chief officials
predict where the storms, etc., will be at a definite future time. In
the United States, the _general_ movement of weather conditions, as
indicated by the barometer, is from west to east, and if a certain
weather condition prevails in the west, it is probable that it will
advance eastward, although with decided modifications. So many
influences modify atmospheric conditions that unfailing predictions
are impossible, but the Weather Bureau predictions prove true in about
eight cases out of ten.

The reports made out at Washington are telegraphed on request to
cities in this country, and are frequently published in the daily
papers, along with the forecast of the local office. A careful study
of these reports enables one to forecast to some extent the probable
weather conditions of the day.

The first impression of a weather map (Fig. 50) with its various lines
and signals is apt to be one of confusion, and the temptation comes to
abandon the task of finding an underlying plan of the weather. If one
will bear in mind a few simple rules, the complexity of the weather
map will disappear and a glance at the map will give one information
concerning general weather conditions just as a glance at the
thermometer in the morning will give some indication of the probable
temperature of the day. (See Laboratory Manual.)

[Illustration: FIG. 50. weather Map]

On the weather map solid lines represent isobars and dotted lines
represent isotherms. The direction of the wind at any point is
indicated by an arrow which flies with the wind; and the state of the
weather--clear, partly cloudy, cloudy, rain, snow, etc.--is indicated
by symbols.

84. Components of the Air. The best known constituent of the air is
oxygen, already familiar to us as the feeder of the fire without and
within the body. Almost one fifth of the air which envelops us is made
up of the life-giving oxygen. This supply of oxygen in the air is
constantly being used up by breathing animals and glowing fires, and
unless there were some constant source of additional supply, the
quantity of oxygen in the air would soon become insufficient to
support animal life. The unfailing constant source of atmospheric
oxygen is plant life (Section 48). The leaves of plants absorb carbon
dioxide from the air, and break it up into oxygen and carbon. The
plant makes use of the carbon but it rejects the oxygen, which passes
back into the atmosphere through the pores of the leaves.

Although oxygen constitutes only one fifth of the atmosphere, it is
one of the most abundant and widely scattered of all substances.
Almost the whole earth, whether it be rich loam, barren clay, or
granite boulder, contains oxygen in some form or other; that is, in
combination with other substances. But nowhere, except in the air
around us, do we find oxygen free and uncombined with other
substances.

A less familiar but more abundant constituent of the atmosphere is the
nitrogen. Almost four fifths of the air around us is made up of
nitrogen. If the atmosphere were composed of oxygen alone, the merest
flicker of a match would set the whole world ablaze. The fact that the
oxygen of the air is diluted as it were with so large a proportion of
nitrogen, prevents fires from sweeping over the world and destroying
everything in their path. Nitrogen does not support combustion, and a
burning match placed in a corked bottle goes out as soon as it has
used up the oxygen in the bottle. The nitrogen in the bottle, not only
does not assist the burning of the match, but it acts as a damper to
the burning.

Free nitrogen, like oxygen, is a colorless, odorless gas. It is not
poisonous; but one would die if surrounded by nitrogen alone, just as
one would die if surrounded by water. The vast supply of nitrogen in
the atmosphere would be useless if the smaller amount of oxygen were
not present to keep the body alive. Nitrogen is so important a factor
in daily life that an entire chapter will be devoted to it later.

Another constituent of the air with which we are familiar is carbon
dioxide. In pure air, carbon dioxide is present in very small
proportion, being continually taken from the air by plants in the
manufacture of their food.

Various other substances are present in the air in very minute
proportions, but of all the substances in the air, oxygen, nitrogen,
and carbon dioxide are the most important.



CHAPTER VIII

GENERAL PROPERTIES OF GASES


85. Bicycle Tires. We know very well that we cannot put more than a
certain amount of water in a tube, but we know equally well that the
amount of air which can be pumped into a bicycle or automobile tire
depends largely upon our muscular energy. A gallon of water remains a
gallon of water and requires a perfectly definite amount of space, but
air can be compressed and compressed, and made to occupy less and less
space. While it is true that air is easily compressed, it is also true
that air is elastic and capable of very rapid and easy expansion. If a
puncture occurs in a tire, the compressed air escapes very quickly;
that is, the compressed air within the tube has taken the first
opportunity offered for expansion.

[Illustration: FIG. 51.--By squeezing the bulb, air is forced out of
the nozzle.]

The fact that air is elastic has added materially to the comfort of
the world. Transportation by bicycles and automobiles has been greatly
facilitated by the use of air tires. In many hospitals, air mattresses
are used in place of hair, feather, or cotton mattresses, and in this
way the bed is kept fresher and cleaner, and can be moved with less
danger of discomfort to the patient. Every time we squeeze the bulb of
an atomizer, we force compressed or condensed air through the
atomizer, and the condensed air pushes the liquid out of the nozzle
(Fig. 51). Thus we see that in the necessities and conveniences of
life compressed air plays an important part.

86. The Danger of Compression. Air under ordinary atmospheric
conditions exerts a pressure of 15 pounds to the square inch. If, now,
large quantities of air are compressed into a small space, the
pressure exerted becomes correspondingly greater. If too much air is
blown into a toy balloon, the balloon bursts because it cannot support
the great pressure exerted by the compressed air within. What is true
of air is true of all gases. Dangerous boiler explosions have occurred
because the boiler walls were not strong enough to withstand the
pressure of the steam (which is water in the form of gas). The
pressure within the boilers of engines is frequently several hundred
pounds to the square inch, and such a pressure needs a strong boiler.

87. How Pressure is Measured in Buildings. In the preceding Section
we saw that undue pressure of a gas may cause explosion. It is
important, therefore, that authorities keep strict watch on gases
confined within pipes and reservoirs, never allowing the pressure to
exceed that which the walls of the reservoir will safely bear.

[Illustration: FIG. 52.--A pressure gauge.]

Pressure in a gas pipe may be measured by a simple instrument called
the pressure gauge: The gauge consists of a bent glass tube containing
mercury, and so made that one end can be fitted to a gas jet (Fig.
52). When the gas cock is closed, the mercury stands at the same level
in both arms, but when the cock is opened, the gas whose pressure is
being measured forces the mercury up the opposite arm. If the pressure
of the gas is small, the mercury changes its level but very little. It
is clear that the height of a column of mercury is a measure of the
gas pressure. Now it is known that one cubic inch of mercury weighs
about half a pound. Hence a column of mercury one inch high indicates
a pressure of about one half pound to the square inch; a column two
inches high indicates a pressure of about one pound to the square
inch, and so on.

This is a very convenient way to measure the pressure of the
illuminating gas in our homes and offices. The gauge is attached to
the gas burner and the pressure is read by means of a scale attached
to the gauge. (See Laboratory Manual.)

In order to have satisfactory illumination, the pressure must be
strong enough to give a steady, broad flame. If the flame from any gas
jet is flickering and weak, it is usually an indication of
insufficient pressure and the gas company should investigate
conditions and see to it that the consumer receives his proper value.

87. The Gas Meter. Most householders are deeply interested in the
actual amount of gas which they consume (gas is charged for according
to the number of cubic feet used), and therefore they should be able
to read the gas meter which indicates their consumption of gas. Such
gas meters are furnished by the companies, and can be read easily.

[Illustration: FIG. 53.--The gas meter indicates the number of cubic
feet of gas consumed.]

The instrument itself is somewhat complex. It will suffice to say that
within the meter box are thin disks which are moved by the stream of
gas that passes them. This movement of the disks is recorded by
clockwork devices on a dial face. In this way, the number of cubic
feet of gas which pass through the meter is automatically registered.

89. The Relation between Pressure and Volume. It was long known that
as the pressure of a gas increases, that is, as it becomes compressed,
its volume decreases, but Robert Boyle was the first to determine the
exact relation between the volume and the pressure of a gas. He did
this in a very simple manner.

Pour mercury into a U-shaped tube until the level of the mercury in
the closed end of the tube is the same as the level in the open end.
The air in the long arm is pressing upon the mercury in that arm, and
is tending to force it up the short arm. The air in the short closed
arm is pressing down upon the mercury in that arm and tending to send
it up the long arm. Since the mercury is at the same level in the two
arms, the pressure in the long arm must be equal to the pressure in
the short arm. But the long arm is open, and the pressure in that arm
is the pressure of the atmosphere. Therefore the pressure in the short
arm must be one atmosphere. Measure the distance _bc_ between the top
of the mercury and the closed end of the tube.

[Illustration: FIGS. 54, 55.--As the pressure on the gas increases,
its volume decreases.]

Pour more mercury into the open end of the tube, and as the mercury
rises higher and higher in the long arm, note carefully the decrease
in the volume of the air in the short arm. Pour mercury into the tube
until the difference in level _bd_ is just equal to the barometric
height, approximately 32 inches. The pressure of the air in the closed
end now supports the pressure of one atmosphere, and in addition, a
column of mercury equal to another atmosphere. If now the air column
in the closed end is measured, its volume will be only one half of its
former volume. By doubling the pressure we have reduced the volume one
half. Similarly, if the pressure is increased threefold, the volume
will be reduced to one third of the original volume.

90. Heat due to Compression. We saw in Section 89 that whenever the
pressure exerted upon a gas is increased, the volume of the gas is
decreased; and that whenever the pressure upon a gas is decreased, the
volume of the gas is increased. If the pressure is changed very
slowly, the change in the temperature of the gas is imperceptible; if,
however, the pressure is removed suddenly, the temperature falls
rapidly, or if the pressure is applied suddenly, the temperature rises
rapidly. When bicycle tires are being inflated, the pump becomes hot
because of the compression of the air.

The amount of heat resulting from compression is surprisingly large;
for example, if a mass of gas at 0° C. is suddenly compressed to one
half its original volume, its temperature rises 87° C.

91. Cooling by Expansion. If a gas expands suddenly, its temperature
falls; for example, if a mass of gas at 87° C. is allowed to expand
rapidly to twice its original volume, its temperature falls to 0° C.
If the compressed air of a bicycle tire is allowed to expand and a
sensitive thermometer is held in the path of the escaping air, the
thermometer will show a decided drop in temperature.

The low temperature obtained by the expansion of air or other gases is
utilized commercially on a large scale. By means of powerful pistons
air is compressed to one third or one fourth its original volume, is
passed through a coil of pipe surrounded with cold water, and is then
allowed to escape into large refrigerating vaults, which thereby have
their temperatures noticeably lowered, and can be used for the
permanent storage of meats, fruits, and other perishable material. In
summer, when the atmospheric temperature is high, the storage and
preservation of foods is of vital importance to factories and cold
storage houses, and but for the low temperature obtainable by the
expansion of compressed gases, much of our food supply would be lost
to use.

92. Unexpected Transformations. If the pressure on a gas is greatly
increased, a sudden transformation sometimes occurs and the gas
becomes a liquid. Then, if the pressure is reduced, a second
transformation occurs, and the liquid evaporates or returns to its
original form as a gas.

In Section 23 we saw that a fall of temperature caused water vapor to
condense or liquefy. If temperature alone were considered, most gases
could not be liquefied, because the temperature at which the average
gas liquefies is so low as to be out of the range of possibility; it
has been calculated, for example, that a temperature of 252° C. below
zero would have to be obtained in order to liquefy hydrogen.

Some gases can be easily transformed into liquids by pressure alone,
some gases can be easily transformed into liquids by cooling alone; on
the other hand, many gases are so difficult to liquefy that both
pressure and low temperature are needed to produce the desired result.
If a gas is cooled and compressed at the same time, liquefaction
occurs much more surely and easily than though either factor alone
were depended upon. The air which surrounds us, and of whose existence
we are scarcely aware, can be reduced to the form of a liquid, but the
pressure exerted upon the portion to be liquefied must be thirty-nine
times as great as the atmospheric pressure, and the temperature must
have been reduced to a very low point.

93. Artificial Ice. Ammonia gas is liquefied by strong pressure and
low temperature and is then allowed to flow into pipes which run
through tanks containing salt water. The reduction of pressure causes
the liquid to evaporate or turn to a gas, and the fall of temperature
which always accompanies evaporation means a lowering of the
temperature of the salt water to 16° or 18° below zero. But immersed
in the salt water are molds containing pure water, and since the
freezing point of water is 0° C, the water in the molds freezes and
can be drawn from the mold as solid cakes of ice.

[Illustration: FIG. 56.--Apparatus for making artificial ice.]

Ammonia gas is driven by the pump _C_ into the coil _D_ (Fig. 56)
under a pressure strong enough to liquefy it, the heat generated by
this compression being carried off by cold water which constantly
circulates through _B_. The liquid ammonia flows through the
regulating valve _V_ into the coil _E_, in which the pressure is kept
low by the pump _C_. The accompanying expansion reduces the
temperature to a very low degree, and the brine which circulates
around the coil _E_ acquires a temperature below the freezing point of
pure water. The cold brine passes from _A_ to a tank in which are
immersed cans filled with water, and within a short time the water in
the cans is frozen into solid cakes of ice.



CHAPTER IX

INVISIBLE OBJECTS


94. Very Small Objects. We saw in Section 84 that gases have a
tendency to expand, but that they can be compressed by the application
of force. This observation has led scientists to suppose that
substances are composed of very minute particles called molecules,
separated by small spaces called pores; and that when a gas is
condensed, the pores become smaller, and that when a gas expands, the
pores become larger.

The fact that certain substances are soluble, like sugar in water,
shows that the molecules of sugar find a lodging place in the spaces
or pores between the molecules of water, in much the same way that
pebbles find lodgment in the chinks of the coal in a coal scuttle. An
indefinite quantity of sugar cannot be dissolved in a given quantity
of liquid, because after a certain amount of sugar has been dissolved
all the pores become filled, and there is no available molecular
space. The remainder of the sugar settles at the bottom of the vessel,
and cannot be dissolved by any amount of stirring.

If a piece of potassium permanganate about the size of a grain of sand
is put into a quart of water, the solid disappears and the water
becomes a deep rich red. The solid evidently has dissolved and has
broken up into minute particles which are too small to be seen, but
which have scattered themselves and lodged in the pores of the water,
thus giving the water its rich color.

There is no visible proof of the existence of molecules and molecular
spaces, because not only are our eyes unable to see them directly, but
even the most powerful microscope cannot make them visible to us. They
are so small that if one thousand of them were laid side by side, they
would make a speck too small to be seen by the eye and too small to be
visible under the most powerful microscope.

We cannot see molecules or molecular pores, but the phenomena of
compression and expansion, solubility and other equally convincing
facts, have led us to conclude that all substances are composed of
very minute particles or molecules separated by spaces called pores.

95. Journeys Made by Molecules. If a gas jet is turned on and not
lighted, an odor of gas soon becomes perceptible, not only throughout
the room, but in adjacent halls and even in distant rooms. An uncorked
bottle of cologne scents an entire room, the odor of a rose or violet
permeates the atmosphere near and far. These simple everyday
occurrences seem to show that the molecules of a gas must be in a
state of continual and rapid motion. In the case of the cologne, some
molecules must have escaped from the liquid by the process of
evaporation and traveled through the air to the nose. We know that the
molecules of a liquid are in motion and are continually passing into
the air because in time the vessel becomes empty. The only way in
which this could happen would be for the molecules of the liquid to
pass from the liquid into the surrounding medium; but this is really
saying that the molecules are in motion.

From these phenomena and others it is reasonably clear that substances
are composed of molecules, and that molecules are not inert, quiet
particles, but that they are in incessant motion, moving rapidly
hither and thither, sometimes traveling far, sometimes near. Even the
log of wood which lies heavy and motionless on our woodpile is made
up of countless billions of molecules each in rapid incessant motion.
The molecules of solid bodies cannot escape so readily as those of
liquids and gases, and do not travel far. The log lies year after year
in an apparently motionless condition, but if one's eyes were keen
enough, the molecules would be seen moving among themselves, even
though they cannot escape into the surrounding medium and make long
journeys as do the molecules of liquids and gases.

96. The Companions of Molecules. Common sense tells us that a
molecule of water is not the same as a molecule of vinegar; the
molecules of each are extremely small and in rapid motion, but they
differ essentially, otherwise one substance would be like every other
substance. What is it that makes a molecule of water differ from a
molecule of vinegar, and each differ from all other molecules? Strange
to say, a molecule is not a simple object, but is quite complex, being
composed of one or more smaller particles, called atoms, and the
number and kind of atoms in a molecule determine the type of the
molecule, and the type of the molecule determines the substance. For
example, a glass of water is composed of untold millions of molecules,
and each molecule is a company of three still smaller particles, one
of which is called the oxygen atom and two of which are alike in every
particular and are called hydrogen atoms.

97. Simple Molecules. Generally molecules are composed of atoms
which are different in kind. For example, the molecule of water has
two different atoms, the oxygen atom and the hydrogen atoms; alcohol
has three different kinds of atoms, oxygen, hydrogen, and carbon.
Sometimes, however, molecules are composed of a group of atoms all of
which are alike. Now there are but seventy or eighty different kinds
of atoms, and hence there can be but seventy or eighty different
substances whose molecules are composed of atoms which are alike. When
the atoms comprising a molecule are all alike, the substance is called
an element, and is said to be a simple substance. Throughout the
length and breadth of this vast world of ours there are only about
eighty known elements. An element is the simplest substance
conceivable, because it has not been separated into anything simpler.
Water is a compound substance. It can be separated into oxygen and
hydrogen.

Gold, silver, and lead are examples of elements, and water, alcohol,
cider, sand, and marble are complex substances, or compounds, as we
are apt to call them. Everything, no matter what its size or shape or
character, is formed from the various combinations into molecules of a
few simple atoms, of which there exist about eighty known different
kinds. But few of the eighty known elements play an important part in
our everyday life. The elements in which we are most interested are
given in the following table, and the symbols by which they are known
are placed in columns to the right:

  |Oxygen       |O    |Copper   |Cu     |Phosphorus |P  |
  |Hydrogen     |H    |Iodine   |I      |Potassium  |K  |
  |Carbon       |C    |Iron     |Fe     |Silver     |Ag |
  |Aluminium Al |Lead |Pb       |Sodium |Na         |   |
  |Calcium      |Ca   |Nickel   |Ni     |Sulphur    |S  |
  |Chlorine     |Cl   |Nitrogen |N      |Tin        |Sn |

We have seen in an earlier experiment that twice as much hydrogen as
oxygen can be obtained from water. Two atoms of the element hydrogen
unite with one atom of the element oxygen to make one molecule of
water. In symbols we express this H_2O. A group of symbols, such as
this, expressing a molecule of a compound is called a _formula_. NaCl
is the formula for sodium chloride, which is the chemical name of
common salt.



CHAPTER X

LIGHT


98. What Light Does for Us. Heat keeps us warm, cooks our food,
drives our engines, and in a thousand ways makes life comfortable and
pleasant, but what should we do without light? How many of us could be
happy even though warm and well fed if we were forced to live in the
dark where the sunbeams never flickered, where the shadows never stole
across the floor, and where the soft twilight could not tell us that
the day was done? Heat and light are the two most important physical
factors in life; we cannot say which is the more necessary, because in
the extreme cold or arctic regions man cannot live, and in the dark
places where the light never penetrates man sickens and dies. Both
heat and light are essential to life, and each has its own part to
play in the varied existence of man and plant and animal.

Light enables us to see the world around us, makes the beautiful
colors of the trees and flowers, enables us to read, is essential to
the taking of photographs, gives us our moving pictures and our magic
lanterns, produces the exquisite tints of stained-glass windows, and
brings us the joy of the rainbow. We do not always realize that light
is beneficial, because sometimes it fades our clothing and our
carpets, and burns our skin and makes it sore. But we shall see that
even these apparently harmful effects of light are in reality of great
value in man's constant battle against disease.

99. The Candle. Natural heat and light are furnished by the sun, but
the absence of the sun during the evening makes artificial light
necessary, and even during the day artificial light is needed in
buildings whose structure excludes the natural light of the sun.
Artificial light is furnished by electricity, by gas, by oil in lamps,
and in numerous other ways. Until modern times candles were the main
source of light, and indeed to-day the intensity, or power, of any
light is measured in candle power units, just as length is measured in
yards; for example, an average gas jet gives a 10 candle power light,
or is ten times as bright as a candle; an ordinary incandescent
electric light gives a 16 candle power light, or furnishes sixteen
times as much light as a candle. Very strong large oil lamps can at
times yield a light of 60 candle power, while the large arc lamps
which flash out on the street corners are said to furnish 1200 times
as much light as a single candle. Naturally all candles do not give
the same amount of light, nor are all candles alike in size. The
candles which decorate our tea tables are of wax, while those which
serve for general use are of paraffin and tallow.

[Illustration: FIG. 57.--A photograph at _a_ receives four times as
much light as when held at _b_.]

100. Fading Illumination. The farther we move from a light, the less
strong, or intense, is the illumination which reaches us; the light of
the street lamp on the corner fades and becomes dim before the middle
of the block is reached, so that we look eagerly for the next lamp.
The light diminishes in brightness much more rapidly than we realize,
as the following simple experiment will show. Let a single candle
(Fig. 57) serve as our light, and at a distance of one foot from the
candle place a photograph. In this position the photograph receives a
definite amount of light from the candle and has a certain brightness.

If now we place a similar photograph directly behind the first
photograph and at a distance of two feet from the candle, the second
photograph receives no light because the first one cuts off all the
light. If, however, the first photograph is removed, the light which
fell on it passes outward and spreads itself over a larger area, until
at the distance of the second photograph the light spreads itself over
four times as large an area as formerly. At this distance, then, the
illumination on the second photograph is only one fourth as strong as
it was on a similar photograph held at a distance of one foot from the
candle.

The photograph or object placed at a distance of one foot from a light
is well illuminated; if it is placed at a distance of two feet, the
illumination is only one fourth as strong, and if the object is placed
three feet away, the illumination is only one ninth as strong. This
fact should make us have thought and care in the use of our eyes. We
think we are sixteen times as well off with our incandescent lights as
our ancestors were with simple candles, but we must reflect that our
ancestors kept the candle near them, "at their elbow," so to speak,
while we sit at some distance from the light and unconcernedly read
and sew.

As an object recedes from a light the illumination which it receives
diminishes rapidly, for the strength of the illumination is inversely
proportional to the square of distance of the object from the light.
Our ancestors with a candle at a distance of one foot from a book were
as well off as we are with an incandescent light four feet away.

101. Money Value of Light. Light is bought and sold almost as
readily as are the products of farm and dairy; many factories,
churches, and apartments pay a definite sum for electric light of a
standard strength, and naturally full value is desired. An instrument
for measuring the strength of a light is called a photometer, and
there are many different varieties, just as there are varieties of
scales which measure household articles. One light-measuring scale
depends upon the law that the intensity of illumination decreases with
the square of the distance of the object from the light. Suppose we
wish to measure the strength of the electric light bulbs in our homes,
in order to see whether we are getting the specified illumination. In
front of a screen place a black rod (Fig. 58) which is illuminated by
two different lights; namely, a standard candle and an incandescent
bulb whose strength is to be measured. Two shadows of the rod will
fall on the screen, one caused by the candle and the other caused by
the incandescent light. The shadow due to the latter source is not so
dark as that due to the candle. Now let the incandescent light be
moved away from the screen until the two shadows are of equal
darkness. If the incandescent light is four times as far away from the
screen as the candle, and the shadows are equal, we know, by Section
100, that its strength is sixteen candle power. If the incandescent
light is four times as far away from the screen as the candle is, its
power must be sixteen times as great, and we know the company is
furnishing the standard amount of light for a sixteen candle power
electric bulb. If, however, the bulb must be moved nearer to the rod
in order that the two shadows may be similar then the light given by
the bulb is less than sixteen candle power, and less than that due the
consumer.

[Illustration: FIG. 58.--The two shadows are equally dark.]

102. How Light Travels. We never expect to see around a corner, and
if we wish to see through pinholes in three separate pieces of
cardboard, we place the cardboards so that the three holes are in a
straight line. When sunlight enters a dark room through a small
opening, the dust particles dancing in the sun show a straight ray. If
a hole is made in a card, and the card is held in front of a light,
the card casts a shadow, in the center of which is a bright spot. The
light, the hole, and the bright spot are all in the same straight
line. These simple observations lead us to think that light travels in
a straight line.

[Illustration: FIG. 59.--The candle cannot be seen unless the three
pinholes are in a strait line.]

We can always tell the direction from which light comes, either by the
shadow cast or by the bright spot formed when an opening occurs in the
opaque object casting the shadow. If the shadow of a tree falls
towards the west, we know the sun must be in the cast; if a bright
spot is on the floor, we can easily locate the light whose rays stream
through an opening and form the bright spot. We know that light
travels in a straight line, and following the path of the beam which
comes to our eyes, we are sure to locate the light.

103. Good and Bad Mirrors. As we walk along the street, we
frequently see ourselves reflected in the shop windows, in polished
metal signboards, in the metal trimmings of wagons and automobiles;
but in mirrors we get the best image of ourselves. We resent the image
given by a piece of tin, because the reflection is distorted and does
not picture us as we really are; a rough surface does not give a fair
representation; if we want a true image of ourselves, we must use a
smooth surface like a mirror as a reflector. If the water in a pond
is absolutely still, we get a clear, true image of the trees, but if
there are ripples on the surface, the reflection is blurred and
distorted. A metal roof reflects so much light that the eyes are
dazzled by it, and a whitewashed fence injures the eyes because of the
glare which comes from the reflected light. Neither of these could be
called mirrors, however, because although they reflect light, they
reflect it so irregularly that not even a suggestion of an image can
be obtained.

Most of us are sufficiently familiar with mirrors to know that the
image is a duplicate of ourselves with regard to size, shape, color,
and expression, but that it appears to be back of the mirror, while we
are actually in front of the mirror. The image appears not only behind
the mirror, but it is also exactly as far back of the mirror as we are
in front of it; if we approach the mirror, the image also draws
nearer; if we withdraw, it likewise recedes.

104. The Path of Light. If a mirror or any other polished surface is
held in the path of a sunbeam, some of the light is reflected, and by
rotating the mirror the reflected sunbeam may be made to take any
path. School children amuse themselves by reflecting sunbeams from a
mirror into their companions' faces. If the companion moves his head
in order to avoid the reflected beam, his tormentor moves or inclines
the mirror and flashes the beam back to his victim's face.

If a mirror is held so that a ray of light strikes it in a
perpendicular direction, the light is reflected backward along the
path by which it came. If, however, the light makes an angle with the
mirror, its direction is changed, and it leaves the mirror along a new
path. By observation we learn that when a beam strikes the mirror and
makes an angle of 30° with the perpendicular, the beam is reflected in
such a way that its new path also makes an angle of 30° with the
perpendicular. If the sunbeam strikes the mirror at an angle of 32°
with the perpendicular, the path of the reflected ray also makes an
angle of 32° with the perpendicular. The ray (_AC_, Fig. 60) which
falls upon the mirror is called the incident ray, and the angle which
the incident ray (_AC_) makes with the perpendicular (_BC_) to the
mirror, at the point where the ray strikes the mirror, is called the
angle of incidence. The angle formed by the reflected ray (_CD_) and
this same perpendicular is called the angle of reflection. Observation
and experiment have taught us that light is always reflected in such a
way that the angle of reflection equals the angle of incidence. Light
is not the only illustration we have of the law of reflection. Every
child who bounces a ball makes use of this law, but he uses it
unconsciously. If an elastic ball is thrown perpendicularly against
the floor, it returns to the sender; if it is thrown against the floor
at an angle (Fig. 61), it rebounds in the opposite direction, but
always in such a way that the angle of reflection equals the angle of
incidence.

[Illustration: FIG. 60.--The ray _AC_ is reflected as _CD_.]

[Illustration: FIG. 61.--A bouncing ball illustrates the law of
reflection.]

105. Why the Image seems to be behind the Mirror. If a candle is
placed in front of a mirror, as in Figure 62, one of the rays of light
which leaves the candle will fall upon the mirror as _AB_ and will be
reflected as _BC_ (in such a way that the angle of reflection equals
the angle of incidence). If an observer stands at _C_, he will think
that the point _A_ of the candle is somewhere along the line _CB_
extended. Such a supposition would be justified from Section 102. But
the candle sends out light in all directions; one ray therefore will
strike the mirror as _AD_ and will be reflected as _DE_, and an
observer at _E_ will think that the point _A_ of the candle is
somewhere along the line _ED_. In order that both observers may be
correct, that is, in order that the light may seem to be in both these
directions, the image of the point _A_ must seem to be at the
intersection of the two lines. In a similar manner it can be shown
that every point of the image of the candle seems to be behind the
mirror.

[Illustration: FIG. 62.--The image is a duplicate of the object, but
appears to be behind the mirror.]

It can be shown by experiment that the distance of the image behind
the mirror is equal to the distance of the object in front of the
mirror.

106. Why Objects are Visible. If the beam of light falls upon a
sheet of paper, or upon a photograph, instead of upon a smooth
polished surface, no definite reflected ray will be seen, but a glare
will be produced by the scattering of the beam of light. The surface
of the paper or photograph is rough, and as a result, it scatters the
beam in every direction. It is hard for us to realize that a smooth
sheet of paper is by no means so smooth as it looks. It is rough
compared with a polished mirror. The law of reflection always holds,
however, no matter what the reflecting surface is,--the angle of
reflection always equals the angle of incidence. In a smooth body the
reflected beams are all parallel; in a rough body, the reflected beams
are inclined to each other in all sorts of ways, and no two beams
leave the paper in exactly the same direction.

[Illustration: FIG. 63.--The surface of the paper, although smooth in
appearance, is in reality rough, and scatters the light in every
direction.]

Hot coals, red-hot stoves, gas flames, and candles shine by their own
light, and are self-luminous. Objects like chairs, tables, carpets,
have no light within themselves and are visible only when they receive
light from a luminous source and reflect that light. We know that
these objects are not self-luminous, because they are not visible at
night unless a lamp or gas is burning. When light from any luminous
object falls upon books, desks, or dishes, it meets rough surfaces,
and hence undergoes diffuse reflection, and is scattered irregularly
in all directions. No matter where the eye is, some reflected rays
enter it, and the various objects are clearly seen.



CHAPTER XI

REFRACTION


107. Bent Rays of Light. A straw in a glass of lemonade seems to be
broken at the surface of the liquid, the handle of a teaspoon in a cup
of water appears broken, and objects seen through a glass of water may
seem distorted and changed in size. When light passes from air into
water, or from any transparent substance into another of different
density, its direction is changed, and it emerges along an entirely
new path (Fig. 64). We know that light rays pass through glass,
because we can see through the window panes and through our
spectacles; we know that light rays pass through water, because we can
see through a glass of clear water; on the other hand, light rays
cannot pass through wood, leather, metal, etc.

[Illustration: FIG. 64.--A straw or stick in water seems broken.]

Whenever light meets a transparent substance obliquely, some of it is
reflected, undergoing a change in its direction; and some of it passes
onward through the medium, but the latter portion passes onward along
a new path. The ray _RO_ (Fig. 65) passes obliquely through the air to
the surface of the water, but, on entering the water, it is bent or
refracted and takes the new path _OS_. The angle _AOR_ is called the
angle of incidence. The angle _POS_ is called the angle of refraction.

[Illustration: FIG. 65.--When the ray _RO_ enters the water, its path
changes to _OS_.]

The angle of refraction is the angle formed by the refracted ray and
the perpendicular to the surface at the point where the light strikes
it.

When light passes from air into water or glass, the refracted ray is
bent toward the perpendicular, so that the angle of refraction is
smaller than the angle of incidence. When a ray of light passes from
water or glass into air, the refracted ray is bent away from the
perpendicular so that the angle of refraction is greater than the
angle of incidence.

The bending or deviation of light in its passage from one substance to
another is called refraction.

108. How Refraction Deceives us. Refraction is the source of many
illusions; bent rays of light make objects appear where they really
are not. A fish at _A_ (Fig. 66) seems to be at _B_. The end of the
stick in Figure 64 seems to be nearer the surface of the water than it
really is.

[Illustration: FIG. 66.--A fish at _A_ seems to be at _B_.]

The light from the sun, moon, and stars can reach us only by passing
through the atmosphere, but in Section 76, we learned that the
atmosphere varies in density from level to level; hence all the light
which travels through the atmosphere is constantly deviated from its
original path, and before the light reaches the eye it has undergone
many changes in direction. Now we learned in Section 102, that the
direction of the rays of light as they enter the eye determines the
direction in which an object is seen; hence the sun, moon, and stars
seem to be along the lines which enter the eye, although in reality
they are not.

109. Uses of Refraction. If it were not for refraction, or the
deviation of light in its passage from medium to medium, the wonders
and beauties of the magic lantern and the camera would be unknown to
us; sun, moon, and stars could not be made to yield up their distant
secrets to us in photographs; the comfort and help of spectacles would
be lacking, spectacles which have helped unfold to many the rare
beauties of nature, such as a clear view of clouds and sunset, of
humming bee and flying bird. Books with their wealth of entertainment
and information would be sealed to a large part of mankind, if glasses
did not assist weak eyes.

By refraction the magnifying glass reveals objects hidden because of
their minuteness, and enlarges for our careful contemplation objects
otherwise barely visible. The watchmaker, unassisted by the magnifying
glass, could not detect the tiny grains of dust or sand which clog the
delicate wheels of our watches. The merchant, with his lens, examines
the separate threads of woolen and silk fabrics to determine the
strength and value of the material. The physician, with his invaluable
microscope, counts the number of infinitesimal corpuscles in the blood
and bases his prescription on that count; he examines the sputum of a
patient to determine whether tuberculosis wastes the system. The
bacteriologist with the same instrument scrutinizes the drinking water
and learns whether the dangerous typhoid germs are present. The
future of medicine will depend somewhat upon the additional secrets
which man is able to force from nature through the use of powerful
lenses, because as lenses have, in the past, been the means of
revealing disease germs, so in the future more powerful lenses may
serve to bring to light germs yet unknown. How refraction accomplishes
these results will be explained in the following Sections.

110. The Window Pane. We have seen that light is bent when it passes
from one medium to another of different density, and that objects
viewed by refracted light do not appear in their proper positions.

When a ray of light passes through a piece of plane glass, such as a
window pane (Fig. 67), it is refracted at the point _B_ toward the
perpendicular, and continues its course through the glass in the new
direction _BC_. On emerging from the glass, the light is refracted
away from the perpendicular and takes the direction _CD_, which is
clearly parallel to its original direction. Hence, when we view
objects through the window, we see them slightly displaced in
position, but otherwise unchanged. The deviation or displacement
caused by glass as thin as window panes is too slight to be noticed,
and we are not conscious that objects are out of position.

[Illustration: FIG. 67.--Objects looked at through a window pane seem
to be in their natural place.]

111. Chandelier Crystals and Prisms. When a ray of light passes
through plane glass, like a window pane, it is shifted somewhat, but
its direction does not change; that is, the emergent ray is parallel
to the incident ray. But when a beam of light passes through a
triangular glass prism, such as a chandelier crystal, its direction is
greatly changed, and an object viewed through a prism is seen quite
out of its true position.

Whenever light passes through a prism, it is bent toward the base of
the prism, or toward the thick portion of the prism, and emerges from
the prism in quite a different direction from that in which it entered
(Fig. 68). Hence, when an object is looked at through a prism, it is
seen quite out of place. In Figure 68, the candle seems to be at _S_,
while in reality it is at _A_.

[Illustration: FIG. 68.--When looked at through the prism, _A_ seems
to be at _S_.]

112. Lenses. If two prisms are arranged as in Figure 69, and two
parallel rays of light fall upon the prisms, the beam _A_ will be bent
downward toward the thickened portion of the prism, and the beam _B_
will be bent upward toward the thick portion of the prism, and after
passing through the prism the two rays will intersect at some point
_F_, called a focus.

[Illustration: FIG. 69.--Rays of light are converged and focused at
_F_.]

If two prisms are arranged as in Figure 70, the ray _A_ will be
refracted upward toward the thick end, and the ray _B_ will be
refracted downward toward the thick end; the two rays, on emerging,
will therefore be widely separated and will not intersect.

[Illustration: FIG. 70.--Rays of light are diverged and do not come to
any real focus.]

Lenses are very similar to prisms; indeed, two prisms placed as in
Figure 69, and rounded off, would make a very good convex lens. A lens
is any transparent material, but usually glass, with one or both sides
curved. The various types of lenses are shown in Figure 71.

[Illustration: FIG. 71.--The different types of lenses.]

The first three types focus parallel rays at some common point _F_, as
in Figure 69. Such lenses are called convex or converging lenses. The
last three types, called concave lenses, scatter parallel rays so that
they do not come to a focus, but diverge widely after passage through
the lens.

113. The Shape and Material of a Lens. The main or principal focus
of a lens, that is, the point at which rays parallel to the base line
_AB_ meet (Fig. 71), depends upon the shape of the lens. For example,
a thick lens, such as _A_ (Fig. 72), focuses the rays very near to the
lens; _B_, which is not so thick, focuses the rays at a greater
distance from the lens; and _C_, which is a very thin lens, focuses
the rays at a considerable distance from the lens. The distance of the
principal focus from the lens is called the focal length of the lens,
and from the diagrams we see that the more convex the lens, the
shorter the focal length.

[Illustration: FIG. 72.--The more curved the lens, the shorter the
focal length, and the nearer the focus is to the lens.]

The position of the principal focus depends not only on the shape of
the lens, but also on the refractive power of the material composing
the lens. A lens made of ice would not deviate the rays of light so
much as a lens of similar shape composed of glass. The greater the
refractive power of the lens, the greater the bending, and the nearer
the principal focus to the lens.

There are many different kinds of glass, and each kind of glass
refracts the light differently. Flint glass contains lead; the lead
makes the glass dense, and gives it great refractive power, enabling
it to bend and separate light in all directions. Cut glass and toilet
articles are made of flint glass because of the brilliant effects
caused by its great refractive power, and imitation gems are commonly
nothing more than polished flint glass.

114. How Lenses Form Images. Suppose we place an arrow, _A_, in
front of a convex lens (Fig. 73). The ray _AC_, parallel to the
principal axis, will pass through the lens and emerge as _DE_. The ray
is always bent toward the thick portion of the lens, both at its
entrance into the lens and its emergence from the lens.

[Illustration: FIG. 73.--The image is larger than the object. By means
of a lens, a watchmaker gets an enlarged image of the dust which clogs
the wheels of his watch.]

In Section 105, we saw that two rays determine the position of any
point of our image; hence in order to locate the image of the top of
the arrow, we need to consider but one more ray from the top of the
object. The most convenient ray to choose would be one passing through
_O_, the optical center of the lens, because such a ray passes through
the lens unchanged in direction, as is clear from Figure 74. The point
where _AC_ and _AO_ meet after refraction will be the position of the
top of the arrow. Similarly it can be shown that the center of the
arrow will be at the point _T_, and we see that the image is larger
than the object. This can be easily proved experimentally. Let a
convex lens be placed near a candle (Fig. 75); move a paper screen
back and forth behind the lens; for some position of the screen a
clear, enlarged image of the candle will be made.

[Illustration: FIG. 74.--Rays above _O_ are bent downward, those below
_O_ are bent upward, and rays through _O_ emerge from the lens
unchanged in direction.]

If the candle or arrow is placed in a new position, say at _MA_ (Fig.
76), the image formed is smaller than the object, and is nearer to the
lens than it was before. Move the lens so that its distance from the
candle is increased, and then find the image on a piece of paper. The
size and position of the image depend upon the distance of the object
from the lens (Fig. _77_). By means of a lens one can easily get on a
visiting card a picture of a distant church steeple.

[Illustration: FIG. 75.--The lens is held in such a position that the
image of the candle is larger than the object.]

[Illustration: FIG. 76.--The image is smaller than the object.]

115. The Value of Lenses. If it were not for the fact that a lens
can be held at such a distance from an object as to make the image
larger than the object, it would be impossible for the lens to assist
the watchmaker in locating the small particles of dust which clog the
wheels of the watch. If it were not for the opposite fact--that a lens
can be held at such a distance from the object as to make an image
smaller than the object, it would be impossible to have a photograph
of a tall tree or building unless the photograph were as large as the
tree itself. When a photographer takes a photograph of a person or a
tree, he moves his camera until the image formed by the lens is of the
desired size. By bringing the camera (really the lens of the camera)
near, we obtain a large-sized photograph; by increasing the distance
between the camera and the object, a smaller photograph is obtained.
The mountain top may be so far distant that in the photograph it will
not appear to be greater than a small stone.

[Illustration: FIG. 77.--The lens is placed in such a position that
the image is about the same size as the object.]

Many familiar illustrations of lenses, or curved refracting surfaces,
and their work, are known to all of us. Fish globes magnify the fish
that swim within. Bottles can be so shaped that they make the olives,
pickles, and peaches that they contain appear larger than they really
are. The fruit in bottles frequently seems too large to have gone
through the neck of the bottle. The deception is due to refraction,
and the material and shape of the bottle furnish a sufficient
explanation.

By using combinations of two or more lenses of various kinds, it is
possible to have an image of almost any desired size, and in
practically any desired position.

116. The Human Eye. In Section 114, we obtained on a movable screen,
by means of a simple lens, an image of a candle. The human eye
possesses a most wonderful lens and screen (Fig. 78); the lens is
called the crystalline lens, and the screen is called the retina. Rays
of light pass from the object through the pupil _P_, go through the
crystalline lens _L_, where they are refracted, and then pass onward
to the retina _R_, where they form a distinct image of the object.

[Illustration: FIG. 78.--The eye.]

We learned in Section 114 that a change in the position of the object
necessitated a change in the position of the screen, and that every
time the object was moved the position of the screen had to be altered
before a clear image of the object could be obtained. The retina of
the eye cannot be moved backward and forward, as the screen was, and
the crystalline lens is permanently located directly back of the iris.
How, then, does it happen that we can see clearly both near and
distant objects; that the printed page which is held in the hand is
visible at one second, and that the church spire on the distant
horizon is visible the instant the eyes are raised from the book? How
is it possible to obtain on an immovable screen by means of a simple
lens two distinct images of objects at widely varying distances?

The answer to these questions is that the crystalline lens changes
shape according to need. The lens is attached to the eye by means of
small muscles, _m_, and it is by the action of these muscles that the
lens is able to become small and thick, or large and thin; that is, to
become more or less curved. When we look at near objects, the muscles
act in such a way that the lens bulges out, and becomes thick in the
middle and of the right curvature to focus the near object upon the
screen. When we look at an object several hundred feet away, the
muscles change their pull on the lens and flatten it until it is of
the proper curvature for the new distance. The adjustment of the
muscles is so quick and unconscious that we normally do not experience
any difficulty in changing our range of view. The ability of the eye
to adjust itself to varying distances is called accommodation. The
power of adjustment in general decreases with age.

117. Farsightedness and Nearsightedness. A farsighted person is one
who cannot see near objects so distinctly as far objects, and who in
many cases cannot see near objects at all. The eyeball of a farsighted
person is very short, and the retina is too close to the crystalline
lens. Near objects are brought to a focus behind the retina instead of
on it, and hence are not visible. Even though the muscles of
accommodation do their best to bulge and thicken the lens, the rays of
light are not bent sufficiently to focus sharply on the retina. In
consequence objects look blurred. Farsightedness can be remedied by
convex glasses, since they bend the light and bring it to a closer
focus. Convex glasses, by bending the rays and bringing them to a
nearer focus, overbalance a short eyeball with its tendency to focus
objects behind the retina.

[Illustration: FIG. 79.--The farsighted eye.]

[Illustration: FIG. 80.--The defect is remedied by convex glasses.]

A nearsighted person is one who cannot see objects unless they are
close to the eye. The eyeball of a nearsighted person is very wide,
and the retina is too far away from the crystalline lens. Far objects
are brought to a focus in front of the retina instead of on it, and
hence are not visible. Even though the muscles of accommodation do
their best to pull out and flatten the lens, the rays are not
separated sufficiently to focus as far back as the retina. In
consequence objects look blurred. Nearsightedness can be remedied by
wearing concave glasses, since they separate the light and move the
focus farther away. Concave glasses, by separating the rays and making
the focus more distant, overbalance a wide eyeball with its tendency
to focus objects in front of the retina.

[Illustration: FIG. 81.--The nearsighted eye. The defect is remedied
by concave glasses.]

118. Headache and Eyes. Ordinarily the muscles of accommodation
adjust themselves easily and quickly; if, however, they do not,
frequent and severe headaches occur as a result of too great muscular
effort toward accommodation. Among young people headaches are
frequently caused by over-exertion of the crystalline muscles. Glasses
relieve the muscles of the extra adjustment, and hence are effective
in eliminating this cause of headache.

An exact balance is required between glasses, crystalline lens, and
muscular activity, and only those who have studied the subject
carefully are competent to treat so sensitive and necessary a part of
the body as the eye. The least mistake in the curvature of the
glasses, the least flaw in the type of glass (for example, the kind of
glass used), means an improper focus, increased duty for the muscles,
and gradual weakening of the entire eye, followed by headache and
general physical discomfort.

119. Eye Strain. The extra work which is thrown upon the nervous
system through seeing, reading, writing, and sewing with defective
eyes is recognized by all physicians as an important cause of disease.
The tax made upon the nervous system by the defective eye lessens the
supply of energy available for other bodily use, and the general
health suffers. The health is improved when proper glasses are
prescribed.

Possibly the greatest danger of eye strain is among school children,
who are not experienced enough to recognize defects in sight. For this
reason, many schools employ a physician who examines the pupils' eyes
at regular intervals.

The following general precautions are worth observing:--

1. Rest the eyes when they hurt, and as far as possible do close work,
such as writing, reading, sewing, wood carving, etc., by daylight.

2. Never read in a very bright or a very dim light.

3. If the light is near, have it shaded.

4. Do not rub the eyes with the fingers.

5. If eyes are weak, bathe them in lukewarm water in which a pinch of
borax has been dissolved.



CHAPTER XII

PHOTOGRAPHY


120. The Magic of the Sun. Ribbons and dresses washed and hung in
the sun fade; when washed and hung in the shade, they are not so apt
to lose their color. Clothes are laid away in drawers and hung in
closets not only for protection against dust, but also against the
well-known power of light to weaken color.

Many housewives lower the window shades that the wall paper may not
lose its brilliancy, that the beautiful hues of velvet, satin, and
plush tapestry may not be marred by loss in brilliancy and sheen.
Bright carpets and rugs are sometimes bought in preference to more
delicately tinted ones, because the purchaser knows that the latter
will fade quickly if used in a sunny room, and will soon acquire a
dull mellow tone. The bright and gay colors and the dull and somber
colors are all affected by the sun, but why one should be affected
more than another we do not know. Thousands of brilliant and dainty
hues catch our eye in the shop and on the street, but not one of them
is absolutely permanent; some may last for years, but there is always
more or less fading in time.

Sunlight causes many strange, unexplained effects. If the two
substances, chlorine and hydrogen, are mixed in a dark room, nothing
remarkable occurs any more than though water and milk were mixed, but
if a mixture of these substances is exposed to sunlight, a violent
explosion occurs and an entirely new substance is formed, a compound
entirely different in character from either of its components.

By some power not understood by man, the sun is able to form new
substances. In the dark, chlorine and hydrogen are simply chlorine and
hydrogen; in the sunlight they combine as if by magic into a totally
different substance. By the same unexplained power, the sun frequently
does just the opposite work; instead of combining two substances to
make one new product, the sun may separate or break down some
particular substance into its various elements. For example, if the
sun's rays fall upon silver chloride, a chemical action immediately
begins, and as a result we have two separate substances, chlorine and
silver. The sunlight separates silver chloride into its constituents,
silver and chlorine.

121. The Magic Wand in Photography. Suppose we coat one side of a
glass plate with silver chloride, just as we might put a coat of
varnish on a chair. We must be very careful to coat the plate in the
dark room,[B] otherwise the sunlight will separate the silver chloride
and spoil our plan. Then lay a horseshoe on the plate for good luck,
and carry the plate out into the light for a second. The light will
separate the silver chloride into chlorine and silver, the latter of
which will remain on the plate as a thin film. All of the plate was
affected by the sun except the portion protected by the horseshoe
which, because it is opaque, would not allow light to pass through and
reach the plate. If now the plate is carried back to the dark room and
the horseshoe is removed, one would expect to see on the plate an
impression of the horseshoe, because the portion protected by the
horseshoe would be covered by silver chloride and the exposed
unprotected portion would be covered by metallic silver. But we are
much disappointed because the plate, when examined ever so carefully,
shows not the slightest change in appearance. The change is there, but
the unaided eye cannot detect the change. Some chemical, the
so-called "developer," must be used to bring out the hidden change and
to reveal the image to our unseeing eyes. There are many different
developers in use, any one of which will effect the necessary
transformation. When the plate has been in the developer for a few
seconds, the silver coating gradually darkens, and slowly but surely
the image printed by the sun's rays appears. But we must not take this
picture into the light, because the silver chloride which was
protected by the horseshoe is still present, and would be strongly
affected by the first glimmer of light, and, as a result, our entire
plate would become similar in character and there would be no contrast
to give an image of the horseshoe on the plate.

[Footnote B: That is, a room from which ordinary daylight is
excluded.]

But a photograph on glass, which must be carefully shielded from the
light and admired only in the dark room, would be neither pleasurable
nor practical. If there were some way by which the hitherto unaffected
silver chloride could be totally removed, it would be possible to take
the plate into any light without fear. To accomplish this, the
unchanged silver chloride is got rid of by the process technically
called "fixing"; that is, by washing off the unreduced silver chloride
with a solution such as sodium thiosulphite, commonly known as hypo.
After a bath in the hypo the plate is cleansed in clear running water
and left to dry. Such a process gives a clear and permanent picture on
the plate.

[Illustration: FIG. 82.--A camera.]

122. The Camera. A camera (Fig. 82) is a light-tight box containing
a movable convex lens at one end and a screen at the opposite end.
Light from the object to be photographed passes through the lens,
falls upon the screen, and forms an image there. If we substitute for
the ordinary screen a plate or film coated with silver chloride or any
other silver salt, the light which falls upon the sensitive plate and
forms an image there will change the silver chloride and produce a
hidden image. If the plate is then removed from the camera in the
dark, and is treated as described in the preceding Section, the image
becomes visible and permanent. In practice some gelatin is mixed with
the silver salt, and the mixture is then poured over the plate or film
in such a way that a thin, even coating is made. It is the presence of
the gelatin that gives plates a yellowish hue. The sensitive plates
are left to dry in dark rooms, and when the coating has become
absolutely firm and dry, the plates are packed in boxes and sent forth
for sale.

Glass plates are heavy and inconvenient to carry, so that celluloid
films have almost entirely taken their place, at least for outdoor
work.

123. Light and Shade. Let us apply the above process to a real
photograph. Suppose we wish to take the photograph of a man sitting in
a chair in his library. If the man wore a gray coat, a black tie, and
a white collar, these details must be faithfully represented in the
photograph. How can the almost innumerable lights and shades be
produced on the plate?

The white collar would send through the lens the most light to the
sensitive plate; hence the silver chloride on the plate would be most
changed at the place where the lens formed an image of the collar. The
gray coat would not send to the lens so much light as the white
collar, hence the silver chloride would be less affected by the light
from the coat than by that from the collar, and at the place where the
lens produced an image of the coat the silver chloride would not be
changed so much as where the collar image is. The light from the face
would produce a still different effect, since the light from the face
is stronger than the light from the gray coat, but less than that from
a white collar. The face in the image would show less changed silver
chloride than the collar, but more than the coat, because the face is
lighter than the coat, but not so light as the collar. Finally, the
silver chloride would be least affected by the dark tie. The wall
paper in the background would affect the plate according to the
brightness of the light which fell directly upon it and which
reflected to the camera. When such a plate has been developed and
fixed, as described in Section 121, we have the so-called negative
(Fig. 83). The collar is very dark, the black tie and gray coat white,
and the white tidy very dark.

[Illustration: FIG. 83.--A negative.]

The lighter the object, such as tidy or collar, the more salt is
changed, or, in other words, the greater the portion of the silver
salt that is affected, and hence the darker the stain on the plate at
that particular spot. The plate shows all gradations of intensity--the
tidy is dark, the black tie is light. The photograph is true as far as
position, form, and expression are concerned, but the actual
intensities are just reversed. How this plate can be transformed into
a photograph true in every detail will be seen in the following
Section.

124. The Perfect Photograph. Bright objects, such as the sky or a
white waist, change much of the silver chloride, and hence appear
dark on the negative. Dark objects, such as furniture or a black coat,
change little of the chloride, and hence appear light on the negative.
To obtain a true photograph, the negative is placed on a piece of
sensitive photographic paper, or paper coated with a silver salt in
the same manner as the plate and films. The combination is exposed to
the light. The dark portions of the negative will act as obstructions
to the passage of light, and but little light will pass through that
part of the negative to the photographic paper, and consequently but
little of the silver salt on the paper will be changed. On the other
hand, the light portion of the negative will allow free and easy
passage of the light rays, which will fall upon the photographic paper
and will change much more of the silver. Thus it is that dark places
in the negative produce light places in the positive or real
photograph (Fig. 84), and that light places in the negative produce
dark places in the positive; all intermediate grades are likewise
represented with their proper gradations of intensity.

[Illustration: FIG. 84.--A positive or true photograph.]

If properly treated, a negative remains good for years, and will serve
for an indefinite number of positives or true photographs.

125. Light and Disease. The far-reaching effect which light has upon
some inanimate objects, such as photographic films and clothes, leads
us to inquire into the relation which exists between light and living
things. We know from daily observation that plants must have light in
order to thrive and grow. A healthy plant brought into a dark room
soon loses its vigor and freshness, and becomes yellow and drooping.
Plants do not all agree as to the amount of light they require, for
some, like the violet and the arbutus, grow best in moderate light,
while others, like the willows, need the strong, full beams of the
sun. But nearly all common plants, whatever they are, sicken and die
if deprived of sunlight for a long time. This is likewise true in the
animal world. During long transportation, animals are sometimes
necessarily confined in dark cars, with the result that many deaths
occur, even though the car is well aired and ventilated and the food
supply good. Light and fresh air put color into pale cheeks, just as
light and air transform sickly, yellowish plants into hardy green
ones. Plenty of fresh air, light, and pure water are the watchwords
against disease.

[Illustration: FIG. 85--Stems and leaves of oxalis growing toward the
light.]

In addition to the plants and animals which we see, there are many
strange unseen ones floating in the atmosphere around us, lying in the
dust of corner and closet, growing in the water we drink, and
thronging decayed vegetable and animal matter. Everyone knows that
mildew and vermin do damage in the home and in the field, but very few
understand that, in addition to these visible enemies of man, there
are swarms of invisible plants and animals some of which do far more
damage, both directly and indirectly, than the seen and familiar
enemies. All such very small plants and animals are known as
_microorganisms_.

Not all microörganisms are harmful; some are our friends and are as
helpful to us as are cultivated plants and domesticated animals. Among
the most important of the microörganisms are bacteria, which include
among their number both friend and foe. In the household, bacteria are
a fruitful source of trouble, but some of them are distinctly friends.
The delicate flavor of butter and the sharp but pleasing taste of
cheese are produced by bacteria. On the other hand, bacteria are the
cause of many of the most dangerous diseases, such as typhoid fever,
tuberculosis, influenza, and la grippe.

By careful observation and experimentation it has been shown
conclusively that sunlight rapidly kills bacteria, and that it is only
in dampness and darkness that bacteria thrive and multiply. Although
sunlight is essential to the growth of most plants and animals, it
retards and prevents the growth of bacteria. Dirt and dust exposed to
the sunlight lose their living bacteria, while in damp cellars and
dark corners the bacteria thrive, increasing steadily in number. For
this reason our houses should be kept light and airy; blinds should be
raised, even if carpets do fade; it is better that carpets and
furniture should fade than that disease-producing bacteria should find
a permanent abode within our dwellings. Kitchens and pantries in
particular should be thoroughly lighted. Bedclothes, rugs, and
clothing should be exposed to the sunlight as frequently as possible;
there is no better safeguard against bacterial disease than light. In
a sick room sunlight is especially valuable, because it not only kills
bacteria, but keeps the air dry, and new bacteria cannot get a start
in a dry atmosphere.



CHAPTER XIII

COLOR


126. The Rainbow. One of the most beautiful and well-known phenomena
in nature is the rainbow, and from time immemorial it has been
considered Jehovah's signal to mankind that the storm is over and that
the sunshine will remain. Practically everyone knows that a rainbow
can be seen only when the sun's rays shine upon a mist of tiny drops
of water. It is these tiny drops which by their refraction and their
scattering of light produce the rainbow in the heavens.

The exquisite tints of the rainbow can be seen if we look at an object
through a prism or chandelier crystal, and a very simple experiment
enables us to produce on the wall of a room the exact colors of the
rainbow in all their beauty.

[Illustration: FIG. 86.--White light is a mixture of lights of rainbow
colors.]

127. How to produce Rainbow Colors. _The Spectrum._ If a beam of
sunlight is admitted into a dark room through a narrow opening in the
shade, and is allowed to fall upon a prism, as shown in Figure 86, a
beautiful band of colors will appear on the opposite wall of the room.
The ray of light which entered the room as ordinary sunlight has not
only been refracted and bent from its straight path, but it has been
spread out into a band of colors similar to those of the rainbow.

Whenever light passes through a prism or lens, it is dispersed or
separated into all the colors which it contains, and a band of colors
produced in this way is called a spectrum. If we examine such a
spectrum we find the following colors in order, each color
imperceptibly fading into the next: violet, indigo, blue, green,
yellow, orange, red.

128. Sunlight or White Light. White light or sunlight can be
dispersed or separated into the primary colors or rainbow hues, as
shown in the preceding Section. What seems even more wonderful is that
these spectral colors can be recombined so as to make white light.

If a prism _B_ (Fig. 87) exactly similar to _A_ in every way is placed
behind _A_ in a reversed position, it will undo the dispersion of _A_,
bending upward the seven different beams in such a way that they
emerge together and produce a white spot on the screen. Thus we see,
from two simple experiments, that all the colors of the rainbow may be
obtained from white light, and that these colors may be in turn
recombined to produce white light.

[Illustration: FIG. 87.--Rainbow colors recombined to form white
light.]

White light is not a simple light, but is composed of all the colors
which appear in the rainbow.

129. Color. If a piece of red glass is held in the path of the
colored beam of light formed as in Section 127, all the colors on the
wall will disappear except the red, and instead of a beautiful
spectrum of all colors there will be seen the red color alone. The red
glass does not allow the passage through it of any light except red
light; all other colors are absorbed by the red glass and do not reach
the eye. Only the red ray passes through the red glass, reaches the
eye, and produces a sensation of color.

If a piece of blue glass is substituted for the red glass, the blue
band remains on the wall, while all the other colors disappear. If
both blue and red pieces of glass are held in the path of the beam, so
that the light must pass through first one and then the other, the
entire spectrum disappears and no color remains. The blue glass
absorbs the various rays with the exception of the blue ones, and the
red glass will not allow these blue rays to pass through it; hence no
light is allowed passage to the eye.

An emerald looks green because it freely transmits green, but absorbs
the other colors of which ordinary daylight is composed. A diamond
appears white because it allows the passage through it of all the
various rays; this is likewise true of water and window panes.

Stained-glass windows owe their charm and beauty to the presence in
the glass of various dyes and pigments which absorb in different
amounts some colors from white light and transmit others. These
pigments or dyes are added to the glass while it is in the molten
state, and the beauty of a stained-glass window depends largely upon
the richness and the delicacy of the pigments used.

130. Reflected Light. _Opaque Objects._ In Section 106 we learned
that most objects are visible to us because of the light diffusely
reflected from them. A white object, such as a sheet of paper, a
whitewashed fence, or a table cloth, absorbs little of the light which
falls upon it, but reflects nearly all, thus producing the sensation
of white. A red carpet absorbs the light rays incident upon it except
the red rays, and these it reflects to the eye.

Any substance or object which reflects none of the rays which fall
upon it, but absorbs all, appears black; no rays reach the eye, and
there is an absence of any color sensation. Coal and tar and soot are
good illustrations of objects which absorb all the light which falls
upon them.

131. How and Why Colors Change. _Matching Colors._ Most women prefer
to shop in the morning and early afternoon when the sunlight
illuminates shops and factories, and when gas and electricity do not
throw their spell over colors. Practically all people know that
ribbons and ties, trimmings and dresses, frequently look different at
night from what they do in the daytime. It is not safe to match colors
by artificial light; cloth which looks red by night may be almost
purple by day. Indeed, the color of an object depends upon the color
of the light which falls upon it. Strange sights are seen on the
Fourth of July when variously colored fireworks are blazing. The child
with a white blouse appears first red, then blue, then green,
according as his powders burn red, blue, or green. The face of the
child changes from its normal healthy hue to a brilliant red and then
to ghastly shades.

Suppose, for example, that a white hat is held at the red end of the
spectrum or in any red light. The characteristics of white objects is
their ability to reflect _all_ the various rays that fall upon them.
Here, however, the only light which falls upon the white hat is red
light, hence the only light which the hat has to reflect is red light
and the hat consequently appears red. Similarly, if a white hat is
placed in a blue light, it will reflect all the light which falls upon
it, namely, blue light, and will appear blue. If a red hat is held in
a red light, it is seen in its proper color. If a red hat is held in a
blue light, it appears black; it cannot reflect any of the blue light
because that is all absorbed and there is no red light to reflect.

A child wearing a green frock on Independence Day seems at night to be
wearing a black frock, if standing near powders burning with red,
blue, or violet light.

132. Pure, Simple Colors--Things as they Seem. To the eye white
light appears a simple, single color. It reveals its compound nature
to us only when passed through a prism, when it shows itself to be
compounded of an infinite number of colors which Sir Isaac Newton
grouped in seven divisions: violet, indigo, blue, green, yellow,
orange, and red.

We naturally ask ourselves whether these colors which compose white
light are themselves in turn compound? To answer that question, let us
very carefully insert a second prism in the path of the rays which
issue from the first prism, carefully barring out the remaining six
kinds of rays. If the red light is compound, it will be broken up into
its constituent parts and will form a typical spectrum of its own,
just as white light did after its passage through a prism. But the red
rays pass through the second prism, are refracted, and bent from this
course, and no new colors appear, no new spectrum is formed. Evidently
a ray of spectrum red is a simple color, not a compound color.

If a similar experiment is made with the remaining spectrum rays, the
result is always the same: the individual spectrum colors remain
simple, pure colors. _The individual spectrum colors are groups of
simple, pure colors._

[Illustration: FIG. 88.--Violet and green give blue. Green, blue, and
red give white.]

133. Colors not as they Seem--Compound Colors. If one half of a
cardboard disk (Fig. 88) is painted green, and the other half violet,
and the disk is slipped upon a toy top, and spun rapidly, the rotating
disk will appear blue; if red and green are used in the same way
instead of green and violet, the rotating disk will appear yellow. A
combination of red and yellow will give orange. The colors formed in
this way do not appear to the eye different from the spectrum colors,
but they are actually very different. The spectrum colors, as we saw
in the preceding Section, are pure, simple colors, while the colors
formed from the rotating disk are in reality compounded of several
totally different rays, although in appearance the resulting colors
are pure and simple.

If it were not that colors can be compounded, we should be limited in
hue and shade to the seven spectral colors; the wealth and beauty of
color in nature, art, and commerce would be unknown; the flowers with
their thousands of hues would have a poverty of color undreamed of;
art would lose its magenta, its lilac, its olive, its lavender, and
would have to work its wonders with the spectral colors alone. By
compounding various colors in different proportions, new colors can be
formed to give freshness and variety. If one third of the rotating
disk is painted blue, and the remainder white, the result is lavender;
if fifteen parts of white, four parts of red, and one part of blue are
arranged on the disk, the result is lilac. Olive is obtained from a
combination of two parts green, one part red, and one part black; and
the soft rich shades of brown are all due to different mixtures of
black, red, orange, or yellow.

134. The Essential Colors. Strange and unexpected facts await us at
every turn in science! If the rotating cardboard disk (Fig. 88) is
painted one third red, one third green, and one third blue, the
resulting color is white. While the mixture of the spectral colors
produces white, it is not necessary to have all of the spectral colors
in order to obtain white; because a mixture of the following colors
alone, red, green, and blue, will give white. Moreover, by the mixture
of these three colors in proper proportions, any color of the
spectrum, such as yellow or indigo or orange, may be obtained. The
three spectral colors, red, green, and blue, are called primary or
essential hues, because all known tints of color may be produced by
the careful blending of blue, green, and red in the proper
proportions; for example, purple is obtained by the blending of red
and blue, and orange by the blending of red and yellow.

135. Color Blindness. The nerve fibers of the eye which carry the
sensation of color to the brain are particularly sensitive to the
primary colors--red, green, blue. Indeed, all color sensations are
produced by the stimulation of three sets of nerves which are
sensitive to the primary colors. If one sees purple, it is because the
optic nerves sensitive to red and blue (purple equals red plus blue)
have carried their separate messages to the brain, and the blending of
the two distinct messages in the brain has given the sensation of
purple. If a red rose is seen, it is because the optic nerves
sensitive to red have been stimulated and have carried the message to
the brain.

A snowy field stimulates equally all three sets of optic nerves--the
red, the green, and the blue. Lavender, which is one part blue and
three parts white, would stimulate all three sets of nerves, but with
a maximum of stimulation for the blue. Equal stimulation of the three
sets would give the impression of white.

A color-blind person has some defect in one or more of the three sets
of nerves which carry the color message to the brain. Suppose the
nerve fibers responsible for carrying the red are totally defective.
If such a person views a yellow flower, he will see it as a green
flower. Yellow contains both red and green, and hence both the red and
green nerve fibers should be stimulated, but the red nerve fibers are
defective and do not respond, the green nerve fibers alone being
stimulated, and the brain therefore interprets green.

A well-known author gives an amusing incident of a dinner party, at
which the host offered stewed tomato for apple sauce. What color
nerves were defective in the case of the host?

In some employments color blindness in an employee would be fatal to
many lives. Engineers and pilots govern the direction and speed of
trains and boats largely by the colored signals which flash out in the
night's darkness or move in the day's bright light, and any mistake in
the reading of color signals would imperil the lives of travelers. For
this reason a rigid test in color is given to all persons seeking such
employment, and the ability to match ribbons and yarns of all ordinary
hues is an unvarying requirement for efficiency.



CHAPTER XIV

HEAT AND LIGHT AS COMPANIONS

    "The night has a thousand eyes,
      And the day but one;
    Yet the light of the bright world dies
      With the dying sun."


136. Most bodies which glow and give out light are hot; the stove
which glows with a warm red is hot and fiery; smoldering wood is black
and lifeless; glowing coals are far hotter than black ones. The
stained-glass window softens and mellows the bright light of the sun,
but it also shuts out some of the warmth of the sun's rays; the shady
side of the street spares our eyes the intense glare of the sun, but
may chill us by the absence of heat. Our illumination, whether it be
oil lamp or gas jet or electric light, carries with it heat; indeed,
so much heat that we refrain from making a light on a warm summer's
night because of the heat which it unavoidably furnishes.

137. Red a Warm Color. We have seen that heat and light usually go
hand in hand. In summer we lower the shades and close the blinds in
order to keep the house cool, because the exclusion of light means the
exclusion of some heat; in winter we open the blinds and raise the
shades in order that the sun may stream into the room and flood it
with light and warmth. The heat of the sun and the light of the sun
seem boon companions.

We can show that when light passes through a prism and is refracted,
forming a spectrum, as in Section 127, it is accompanied by heat. If
we hold a sensitive thermometer in the violet end of the spectrum so
that the violet rays fall upon the bulb, the reading of the mercury
will be practically the same as when the thermometer is held in any
dark part of the room; if, however, the thermometer is slowly moved
toward the red end of the spectrum, a change occurs and the mercury
rises slowly but steadily, showing that heat rays are present at the
red end of the spectrum. This agrees with the popular notion, formed
independently of science, which calls the reds the warm colors. Every
one of us associates red with warmth; in the summer red is rarely
worn, it looks hot; but in winter red is one of the most pleasing
colors because of the sense of warmth and cheer it brings.

_All light rays are accompanied by a small amount of heat, but the red
rays carry the most._

What seems perhaps the most unexpected thing, is that the temperature,
as indicated by a sensitive thermometer, continues to rise if the
thermometer is moved just beyond the red light of the spectrum. There
actually seems to be more heat beyond the red than in the red, but if
the thermometer is moved too far away, the temperature again falls.
Later we shall see what this means.

138. The Energy of the Sun. It is difficult to tell how much of the
energy of the sun is light and how much is heat, but it is easy to
determine the combined effect of heat and light.

[Illustration: FIG. 89.--The energy of the sun can be measured in heat
units.]

Suppose we allow the sun's rays to fall perpendicularly upon a metal
cylinder coated with lampblack and filled with a known quantity of
water (Fig. 89); at the expiration of a few hours the temperature of
the water will be considerably higher. Lampblack is a good absorber of
heat, and it is used as a coating in order that all the light rays
which fall upon the cylinder may be absorbed and none lost by
reflection.

Light and heat rays fall upon the lampblack, pass through the
cylinder, and heat the water. We know that the red light rays have the
largest share toward heating the water, because if the cylinder is
surrounded by blue glass which absorbs the red rays and prevents their
passage into the water, the temperature of the water begins to fall.
That the other light rays have a small share would have been clear
from the preceding Section.

All the energy of the sunshine which falls upon the cylinder, both as
heat and as light, is absorbed in the form of heat, and the total
amount of this energy can be calculated from the increase in the
temperature of the water. The energy which heated the water would have
passed onward to the surface of the earth if its path had not been
obstructed by the cylinder of water; and we can be sure that the
energy which entered the water and changed its temperature would
ordinarily have heated an equal area of the earth's surface; and from
this, we can calculate the energy falling upon the entire surface of
the earth during any one day.

Computations based upon this experiment show that the earth receives
daily from the sun the equivalent of 341,000,000,000 horse power--an
amount inconceivable to the human mind.

Professor Young gives a striking picture of what this energy of the
sun could do. A solid column of ice 93,000,000 miles long and 2-1/4
miles in diameter could be melted in a single second if the sun could
concentrate its entire power on the ice.

While the amount of energy received daily from the sun by the earth is
actually enormous, it is small in comparison with the whole amount
given out by the sun to the numerous heavenly bodies which make up the
universe. In fact, of the entire outflow of heat and light, the earth
receives only one part in two thousand million, and this is a very
small portion indeed.

139. How Light and Heat Travel from the Sun to Us. Astronomers tell
us that the sun--the chief source of heat and light--is 93,000,000
miles away from us; that is, so far distant that the fastest express
train would require about 176 years to reach the sun. How do heat and
light travel through this vast abyss of space?

[Illustration: FIG. 90.--Waves formed by a pebble.]

A quiet pool and a pebble will help to make it clear to us. If we
throw a pebble into a quiet pool (Fig. 90), waves or ripples form and
spread out in all directions, gradually dying out as they become more
and more distant from the pebble. It is a strange fact that while we
see the ripple moving farther and farther away, the particles of water
are themselves not moving outward and away, but are merely bobbing up
and down, or are vibrating. If you wish to be sure of this, throw the
pebble near a spot where a chip lies quiet on the smooth pond. After
the waves form, the chip rides up and down with the water, but does
not move outward; if the water itself were moving outward, it would
carry the chip with it, but the water has no forward motion, and hence
the chip assumes the only motion possessed by the water, that is, an
up-and-down motion. Perhaps a more simple illustration is the
appearance of a wheat field or a lawn on a windy day; the wind sweeps
over the grass, producing in the grass a wave like the water waves of
the ocean, but the blades of grass do not move from their accustomed
place in the ground, held fast as they are by their roots.

If a pebble is thrown into a quiet pool, it creates ripples or waves
which spread outward in all directions, but which soon die out,
leaving the pool again placid and undisturbed. If now we could quickly
withdraw the pebble from the pool, the water would again be disturbed
and waves would form. If the pebble were attached to a string so that
it could be dropped into the water and withdrawn at regular intervals,
the waves would never have a chance to disappear, because there would
always be a regularly timed definite disturbance of the water. Learned
men tell us that all hot bodies and all luminous bodies are composed
of tiny particles, called molecules, which move unceasingly back and
forth with great speed. In Section 95 we saw that the molecules of all
substances move unceasingly; their speed, however, is not so great,
nor are their motions so regularly timed as are those of the
heat-giving and the light-giving particles. As the particles of the
hot and luminous bodies vibrate with great speed and force they
violently disturb the medium around them, and produce a series of
waves similar to those produced in the water by the pebble. If,
however, a pebble is thrown into the water very gently, the
disturbance is slight, sometimes too slight to throw the water into
waves; in the same way objects whose molecules are in a state of
gentle motion do not produce light.

The particles of heat-giving and light-giving bodies are in a state of
rapid vibration, and thereby disturb the surrounding medium, which
transmits or conveys the disturbance to the earth or to other objects
by a train of waves. When these waves reach their destination, the
sensation of light or heat is produced.

We see the water waves, but we can never see with the eye the heat and
light waves which roll in to us from that far-distant source, the sun.
We can be sure of them only through their effect on our bodies, and by
the visible work they do.

140. How Heat and Light Differ. If heat and light are alike due to
the regular, rapid motion of the particles of a body, and are
similarly conveyed by waves, how is it, then, that heat and light are
apparently so different?

Light and heat differ as much as the short, choppy waves of the ocean
and the slow, long swell of the ocean, but not more so. The sailor
handles his boat in one way in a choppy sea and in a different way in
a rolling sea, for he knows that these two kinds of waves act
dissimilarly. The long, slow swell of the ocean would correspond with
the longer, slower waves which travel out from the sun, and which on
reaching us are interpreted as heat. The shorter, more frequent waves
of the ocean would typify the short, rapid waves which leave the sun,
and which on reaching us are interpreted as light.



CHAPTER XV

ARTIFICIAL LIGHTING


141. We seldom consider what life would be without our wonderful
methods of illumination which turn night into day, and prolong the
hours of work and pleasure. Yet it was not until the nineteenth
century that the marvelous change was made from the short-lived candle
to the more enduring oil lamp. Before the coming of the lamp, even in
large cities like Paris, the only artificial light to guide the
belated traveler at night was the candle required to be kept burning
in an occasional window.

With the invention of the kerosene lamp came more efficient lighting
of home and street, and with the advent of gas and electricity came a
light so effective that the hours of business, manufacture, and
pleasure could be extended far beyond the setting of the sun.

The production of light by candle, oil, and gas will be considered in
the following paragraphs, while illumination by electricity will be
reserved for a later Chapter.

142. The Candle. Candles were originally made by dipping a wick into
melting tallow, withdrawing it, allowing the adhered tallow to harden,
and repeating the dipping until a satisfactory thickness was obtained.
The more modern method consists in pouring a fatty preparation into a
mold, at the center of which a wick has been placed.

The wick, when lighted, burns for a brief interval with a faint,
uncertain light; almost immediately, however, the intensity of the
light increases and the illumination remains good as long as the
candle lasts. The heat of the burning tallow melts more of the tallow
near it, and this liquid fat is quickly sucked up into the burning
wick. The heat of the flame is sufficient to change most of this
liquid into a gas, that is, to vaporize the liquid, and furthermore to
set fire to the gas thus formed. These heated gases burn with a bright
yellow flame.

143. The Oil Lamp. The simple candle of our ancestors was now
replaced by the oil lamp, which gave a brighter, steadier, and more
permanent illumination. The principle of the lamp is similar to that
of the candle, except that the wick is saturated with kerosene or oil
rather than with fat. The heat from the burning wick is sufficient to
change the oil into a gas and then to set fire to the gas. By placing
a chimney over the burning wick, a constant and uniform draught of air
is maintained around the blazing gases, and hence a steady,
unflickering light is obtained. Gases and carbon particles are set
free by the burning wick. In order that the gases may burn and the
solid particle glow, a plentiful supply of oxygen is necessary. If the
quantity of air is insufficient, the carbon particles remain unburned
and form soot. A lamp "smokes" when the air which reaches the wick is
insufficient to burn the rapidly formed carbon particles; this
explains the danger of turning a lamp wick too high and producing more
carbon particles than can be oxidized by the air admitted through the
lamp chimney.

One great disadvantage of oil lamps and oil stoves is that they cannot
be carried safely from place to place. It is almost impossible to
carry a lamp without spilling the oil. The flame soon spreads from the
wick to the overflowing oil and in consequence the lamp blazes and an
explosion may result. Candles, on the other hand, are safe from
explosion; the dripping grease is unpleasant but not dangerous.

The illumination from a shaded oil lamp is soft and agreeable, but the
trimming of the wicks, the refilling of bowls, and the cleaning of
chimneys require time and labor. For this reason, the introduction of
gas met with widespread success. The illumination from an ordinary gas
jet is stronger than that from an ordinary lamp, and the stronger
illumination added to the greater convenience has made gas a very
popular source of light.

144. Gas Burners and Gas Mantles. For a long time, the only gas
flame used was that in which the luminosity resulted in heating
particles of carbon to incandescence. Recently, however, that has been
widely replaced by use of a Bunsen flame upon an incandescent mantle,
such as the Welsbach. The principle of the incandescent mantle is very
simple. When certain substances, such as thorium and cerium, are
heated, they do not melt or vaporize, but glow with an intense bright
light. Welsbach made use of this fact to secure a burner in which the
illumination depends upon the glowing of an incandescent, solid
mantle, rather than upon the blazing of a burning gas. He made a
cylindrical mantle of thin fabric, and then soaked it in a solution of
thorium and cerium until it became saturated with the chemical. The
mantle thus impregnated with thorium and cerium is placed on the gas
jet, but before the gas is turned on, a lighted match is held to the
mantle in order to burn away the thin fabric. After the fabric has
been burned away, there remains a coarse gauze mantle of the desired
chemicals. If now the gas cock is opened, the escaping gas is ignited,
the heat of the flame will raise the mantle to incandescence and will
produce a brilliant light. A very small amount of burning gas is
sufficient to raise the mantle to incandescence, and hence, by the use
of a mantle, intense light is secured at little cost. The mantle saves
us gas, because the cock is usually "turned on full" whether we use a
plain burner or a mantle burner. But, nevertheless, gas is saved,
because when the mantle is adjusted to the gas jet, the pressure of
the gas is lessened by a mechanical device and hence less gas escapes
and burns. By actual experiment, it has been found that an ordinary
burner consumes about five times as much gas per candle power as the
best incandescent burner, and hence is about five times as expensive.
One objection to the mantles is their tendency to break. But if the
mantles are carefully adjusted on the burner and are not roughly
jarred in use, they last many months; and since the best quality cost
only twenty-five cents, the expense of renewing the mantles is slight.

145. Gas for Cooking. If a cold object is held in the bright flame
of an ordinary gas jet, it becomes covered with soot, or particles of
unburned carbon. Although the flame is surrounded by air, the central
portion of it does not receive sufficient oxygen to burn up the
numerous carbon particles constantly thrown off by the burning gas,
and hence many carbon particles remain in the flame as glowing,
incandescent masses. That some unburned carbon is present in a flame
is shown by the fact that whenever a cold object is held in the flame,
it becomes "smoked" or covered with soot. If enough air were supplied
to the flame to burn up the carbon as fast as it was set free, there
would be no deposition of soot on objects held over the flame or in
it, because the carbon would be transformed into gaseous matter.

Unburned carbon would be objectionable in cooking stoves where
utensils are constantly in contact with the flame, and for this reason
cooking stoves are provided with an arrangement by means of which
additional air is supplied to the burning gas in quantities adequate
to insure complete combustion of the rapidly formed carbon particles.
An opening is made in the tube through which gas passes to the burner,
and as the gas moves past this opening, it carries with it a draft of
air. These openings are visible on all gas stoves, and should be kept
clean and free of clogging, in order to insure complete combustion. So
long as the supply of air is sufficient, the flame burns with a dull
blue color, but when the supply falls below that needed for complete
burning of the carbon, the blue color disappears, and a yellow flame
takes its place, and with the yellow flame the deposition of soot is
inevitable.

146. By-products of Coal Gas. Many important products besides
illuminating gas are obtained from the distillation of soft coal.
Ammonia is made from the liquids which collect in the condensers;
anilin, the source of exquisite dyes, is made from the thick, tarry
distillate, and coke is the residue left in the clay retorts. The coal
tar yields not only anilin, but also carbolic acid and naphthalene,
both of which are commercially valuable, the former as a widely used
disinfectant, and the latter as a popular moth preventive.

From a ton of good gas-producing coal can be obtained about 10,000
cubic feet of illuminating gas, and as by-products 6 pounds of
ammonia, 12 gallons of coal tar, and 1300 pounds of coke.

147. Natural Gas. Animal and vegetable matter buried in the depth of
the earth sometimes undergoes natural distillation, and as a result
gas is formed. The gas produced in this way is called natural gas. It
is a cheap source of illumination, but is found in relatively few
localities and only in limited quantity.

148. Acetylene. In 1892 it was discovered that lime and coal fused
together in the intense heat of the electric furnace formed a
crystalline, metallic-looking substance called calcium carbide. As a
result of that discovery, this substance was soon made on a large
scale and sold at a moderate price. The cheapness of calcium carbide
has made it possible for the isolated farmhouse to discard oil lamps
and to have a private gas system. When the hard, gray crystals of
calcium carbide are put in water, they give off acetylene, a colorless
gas which burns with a brilliant white flame. If bits of calcium
carbide are dropped into a test tube containing water, bubbles of gas
will be seen to form and escape into the air, and the escaping gas may
be ignited by a burning match held near the mouth of the test tube.
When chemical action between the water and carbide has ceased, and gas
bubbles have stopped forming, slaked lime is all that is left of the
dark gray crystals which were put into the water.

When calcium carbide is used as a source of illumination, the crystals
are mechanically dropped into a tank containing water, and the gas
generated is automatically collected in a small sliding tank, whence
it passes through pipes to the various rooms. The slaked lime, formed
while the gas was generated, collects at the bottom of the tanks and
is removed from time to time.

The cost of an acetylene generator is about $50 for a small house, and
the cost of maintenance is not more than that of lamps. The generator
does not require filling oftener than once a week, and the labor is
less than that required for oil lamps. In a house in which there were
twenty burners, the tanks were filled with water and carbide but once
a fortnight. Acetylene is seldom used in large cities, but it is very
widely used in small communities and is particularly convenient in
more or less remote summer residences.

Electric Lights. The most recent and the most convenient lighting is
that obtained by electricity. A fine, hairlike filament within a glass
bulb is raised to incandescence by the heat of an electric current.
This form of illumination will be considered in connection with
electricity.



CHAPTER XVI

MAN'S WAY OF HELPING HIMSELF


149. Labor-saving Devices. To primitive man belonged more especially
the arduous tasks of the out-of-door life: the clearing of paths
through the wilderness; the hauling of material; the breaking up of
the hard soil of barren fields into soft loam ready to receive the
seed; the harvesting of the ripe grain, etc.

[Illustration: FIG. 91.--Prying a stone out of the ground.]

The more intelligent races among men soon learned to help themselves
in these tasks. For example, our ancestors in the field soon learned
to pry stones out of the ground (Fig. 91) rather than to undertake the
almost impossible task of lifting them out of the earth in which they
were embedded; to swing fallen trees away from a path by means of rope
attached to one end rather than to attempt to remove them
single-handed; to pitch hay rather than to lift it; to clear a field
with a rake rather than with the hands; to carry heavy loads in
wheelbarrows (Fig. 92) rather than on the shoulders; to roll barrels
up a plank (Fig. 93) and to raise weights by ropes. In every case,
whether in the lifting of stones, or the felling of trees, or the
transportation of heavy weights, or the digging of the ground, man
used his brain in the invention of mechanical devices which would
relieve muscular strain and lighten physical labor.

If all mankind had depended upon physical strength only, the world
to-day would be in the condition prevalent in parts of Africa, Asia,
and South America, where the natives loosen the soil with their hands
or with crude implements (Fig. 94), and transport huge weights on
their shoulders and heads.

[Illustration: FIG. 92.--The wheelbarrow lightens labor.]

Any mechanical device (Figs. 95 and 96), whereby man's work can be
more conveniently done, is called a machine; the machine itself never
does any work--it merely enables man to use his own efforts to better
advantage.

[Illustration: FIG. 93.--Rolling barrels up a plank.]

150. When do we Work? Whenever, as a result of effort or force, an
object is moved, work is done. If you lift a knapsack from the floor
to the table, you do work because you use force and move the knapsack
through a distance equal to the height of the table. If the knapsack
were twice as heavy, you would exert twice as much force to raise it
to the same height, and hence you would do double the work. If you
raised the knapsack twice the distance,--say to your shoulders
instead of to the level of the table,--you would do twice the work,
because while you would exert the same force you would continue it
through double the distance.

[Illustration: FIG. 94.--Crude method of farming.]

Lifting heavy weights through great distances is not the only way in
which work is done. Painting, chopping wood, hammering, plowing,
washing, scrubbing, sewing, are all forms of work. In painting, the
moving brush spreads paint over a surface; in chopping wood, the
descending ax cleaves the wood asunder; in scrubbing, the wet mop
rubbed over the floor carries dirt away; in every conceivable form of
work, force and motion occur.

A man does work when he walks, a woman does work when she rocks in a
chair--although here the work is less than in walking. On a windy day
the work done in walking is greater than normal. The wind resists our
progress, and we must exert more force in order to cover the same
distance. Walking through a plowed or rough field is much more tiring
than to walk on a smooth road, because, while the distance covered may
be the same, the effort put forth is greater, and hence more work is
done. Always the greater the resistance encountered, the greater the
force required, and hence the greater the work done.

The work done by a boy who raises a 5-pound knapsack to his shoulder
would be 5x4, or 20, providing his shoulders were 4 feet from the
ground.

The amount of work done depends upon the force used and the distance
covered (sometimes called displacement), and hence we can say that

       Work = force multiplied by distance,
  or  _W = f × d_.

151. Machines. A glance into our machine shops, our factories, and
even our homes shows how widespread is the use of complex machinery.
But all machines, however complicated in appearance, are in reality
but modifications and combinations of one or more of four simple
machines devised long ago by our remote ancestors. These simple
devices are known to-day, as (1) the lever, represented by a crowbar,
a pitchfork; (2) the inclined plane, represented by the plank upon
which barrels are rolled into a wagon; (3) the pulley, represented by
almost any contrivance for the raising of furniture to upper stories;
(4) the wheel and axle, represented by cogwheels and coffee grinders.

[Illustration: FIG. 95.--Primitive method of grinding corn.]

Suppose a 600-pound bowlder which is embedded in the ground is needed
for the tower of a building. The problem of the builder is to get the
heavy bowlder out of the ground, to load it on a wagon for
transportation, and finally to raise it to the tower. Obviously, he
cannot do this alone; the greatest amount of force of which he is
capable would not suffice to accomplish any one of these tasks. How
then does he help himself and perform the impossible? Simply, by the
use of some of the machine types mentioned above, illustrations of
which are known in a general way to every schoolboy. The very knife
with which a stick is whittled is a machine.

[Illustration: FIG. 96.--Separating rice grains by flailing.]

[Illustration: FIG. 97.--The principle of the lever.]

152. The Lever. Balance a foot rule, containing a hole at its middle
point _F_, as shown in Figure 97. If now a weight of 1 pound is
suspended from the bar at some point, say 12, the balance is
disturbed, and the bar swings about the point _F_ as a center. The
balance can be regained by suspending an equivalent weight at the
opposite end of the bar, or by applying a 2-pound weight at a point 3
inches to the left of _F_. In the latter case a force of 1 pound
actually balances a force of 2 pounds, but the 1-pound weight is twice
as far from the point of suspension as is the 2-pound weight. The
small weight makes up in distance what it lacks in magnitude.

Such an arrangement of a rod or bar is called a lever. In any form of
lever there are only three things to be considered: the point where
the weight rests, the point where the force acts, and the point called
the fulcrum about which the rod rotates.

The distance from the force to the fulcrum is called the force arm.
The distance from the weight to the fulcrum is called the weight arm;
and it is a law of levers, as well as of all other machines, that the
force multiplied by the length of the force arm must equal the weight
multiplied by the length of the weight arm.

  Force × force arm = weight × weight arm.

A force of 1 pound at a distance of 6, or with a force arm 6, will
balance a weight of 2 pounds with a weight arm 3; that is,

  1 × 6 = 2 × 3.

Similarly a force of 10 pounds may be made to sustain a weight of 100
pounds, providing the force arm is 10 times longer than the weight
arm; and a force arm of 800 pounds, at a distance of 10 feet from the
fulcrum, may be made to sustain a weight of 8000 pounds, providing the
weight is 1 foot from the fulcrum.

153. Applications of the Lever. By means of a lever, a 600-pound
bowlder can be easily pried out of the ground. Let the lever, any
strong metal bar, be supported on a stone which serves as fulcrum;
then if a man exerts his force at the end of the rod somewhat as in
Figure 91 (p. 154), the force arm will be the distance from the stone
or fulcrum to the end of the bar, and the weight arm will be the
distance from the fulcrum to the bowlder itself. The man pushes down
with a force of 100 pounds, but with that amount succeeds in prying up
the 600-pound bowlder. If, however, you look carefully, you will see
that the force arm is 6 times as long as the weight arm, so that the
smaller force is compensated for by the greater distance through which
it acts.

At first sight it seems as though the man's work were done for him by
the machine. But this is not so. The man must lower his end of the
lever 3 feet in order to raise the bowlder 6 inches out of the ground.
He does not at any time exert a large force, but he accomplishes his
purpose by exerting a small force continuously through a
correspondingly greater distance. He finds it easier to exert a force
of 100 pounds continuously until his end has moved 3 feet rather than
to exert a force of 600 pounds on the bowlder and move it 6 inches.

By the time the stone has been raised the man has done as much work as
though the stone had been raised directly, but his inability to put
forth sufficient muscular force to raise the bowlder directly would
have rendered impossible a result which was easily accomplished when
through the medium of the lever he could extend his small force
through greater distance.

154. The Wheelbarrow as a Lever. The principle of the lever is
always the same; but the relative position of the important points may
vary. For example, the fulcrum is sometimes at one end, the force at
the opposite end, and the weight to be lifted between them.

[Illustration: FIG. 98.--A slightly different form of lever.]

Suspend a stick with a hole at its center as in Figure 98, and hang a
4-pound weight at a distance of 1 foot from the fulcrum, supporting
the load by means of a spring balance 2 feet from the fulcrum. The
pointer on the spring balance shows that the force required to balance
the 4-pound load is but 2 pounds.

The force is 2 feet from the fulcrum, and the weight (4) is 1 foot
from the fulcrum, so that

  Force × distance = Weight × distance,
  or               2 × 2 = 4 × 1.

Move the 4-pound weight so that it is very near the fulcrum, say but 6
inches from it; then the spring balance registers a force only one
fourth as great as the weight which it suspends. In other words a
force of 1 at a distance of 24 inches (2 feet) is equivalent to a
force of 4 at a distance of 6 inches.

[Illustration: FIG. 99.--The wheelbarrow lightened labor.]

One of the most useful levers of this type is the wheelbarrow (Fig.
99). The fulcrum is at the wheel, the force is at the handles, the
weight is on the wheelbarrow. If the load is halfway from the fulcrum
to the man's hands, the man will have to lift with a force equal to
one half the load. If the load is one fourth as far from the fulcrum
as the man's hands, he will need to lift with a force only one fourth
as great as that of the load.

[Illustration: FIG. 100.--A modified wheelbarrow.]

This shows that in loading a wheelbarrow, it is important to arrange
the load as near to the wheel as possible.

[Illustration: FIG. 101.--The nutcracker is a lever.]

The nutcracker (Fig. 101) is an illustration of a double lever of the
wheelbarrow kind; the nearer the nut is to the fulcrum, the easier the
cracking.

[Illustration: FIG. 102.--The hand exerts a small force over a long
distance and draws out a nail.]

Hammers (Fig. 102), tack-lifters, scissors, forceps, are important
levers, and if you will notice how many different levers (fig. 103)
are used by all classes of men, you will understand how valuable a
machine this simple device is.

155. The Inclined Plane. A man wishes to load the 600-pound bowlder
on a wagon, and proceeds to do it by means of a plank, as in Figure
93. Such an arrangement is called an inclined plane.

The advantage of an inclined plane can be seen by the following
experiment. Select a smooth board 4 feet long and prop it so that the
end _A_ (Fig. 104) is 1 foot above the level of the table; the length
of the incline is then 4 times as great as its height. Fasten a metal
roller to a spring balance and observe its weight. Then pull the
roller uniformly upward along the plank and notice what the pull is on
the balance, being careful always to hold the balance parallel to the
incline.

When the roller is raised along the incline, the balance registers a
pull only one fourth as great as the actual weight of the roller. That
is, when the roller weighs 12, a force of 3 suffices to raise it to
the height _A_ along the incline; but the smaller force must be
applied throughout the entire length of the incline. In many cases, it
is preferable to exert a force of 30 pounds, for example, over the
distance _CA_ than a force of 120 pounds over the shorter distance
_BA_.

[Illustration: FIG. 103.--Primitive man tried to lighten his task by
balancing his burden.]

Prop the board so that the end _A_ is 2 feet above the table level;
that is, arrange the inclined plane in such a way that its length is
twice as great as its height. In that case the steady pull on the
balance will be one half the weight of the roller; or a force of 6
pounds will suffice to raise the 12-pound roller.

[Illustration: FIG. 104.--Less force is required to raise the roller
along the incline than to raise it to _A_ directly.]

The steeper the incline, the more force necessary to raise a weight;
whereas if the incline is small, the necessary lifting force is
greatly reduced. On an inclined plane whose length is ten times its
height, the lifting force is reduced to one tenth the weight of the
load. The advantage of an incline depends upon the relative length and
height, or is equal to the ratio of the length to the height.

156. Application. By the use of an inclined plank a strong man can
load the 600-pound bowlder on a wagon. Suppose the floor of the wagon
is 2 feet above the ground, then if a 6-foot plank is used, 200 pounds
of force will suffice to raise the bowlder; but the man will have to
push with this force against the bowlder while it moves over the
entire length of the plank.

Since work is equal to force multiplied by distance, the man has done
work represented by 200 × 6, or 1200. This is exactly the amount of
work which would have been necessary to raise the bowlder directly. A
man of even enormous strength could not lift such a weight (600 lb.)
even an inch directly, but a strong man can furnish the smaller force
(200) over a distance of 6 feet; hence, while the machine does not
lessen the total amount of work required of a man, it creates a new
distribution of work and makes possible, and even easy, results which
otherwise would be impossible by human agency.

157. Railroads and Highways. The problem of the incline is an
important one to engineers who have under their direction the
construction of our highways and the laying of our railroad tracks. It
requires tremendous force to pull a load up grade, and most of us are
familiar with the struggling horse and the puffing locomotive. For
this reason engineers, wherever possible, level down the steep places,
and reduce the strain as far as possible.

[Illustration: FIG. 105.--A well-graded railroad bed.]

The slope of the road is called its grade, and the grade itself is
simply the number of feet the hill rises per mile. A road a mile long
(5280 feet) has a grade of 132 if the crest of the hill is 132 feet
above the level at which the road started.

[Illustration: FIG. 106.--A long, gradual ascent is better than a
shorter, steeper one.]

In such an incline, the ratio of length to height is 5280 ÷ 132, or
40; and hence in order to pull a train of cars to the summit, the
engine would need to exert a continuous pull equal to one fortieth of
the combined weight of the train.

If, on the other hand, the ascent had been gradual, so that the grade
was 66 feet per mile, a pull from the engine of one eightieth of the
combined weight would have sufficed to land the train of cars at the
crest of the grade.

Because of these facts, engineers spend great sums in grading down
railroad beds, making them as nearly level as possible. In mountainous
regions, the topography of the land prevents the elimination of all
steep grades, but nevertheless the attempt is always made to follow
the easiest grades.

158. The Wedge. If an inclined plane is pushed underneath or within
an object, it serves as a wedge. Usually a wedge consists of two
inclined planes (Fig. 107).

[Illustration: FIG. 107.--By means of a wedge, the stump is split.]

A chisel and an ax are illustrations of wedges. Perhaps the most
universal form of a wedge is our common pin. Can you explain how this
is a wedge?

159. The Screw. Another valuable and indispensable form of the
inclined plane is the screw. This consists of a metal rod around which
passes a ridge, and Figure 108 shows clearly that a screw is simply a
rod around which (in effect) an inclined plane has been wrapped.

[Illustration: FIG. 108--A screw as a simple machine.]

The ridge encircling the screw is called the thread, and the distance
between two successive threads is called the pitch. It is easy to see
that the closer the threads and the smaller the pitch, the greater the
advantage of the screw, and hence the less force needed in overcoming
resistance. A corkscrew is a familiar illustration of the screw.

160. Pulleys. The pulley, another of the machines, is merely a
grooved wheel around which a cord passes. It is sometimes more
convenient to move a load in one direction rather than in another, and
the pulley in its simplest form enables us to do this. In order to
raise a flag to the top of a mast, it is not necessary to climb the
mast, and so pull up the flag; the same result is accomplished much
more easily by attaching the flag to a movable string, somewhat as in
Figure 109, and pulling from below. As the string is pulled down, the
flag rises and ultimately reaches the desired position.

If we employ a stationary pulley, as in Figure 109, we do not change
the force, because the force required to balance the load is as large
as the load itself. The only advantage is that a force in one
direction may be used to produce motion in another direction. Such a
pulley is known as a fixed pulley.

[Illustration: FIG. 109.--By means of a pulley, a force in one
direction produces motion in the opposite direction.]

161. Movable Pulleys. By the use of a movable pulley, we are able to
support a weight by a force equal to only one half the load. In Figure
109, the downward pull of the weight and the downward pull of the hand
are equal; in Figure 110, the spring balance supports only one half
the entire load, the remaining half being borne by the hook to which
the string is attached. The weight is divided equally between the two
parts of the string which passes around the pulley, so that each
strand bears only one half of the burden.

We have seen in our study of the lever and the inclined plane that an
increase in force is always accompanied by a decrease in distance, and
in the case of the pulley we naturally look for a similar result. If
you raise the balance (Fig. 110) 12 feet, you will find that the
weight rises only 6 feet; if you raise the balance 24 inches, you will
find that the weight rises 12 inches. You must exercise a force of
100 pounds over 12 feet of space in order to raise a weight of 200
pounds a distance of 6 feet. When we raise 100 pounds through 12 feet
or 200 pounds through 6 feet the total work done is the same; but the
pulley enables those who cannot furnish a force of 200 pounds for the
space of 6 feet to accomplish the task by furnishing 100 pounds for
the space of 12 feet.

[Illustration: FIG. 110.--A movable pulley lightens labor.]

162. Combination of Pulleys. A combination of pulleys called block
and tackle is used where very heavy loads are to be moved. In Figure
111 the upper block of pulleys is fixed, the lower block is movable,
and one continuous rope passes around the various pulleys. The load is
supported by 6 strands, and each strand bears one sixth of the load.
If the hand pulls with a force of 1 pound at _P_, it can raise a load
of 6 pounds at _W_, but the hand will have to pull downward 6 feet at
_P_ in order to raise the load at _W_ 1 foot. If 8 pulleys were used,
a force equivalent to one eighth of the load would suffice to move
_W_, but this force would have to be exerted over a distance 8 times
as great as that through which _W_ was raised.

[Illustration: FIG. 111.--An effective arrangement of pulleys known as
block and tackle.]

163. Practical Application. In our childhood many of us saw with
wonder the appearance and disappearance of flags flying at the tops
of high masts, but observation soon taught us that the flags were
raised by pulleys. In tenements, where there is no yard for the family
washing, clothes often appear flapping in mid-air. This seems most
marvelous until we learn that the lines are pulled back and forth by
pulleys at the window and at a distant support. By means of pulleys,
awnings are raised and lowered, and the use of pulleys by furniture
movers, etc., is familiar to every wide-awake observer on the streets.

164. Wheel and Axle. The wheel and axle consists of a large wheel
and a small axle so fastened that they rotate together.

[Illustration: FIG. 112.--The wheel and axle.]

When the large wheel makes one revolution, _P_ falls a distance equal
to the circumference of the wheel. While _P_ moves downward, _W_
likewise moves, but its motion is upward, and the distance it moves is
small, being equal only to the circumference of the small axle. But a
small force at _P_ will sustain a larger force at _W_; if the
circumference of the large wheel is 40 inches, and that of the small
wheel 10 inches, a load of 100 at _W_ can be sustained by a force of
25 at _P_. The increase in force of the wheel and axle depends upon
the relative size of the two parts, that is, upon the circumference of
wheel as compared with circumference of axle, and since the ratio
between circumference and radius is constant, the ratio of the wheel
and axle combination is the ratio of the long radius to the short
radius.

For example, in a wheel and axle of radii 20 and 4, respectively, a
given weight at _P_ would balance 5 times as great a load at _W_.

165. Application. _Windlass, Cogwheels._ In the old-fashioned
windlass used in farming districts, the large wheel is replaced by a
handle which, when turned, describes a circle. Such an arrangement is
equivalent to wheel and axle (Fig. 112); the capstan used on shipboard
for raising the anchor has the same principle. The kitchen coffee
grinder and the meat chopper are other familiar illustrations.

Cogwheels are modifications of the wheel and axle. Teeth cut in _A_
fit into similar teeth cut in _B_, and hence rotation of _A_ causes
rotation of _B_. Several revolutions of the smaller wheel, however,
are necessary in order to turn the larger wheel through one complete
revolution; if the radius of _A_ is one half that of _B_, two
revolutions of _A_ will correspond to one of _B_; if the radius of _A_
is one third that of _B_, three revolutions of _A_ will correspond to
one of _B_.

[Illustration: FIG. 113.--Cogwheels.]

Experiment demonstrates that a weight _W_ attached to a cogwheel of
radius 3 can be raised by a force _P_, equal to one third of _W_
applied to a cogwheel of radius 1. There is thus a great increase in
force. But the speed with which _W_ is raised is only one third the
speed with which the small wheel rotates, or increase in power has
been at the decrease of speed.

This is a very common method for raising heavy weights by small force.

Cogwheels can be made to give speed at the decrease of force. A heavy
weight _W_ attached to _B_ will in its slow fall cause rapid rotation
of _A_, and hence rapid rise of _P_. It is true that _P_, the load
raised, will be less than _W_, the force exerted, but if speed is our
aim, this machine serves our purpose admirably.

An extremely important form of wheel and axle is that in which the two
wheels are connected by belts as in Figure 114. Rotation of _W_
induces rotation of _w_, and a small force at _W_ is able to overcome
a large force at _w_. An advantage of the belt connection is that
power at one place can be transmitted over a considerable distance and
utilized in another place.

[Illustration: FIG. 114.--By means of a belt, motion can be
transferred from place to place.]

166. Compound Machines. Out of the few simple machines mentioned in
the preceding Sections has developed the complex machinery of to-day.
By a combination of screw and lever, for example, we obtain the
advantage due to each device, and some compound machines have been
made which combine all the various kinds of simple machines, and in
this way multiply their mechanical advantage many fold.

A relatively simple complex machine called the crane (Fig. 116) maybe
seen almost any day on the street, or wherever heavy weights are being
lifted. It is clear that a force applied to turn wheel 1 causes a
slower rotation of wheel 3, and a still slower rotation of wheel 4,
but as 4 rotates it winds up a chain and slowly raises _Q_. A very
complex machine is that seen in Figure 117.

[Illustration: FIG. 115.--A simple derrick for raising weights.]

[Illustration: FIG. 116.--A traveling crane.]

167. Measurement of Work. In Section 150, we learned that the amount
of work done depends upon the force exerted, and the distance
covered, or that _W_ = force × distance. A man who raises 5 pounds a
height of 5 feet does far more work than a man who raises 5 ounces a
height of 5 inches, but the product of force by distance is 25 in each
case. There is difficulty because we have not selected an arbitrary
unit of work. The unit of work chosen and in use in practical affairs
is the foot pound, and is defined as the work done when a force of 1
pound acts through a distance of 1 foot. A man who moves 8 pounds
through 6 feet does 48 foot pounds of work, while a man who moves 8
ounces (1/2 pound) through 6 inches (1/2 foot) does only one fourth of
a foot pound of work.

[Illustration: FIG. 117.--A farm engine putting in a crop.]

168. The Power or the Speed with which Work is Done. A man can load
a wagon more quickly than a growing boy. The work done by the one is
equal to the work done by the other, but the man is more powerful,
because the time required for a given task is very important. An
engine which hoists a 50-pound weight in 1 second is much more
powerful than a man who requires 50 seconds for the same task; hence
in estimating the value of a working agent, whether animal or
mechanical, we must consider not only the work done, but the speed
with which it is done.

The rate at which a machine is able to accomplish a unit of work is
called _power_, and the unit of power customarily used is the horse
power. Any power which can do 550 foot pounds of work per second is
said to be one horse power (H.P.). This unit was chosen by James Watt,
the inventor of a steam engine, when he was in need of a unit with
which to compare the new source of power, the engine, with his old
source of power, the horse. Although called a horse power it is
greater than the power of an average horse.

An ordinary man can do one sixth of a horse power. The average
locomotive of a railroad has more than 500 H.P., while the engines of
an ocean liner may have as high as 70,000 H.P.

169. Waste Work and Efficient Work. In our study of machines we
omitted a factor which in practical cases cannot be ignored, namely,
friction. No surface can be made perfectly smooth, and when a barrel
rolls over an incline, or a rope passes over a pulley, or a cogwheel
turns its neighbor, there is rubbing and slipping and sliding. Motion
is thus hindered, and the effective value of the acting force is
lessened. In order to secure the desired result it is necessary to
apply a force in excess of that calculated. This extra force, which
must be supplied if friction is to be counteracted, is in reality
waste work.

If the force required by a machine is 150 pounds, while that
calculated as necessary is 100 pounds, the loss due to friction is 50
pounds, and the machine, instead of being thoroughly efficient, is
only two thirds efficient.

Machinists make every effort to eliminate from a machine the waste due
to friction, leveling and grinding to the most perfect smoothness and
adjustment every part of the machine. When the machine is in use,
friction may be further reduced by the use of lubricating oil.
Friction can never be totally eliminated, however, and machines of
even the finest construction lose by friction some of their
efficiency, while poorly constructed ones lose by friction as much as
one half of their efficiency.

170. Man's Strength not Sufficient for Machines. A machine, an inert
mass of metal and wood, cannot of itself do any work, but can only
distribute the energy which is brought to it. Fortunately it is not
necessary that this energy should be contributed by man alone, because
the store of energy possessed by him is very small in comparison with
the energy required to run locomotives, automobiles, sawmills, etc.
Perhaps the greatest value of machines lies in the fact that they
enable man to perform work by the use of energy other than his own.

[Illustration: FIG. 118.--Man's strength is not sufficient for heavy
work.]

Figure 118 shows one way in which a horse's energy can be utilized in
lifting heavy loads. Even the fleeting wind has been harnessed by man,
and, as in the windmill, made to work for him (Fig. 119). One sees
dotted over the country windmills large and small, and in Holland, the
country of windmills, the landowner who does not possess a windmill is
poor indeed.

For generations running water from rivers, streams, and falls has
served man by carrying his logs downstream, by turning the wheels of
his mill, etc.; and in our own day running water is used as an
indirect source of electric lights for street and house, the energy of
the falling water serving to rotate the armature of a dynamo (Section
310).

[Illustration: FIG. 119.--The windmill pumps water into the troughs
where cattle drink.]

A more constant source of energy is that available from the burning of
fuel, such as coal and oil. The former is the source of energy in
locomotives, the latter in most automobiles.

In the following Chapter will be given an account of water, wind, and
fuel as machine feeders.



CHAPTER XVII

THE POWER BEHIND THE ENGINE


171. Small boys soon learn the power of running water; swimming or
rowing downstream is easy, while swimming or rowing against the
current is difficult, and the swifter the water, the easier the one
and the more difficult the other; the river assists or opposes us as
we go with it or against it. The water of a quiet pool or of a gentle
stream cannot do work, but water which is plunging over a precipice or
dam, or is flowing down steep slopes, may be made to saw wood, grind
our corn, light our streets, run our electric cars, etc. A waterfall,
or a rapid stream, is a great asset to any community, and for this
reason should be carefully guarded. Water power is as great a source
of wealth as a coal bed or a gold mine.

The most tremendous waterfall in our country is Niagara Falls, which
every minute hurls millions of gallons of water down a 163-foot
precipice. The energy possessed by such an enormous quantity of water
flowing at such a tremendous speed is almost beyond everyday
comprehension, and would suffice to run the engines of many cities far
and near. Numerous attempts to buy from the United States the right to
utilize some of this apparently wasted energy have been made by
various commercial companies. It is fortunate that these negotiations
have been largely fruitless, because much deviation of the water for
commercial uses and the installation of machinery in the vicinity of
the famous falls would greatly detract from the beauty of this
world-known scene, and would rob our country of a natural beauty
unequaled elsewhere.

[Illustration: FIG. 120.--A mountain stream turns the wheels of the
mill.]

172. Water Wheels. In Figure 120 the water of a small but rapid
mountain stream is made to rotate a large wheel, which in turn
communicates its motion through belts to a distant sawmill or grinder.
In more level regions huge dams are built which hold back the water
and keep it at a higher level than the wheel; from the dam the water
is conveyed in pipes (flumes) to the paddle wheel which it turns.
Cogwheels or belts connect the paddle wheel with the factory
machinery, so that motion of the paddle wheel insures the running of
the machinery.

[Illustration: FIG. 121.--The Pelton water wheel.]

One of the most efficient forms of water wheels is that shown in
Figure 121, and called the Pelton wheel. Water issues in a narrow jet
similar to that of the ordinary garden hose and strikes with great
force against the lower part of the wheel, thereby causing rotation of
the wheel. Belts transfer this motion to the machinery of factory or
mill.

173. Turbines. The most efficient form of water motor is the
turbine, a strong metal wheel shaped somewhat like a pin wheel,
inclosed in a heavy metal case.

[Illustration: FIG. 122--A turbine at Niagara Falls.]

Water is conveyed from a reservoir or dam through a pipe (penstock) to
the turbine case, in which is placed the heavy metal turbine wheel
(Fig. 122). The force of the water causes rotation of the turbine and
of the shaft which is rigidly fastened to it. The water which flows
into the turbine case causes rotation of the wheel, escapes from the
case through openings, and flows into the tail water.

The power which a turbine can furnish depends upon the quantity of
water and the height of the fall, and also upon the turbine wheel
itself. One of the largest turbines known has a horse power of about
20,000; that is, it is equivalent, approximately, to 20,000 horses.

174. How much is a Stream Worth? The work which a stream can perform
may be easily calculated. Suppose, for example, that 50,000 pounds of
water fall over a 22-foot dam every second; the power of such a stream
would be 1,100,000 foot pounds per second or 2000 H.P. Naturally, a
part of this power would be lost to use by friction within the
machinery and by leakage, so that the power of a turbine run by a 2000
H.P. stream would be less than that value.

Of course, the horse power to be obtained from a stream determines the
size of the paddle wheel or turbine which can be run by it. It would
be possible to construct a turbine so large that the stream would not
suffice to turn the wheel; for this reason, the power of a stream is
carefully determined before machine construction is begun, and the
size of the machinery depends upon the estimates of the water power
furnished by expert engineers.

A rough estimate of the volume of a stream may be made by the method
described below:--

Suppose we allow a stream of water to flow through a rectangular
trough; the speed with which the water flows through the trough can be
determined by noting the time required for a chip to float the length
of the trough; if the trough is 10 feet long and the time required is
5 seconds, the water has a velocity of 2 feet per second.

[Illustration: FIG. 123.--Estimating the quantity of water which flows
through the trough each second.]

The quantity of water which flows through the trough each second
depends upon the dimensions of the trough and the velocity of the
water. Suppose the trough is 5 feet wide and 3 feet high, or has a
cross section of 15 square feet. If the velocity of the water were 1
foot per second, then 15 cubic feet of water would pass any given
point each second, but since the velocity of the water is 2 feet per
second, 30 cubic feet will represent the amount of water which will
flow by a given point in one second.

175. Quantity of Water Furnished by a River. Drive stakes in the
river at various places and note the time required for a chip to float
from one stake to another. If we know the distance between the stakes
and the time required for the chip to float from one stake to another,
the velocity of the water can be readily determined.

The width of the stream from bank to bank is easily measured, and the
depth is obtained in the ordinary way by sounding; it is necessary to
take a number of soundings because the bed of the river is by no means
level, and soundings taken at only one level would not give an
accurate estimate. If the soundings show the following depths: 30, 25,
20, 32, 28, the average depth could be taken as 30 + 25 + 20 + 32 + 28
÷ 5, or 27 feet. If, as a result of measuring, the river at a given
point in its course is found to be 27 feet deep and 60 feet wide, the
area of a cross section at that spot would be 1620 square feet, and if
the velocity proved to be 6 feet per second, then the quantity of
water passing in any one second would be 1620 × 6, or 9720 cubic feet.
By experiment it has been found that 1 cu. ft. of water weighs about
62.5 lb. The weight of the water passing each second would therefore
be 62.5 × 9720, or 607,500 lb. If this quantity of water plunges over
a 10-ft. dam, it does 607,500 × 10, or 6,075,000 foot pounds of work
per second, or 11,045 H.P. Such a stream would be very valuable for
the running of machinery.

176. Windmills. Those of us who have spent our vacation days in the
country know that there is no ready-made water supply there as in the
cities, but that as a rule the farmhouses obtain their drinking water
from springs and wells. In poorer houses, water is laboriously
carried in buckets from the spring or is lifted from the well by the
windlass. In more prosperous houses, pumps are installed; this is an
improvement over the original methods, but the quantity of water
consumed by the average family is so great as to make the task of
pumping an arduous one.

The average amount of water used per day by one person is 25 gallons.
This includes water for drinking, cooking, dish washing, bathing,
laundry. For a family of five, therefore, the daily consumption would
be 125 gallons; if to this be added the water for a single horse, cow,
and pig, the total amount needed will be approximately 150 gallons per
day. A strong man can pump that amount from an ordinary well in about
one hour, but if the well is deep, more time and strength are
required.

The invention of the windmill was a great boon to country folks
because it eliminated from their always busy life one task in which
labor and time were consumed.

177. The Principle of the Windmill. The toy pin wheel is a windmill
in miniature. The wind strikes the sails, and causes rotation; and the
stronger the wind blows, the faster will the wheel rotate. In
windmills, the sails are of wood or steel, instead of paper, but the
principle is identical.

[Illustration: FIG. 124.--The toy pin wheel is a miniature windmill.]

As the wheel rotates, its motion is communicated to a mechanical
device which makes use of it to raise and lower a plunger, and hence
as long as the wind turns the windmill, water is raised. The water
thus raised empties into a large tank, built either in the windmill
tower or in the garret of the house, and from the tank the water
flows through pipes to the different parts of the house. On very windy
days the wheel rotates rapidly, and the tank fills quickly; in order
to guard against an overflow from the tank a mechanical device is
installed which stops rotation of the wheel when the tank is nearly
full. The supply tank is usually large enough to hold a supply of
water sufficient for several days, and hence a continuous calm of a
day or two does not materially affect the house flow. When once built,
a windmill practically takes care of itself, except for oiling, and is
an efficient and cheap domestic possession.

[Illustration: FIG. 125.--The windmill pumps water into the tank.]

178. Steam as a Working Power. If a delicate vane is held at an
opening from which steam issues, the pressure of the steam will cause
rotation of the vane (Fig. 126), and if the vane is connected with a
machine, work can be obtained from the steam.

When water is heated in an open vessel, the pressure of its steam is
too low to be of practical value, but if on the contrary water is
heated in an almost closed vessel, its steam pressure is considerable.
If steam at high pressure is directed by nozzles against the blades of
a wheel, rapid rotation of the wheel ensues just as it did in Figure
121, although in this case steam pressure replaces water pressure.
After the steam has spent itself in turning the turbine, it condenses
into water and makes its escape through openings in an inclosing case.
In Figure 127 the protecting case is removed, in order that the form
of the turbine and the positions of the nozzles may be visible.

[Illustration: FIG. 126.--Steam as a source of power.]

[Illustration: FIG. 127.--Steam turbine with many blades and 4
nozzles.]

A single large turbine wheel may have as many as 800,000 sails or
blades, and steam may pour out upon these from many nozzles.

The steam turbine is very much more efficient than its forerunner, the
steam engine. The installation of turbines on ocean liners has been
accompanied by great increase in speed, and by an almost corresponding
decrease in the cost of maintenance.

179. Steam Engines. A very simple illustration of the working of a
steam engine is given in Figure 128. Steam under pressure enters
through the opening _F_, passes through _N_, and presses upon the
piston _M_. As a result _M_ moves downward, and thereby induces
rotation in the large wheel _L_.

[Illustration: FIG. 128.--The principle of the steam engine.]

As _M_ falls it drives the air in _D_ out through _O_ and _P_ (the
opening _P_ is not visible in the diagram).

As soon as this is accomplished, a mechanical device draws up the rod
_E_, which in turn closes the opening _N_, and thus prevents the steam
from passing into the part of _D_ above _M_.

But when the rod _E_ is in such a position that _N_ is closed, _O_ on
the other hand is open, and steam rushes through it into _D_ and
forces up the piston. This up-and-down motion of the piston causes
continuous rotation of the wheel _L_. If the fire is hot, steam is
formed quickly, and the piston moves rapidly; if the fire is low,
steam is formed slowly, and the piston moves less rapidly.

The steam engine as seen on our railroad trains is very complex, and
cannot be discussed here; in principle, however, it is identical with
that just described. Figure 129 shows a steam harvester at work on a
modern farm.

[Illustration: FIG. 129.--Steam harvester at work.]

In both engine and turbine the real source of power is not the steam
but the fuel, such as coal or oil, which converts the water into
steam.

180. Gas Engines. Automobiles have been largely responsible for the
gas engine. To carry coal for fuel and water for steam would be
impracticable for most motor cars. Electricity is used in some cars,
but the batteries are heavy, expensive, and short-lived, and are not
always easily replaceable. For this reason gasoline is extensively
used, and in the average automobile the source of power is the force
generated by exploding gases.

It was discovered some years ago that if the vapor of gasoline or
naphtha was mixed with a definite quantity of air, and a light was
applied to the mixture, an explosion would result. Modern science uses
the force of such exploding gases for the accomplishment of work, such
as running of automobiles and launches.

In connection with the gasoline supply is a carburetor or sprayer,
from which the cylinder _C_ (Fig. 130) receives a fine mist of
gasoline vapor and air. This mixture is ignited by an automatic,
electric sparking device, and the explosion of the gases drives the
piston _P_ to the right. In the 4-cycle type of gas engines (Fig.
130)--the kind used in automobiles--the four strokes are as follows:
1. The mixture of gasoline and air enters the cylinder as the piston
moves to the right. 2. The valves being closed, the mixture is
compressed as the piston moves to the left. 3. The electric spark
ignites the compressed mixture and drives the piston to the right. 4.
The waste gas is expelled as the piston moves to the left. The exhaust
valve is then closed, the inlet valve opened, and another cycle of
four strokes begins.

[Illustration: FIG. 130.--The gas engine.]

The use of gasoline in launches and automobiles is familiar to many.
Not only are launches and automobiles making use of gas power, but the
gasoline engine has made it possible to propel aëroplanes through the
air.



CHAPTER XVIII

PUMPS AND THEIR VALUE TO MAN


181. "As difficult as for water to run up a hill!" Is there any one
who has not heard this saying? And yet most of us accept as a matter
of course the stream which gushes from our faucet, or give no thought
to the ingenuity which devised a means of forcing water upward through
pipes. Despite the fact that water flows naturally down hill, and not
up, we find it available in our homes and office buildings, in some of
which it ascends to the fiftieth floor; and we see great streams of it
directed upon the tops of burning buildings by firemen in the streets
below.

In the country, where there are no great central pumping stations,
water for the daily need must be raised from wells, and the supply of
each household is dependent upon the labor and foresight of its
members. The water may be brought to the surface either by laboriously
raising it, bucket by bucket, or by the less arduous method of
pumping. These are the only means possible; even the windmill does not
eliminate the necessity for the pump, but merely replaces the energy
used by man in working it.

In some parts of our country we have oil beds or wells. But if this
underground oil is to be of service to man, it must be brought to the
surface, and this is accomplished, as in the case of water, by the use
of pumps.

An old tin can or a sponge may serve to bale out water from a leaking
rowboat, but such a crude device would be absurd if employed on our
huge vessels of war and commerce. Here a rent in the ship's side would
mean inevitable loss were it not possible to rid the ship of the
inflowing water by the action of strong pumps.

Another and very different use to which pumps are put is seen in the
compression of gases. Air is forced into the tires of bicycles and
automobiles until they become sufficiently inflated to insure comfort
in riding. Some present-day systems of artificial refrigeration
(Section 93) could not exist without the aid of compressed gases.

Compressed air has played a very important role in mining, being sent
into poorly ventilated mines to improve the condition of the air, and
to supply to the miners the oxygen necessary for respiration. Divers
and men who work under water carry on their backs a tank of compressed
air, and take from it, at will, the amount required.

There are many forms of pumps, and they serve widely different
purposes, being essential to the operation of many industrial
undertakings. In the following Sections some of these forms will be
studied.

[Illustration: FIG. 131.--Carrying water home from the spring.]

182. The Air as Man's Servant. Long before man harnessed water for
turbines, or steam for engines, he made the air serve his purpose, and
by means of it raised water from hidden underground depths to the
surface of the earth; likewise, by means of it, he raised to his
dwelling on the hillside water from the stream in the valley below.
Those who live in cities where running water is always present in the
home cannot realize the hardship of the days when this "ready-made"
supply did not exist, but when man laboriously carried to his
dwelling, from distant spring and stream, the water necessary for the
daily need.

What are the characteristics of the air which have enabled man to
accomplish these feats? They are well known to us and may be briefly
stated as follows:--

(1) Air has weight, and 1 cubic foot of air, at atmospheric pressure,
weighs 1-1/4 ounces.

(2) The air around us presses with a force of about 15 pounds upon
every square inch of surface that it touches.

(3) Air is elastic; it can be compressed, as in the balloon or bicycle
tire, but it expands immediately when pressure is reduced. As it
expands and occupies more space, its pressure falls and it exerts less
force against the matter with which it comes in contact. If, for
example, 1 cubic foot of air is allowed to expand and occupy 2 cubic
feet of space, the pressure which it exerts is reduced one half. When
air is compressed, its pressure increases, and it exerts a greater
force against the matter with which it comes in contact. If 2 cubic
feet of air are compressed to 1 cubic foot, the pressure of the
compressed air is doubled. (See Section 89.)

[Illustration: FIG. 132.--The atmosphere pressing downward on _a_
pushes water after the rising piston _b_.]

183. The Common Pump or Lifting Pump. Place a tube containing a
close-fitting piston in a vessel of water, as shown in Figure 132.
Then raise the piston with the hand and notice that the water rises in
the piston tube. The rise of water in the piston tube is similar to
the raising of lemonade through a straw (Section 77). The atmosphere
presses with a force of 15 pounds upon every square inch of water in
the large vessel, and forces some of it into the space left vacant by
the retreating piston. The common pump works in a similar manner. It
consists of a piston or plunger which moves back and forth in an
air-tight cylinder, and contains an outward opening valve through
which water and air can pass. From the bottom of the cylinder a tube
runs down into the well or reservoir, and water from the well has
access to the cylinder through another outward-moving valve. In
practice the tube is known as the suction pipe, and its valve as the
suction valve.

In order to understand the action of a pump, we will suppose that no
water is in the pump, and we will pump until a stream issues from the
spout. The various stages are represented diagrammatically by Figure
133. In (1) the entire pump is empty of water but full of air at
atmospheric pressure, and both valves are closed. In (2) the plunger
is being raised and is lifting the column of air that rests on it. The
air and water in the inlet pipe, being thus partially relieved of
downward pressure, are pushed up by the atmospheric pressure on the
surface of the water in the well. When the piston moves downward as in
(3), the valve in the pipe closes by its own weight, and the air in
the cylinder escapes through the valve in the plunger. In (4) the
piston is again rising, repeating the process of (2). In (5) the
process of (3) is being repeated, but water instead of air is escaping
through the valve in the plunger. In (6) the process of (2) is being
repeated, but the water has reached the spout and is flowing out.

[Illustration: FIG. 133. Diagram of the process of pumping.]

After the pump is in condition (6), motion of the plunger is followed
by a more or less regular discharge of water through the spout, and
the quantity of water which gushes forth depends upon the speed with
which the piston is moved. A strong man giving quick strokes can
produce a large flow; a child, on the other hand, is able to produce
only a thin stream. Whoever pumps must exert sufficient force to lift
the water from the surface of the well to the spout exit. For this
reason the pump has received the name of _lifting pump_.

[Illustration: FIG. 134.--Force pump.]

184. The Force Pump. In the common pump, water cannot not be raised
higher than the spout. In many cases it is desirable to force water
considerably above the pump itself, as, for instance, in the fire
hose; under such circumstances a type of pump is employed which has
received the name of _force pump_. This differs but little from the
ordinary lift pump, as a reference to Figure 134 will show. Here both
valves are placed in the cylinder, and the piston is solid, but the
principle is the same as in the lifting pump.

An upward motion of the plunger allows water to enter the cylinder,
and the downward motion of the plunger drives water through _E_. (Is
this true for the lift pump as well?) Since only the downward motion
of the plunger forces water through _E_, the discharge is intermittent
and is therefore not practical for commercial purposes. In order to
convert this intermittent discharge into a steady stream, an air
chamber is installed near the discharge tube, as in Figure 135. The
water forced into the air chamber by the downward-moving piston
compresses the air and increases its pressure. The pressure of the
confined air reacts against the water and tends to drive it out of the
chamber. Hence, even when the plunger is moving upward, water is
forced through the pipe because of the pressure of the compressed
air. In this way a continuous flow is secured.

[Illustration: FIG 135.--The air chamber _A_ insures a continuous flow
of water.]

The height to which the water can be forced in the pipe depends upon
the size and construction of the pump and upon the force with which
the plunger can be moved. The larger the stream desired and the
greater the height to be reached, the stronger the force needed and
the more powerful the construction necessary.

The force pump gets its name from the fact that the moving piston
drives or forces the water through the discharge tube.

185. Irrigation and Drainage. History shows that the lifting pump
has been used by man since the fourth century before Christ; for many
present-day enterprises this ancient form of pump is inconvenient and
impracticable, and hence it has been replaced in many cases by more
modern types, such as rotary and centrifugal pumps (Fig. 136). In
these forms, rapidly rotating wheels lift the water and drive it
onward into a discharge pipe, from which it issues with great force.
There is neither piston nor valve in these pumps, and the quantity of
water raised and the force with which it is driven through the pipes
depends solely upon the size of the wheels and the speed with which
they rotate.

Irrigation, or the artificial watering of land, is of the greatest
importance in those parts of the world where the land is naturally too
dry for farming. In the United States, approximately two fifths of the
land area is so dry as to be worthless for agricultural purposes
unless artificially watered. In the West, several large irrigating
systems have been built by the federal government, and at present
about ten million acres of land have been converted from worthless
farms into fields rich in crops. Many irrigating systems use
centrifugal pumps to force water over long distances and to supply it
in quantities sufficient for vast agricultural needs. In many regions,
the success of a farm or ranch depends upon the irrigation furnished
in dry seasons, or upon man's ability to drive water from a region of
abundance to a remote region of scarcity.

[Illustration: FIG. 136.--Centrifugal pump with part of the casing]
cut away to show the wheel.

[Illustration: FIG. 137.--Agriculture made possible by irrigation.]

The draining of land is also a matter of considerable importance;
swamps and marshes which were at one time considered useless have been
drained and then reclaimed and converted into good farming land. The
surplus water is best removed by centrifugal pumps, since sand and
sticks which would clog the valves of an ordinary pump are passed
along without difficulty by the rotating wheel.

[Illustration: FIG. 138.--Rice for its growth needs periodical
flooding, and irrigation often supplies the necessary water.]

186. Camping.--Its Pleasures and its Dangers. The allurement of a
vacation camp in the heart of the woods is so great as to make many
campers ignore the vital importance of securing a safe water supply. A
river bank may be beautiful and teeming with diversions, but if the
river is used as a source of drinking water, the results will almost
always be fatal to some. The water can be boiled, it is true, but few
campers are willing to forage for the additional wood needed for this
apparently unnecessary requirement; then, too, boiled water does not
cool readily in summer, and hence is disagreeable for drinking
purposes.

The only safe course is to abandon the river as a source of drinking
water, and if a spring cannot be found, to drive a well. In many
regions, especially in the neighborhood of streams, water can be
found ten or fifteen feet below the surface. Water taken from such a
depth has filtered through a bed of soil, and is fairly safe for any
purpose. Of course the deeper the well, the safer will be the water.
With the use of such a pump as will be described, campers can, without
grave danger, throw dish water, etc., on the ground somewhat remote
from the camp; this may not injure their drinking water because the
liquids will slowly seep through the ground, and as they filter
downward will lose their dangerous matter. All the water which reaches
the well pipes will have filtered through the soil bed and therefore
will probably be safe.

But while the careless disposal of wastes may not spoil the drinking
water (in the well to be described), other laws of health demand a
thoughtful disposal of wastes. The malarial mosquito and the typhoid
fly flourish in unhygienic quarters, and the only way to guard against
their dangers is to allow them neither food nor breeding place.

The burning of garbage, the discharge of waters into cesspools, or, in
temporary camps, the discharge of wastes to distant points through the
agency of a cheap sewage pipe will insure safety to campers, will
lessen the trials of flies and mosquitoes, and will add but little to
the expense.

187. A Cheap Well for Campers. A two-inch galvanized iron pipe with
a strong, pointed end containing small perforations is driven into the
ground with a sledge hammer. After it has penetrated for a few feet,
another length is added and the whole is driven down, and this is
repeated until water is reached. A cheap pump is then attached to the
upper end of the drill pipe and serves to raise the water. During the
drilling, some soil particles get into the pipe through the
perforations, and these cloud the water at first; but after the pipe
has once been cleaned by the upward-moving water, the supply remains
clear. The flow from such a well is naturally small; first, because
water is not abundant near the surface of the earth, and second,
because cheap pumps are poorly constructed and cannot raise a large
amount. But the supply will usually be sufficient for the needs of
simple camp life, and many a small farm uses this form of well, not
only for household purposes, but for watering the cattle in winter.

If the cheapness of such pumps were known, their use would be more
general for temporary purposes. The cost of material need not exceed
$5 for a 10-foot well, and the driving of the pipe could be made as
much a part of the camping as the pitching of the tent itself. If the
camping site is abandoned at the close of the vacation, the pump can
be removed and kept over winter for use the following summer in
another place. In this way the actual cost of the water supply can be
reduced to scarcely more than $3, the removable pump being a permanent
possession. In rocky or mountain regions the driven well is not
practicable, because the driving point is blunted and broken by the
rock and cannot pierce the rocky beds of land.

[Illustration: FIG. 139--A driven well.]

[Illustration: FIG. 140.--Diagram showing how supplying a city with
good water lessens sickness and death. The lines _b_ show the relative
number of people who died of typhoid fever before the water was
filtered; the lines _a_ show the numbers who died after the water was
filtered. The figures are the number of typhoid deaths occurring
yearly out of 100,000 inhabitants.]

188. Our Summer Vacation. It has been asserted by some city health
officials that many cases of typhoid fever in cities can be traced to
the unsanitary conditions existing in summer resorts. The drinking
water of most cities is now under strict supervision, while that of
isolated farms, of small seaside resorts, and of scattered mountain
hotels is left to the care of individual proprietors, and in only too
many instances receives no attention whatever. The sewage disposal is
often inadequate and badly planned, and the water becomes dangerously
contaminated. A strong, healthy person, with plenty of outdoor
exercise and with hygienic habits, may be able to resist the disease
germs present in the poor water supply; more often the summer guests
carry back with them to their winter homes the germs of disease, and
these gain the upper hand under the altered conditions of city and
business life. It is not too much to say that every man and woman
should know the source of his summer table water and the method of
sewage disposal. If the conditions are unsanitary, they cannot be
remedied at once, but another resort can be found and personal danger
can be avoided. Public sentiment and the loss of trade will go far in
furthering an effort toward better sanitation.

In the driven well, water cannot reach the spout unless it has first
filtered through the soil to the depth of the driven pipe; after such
a journey it is fairly safe, unless very large quantities of sewage
are present; generally speaking, such a depth of soil is able to
filter satisfactorily the drainage of the limited number of people
which a driven well suffices to supply.

[Illustration: FIG. 141.--A deep well with the piston in the water.]

Abundant water is rarely reached at less than 75 feet, and it would
usually be impossible to drive a pipe to such a depth. When a large
quantity of water is desired, strong machines drill into the ground
and excavate an opening into which a wide pipe can be lowered. I
recently spent a summer in the Pocono Mountains and saw such a well
completed. The machine drilled to a depth of 250 feet before much
water was reached and to over 300 feet before a flow was obtained
sufficient to satisfy the owner. The water thus obtained was to be the
sole water supply of a hotel accommodating 150 persons; the proprietor
calculated that the requirements of his guests, for bath, toilet,
laundry, kitchen, etc., and the domestics employed to serve them,
together with the livery at their disposal, demanded a flow of 10
gallons per minute. The ground was full of rock and difficult to
penetrate, and it required 6 weeks of constant work for two skilled
men to drill the opening, lower the suction pipe, and install the
pump, the cost being approximately $700.

[Illustration: FIG. 142.--Showing how drinking water can be
contaminated from cesspool _(c)_ and wash water _(w)_.]

The water from such a well is safe and pure except under the
conditions represented in Figure 142. If sewage or slops be poured
upon the ground in the neighborhood of the well, the liquid will seep
through the ground and some may make its way into the pump before it
has been purified by the earth. The impure liquid will thus
contaminate the otherwise pure water and will render it decidedly
harmful. For absolute safety the sewage discharge should be at least
75 feet from the well, and in large hotels, where there is necessarily
a large quantity of sewage, the distance should be much greater. As
the sewage seeps through the ground it loses its impurities, but the
quantity of earth required to purify it depends upon its abundance; a
small depth of soil cannot take care of an indefinite amount of
sewage. Hence, the greater the number of people in a hotel, or the
more abundant the sewage, the greater should be the distance between
well and sewer.

By far the best way to avoid contamination is to see to it that the
sewage discharges into the ground _below_ the well; that is, to dig
the well in such a location that the sewage drainage will be away from
the well.

In cities and towns and large summer communities, the sewage of
individual buildings drains into common tanks erected at public
expense; the contents of these are discharged in turn into harbors and
streams, or are otherwise disposed of at great expense, although they
contain valuable substances. It has been estimated that the drainage
or sewage of England alone would be worth $ 80,000,000 a year if used
as fertilizer.

A few cities, such as Columbus and Cleveland, Ohio, realize the need
of utilizing this source of wealth, and by chemical means deodorize
their sewage and change it into substances useful for agricultural and
industrial purposes. There is still a great deal to be learned on this
subject, and it is possible that chemically treated sewage may be made
a source of income to a community rather than an expense.

189. Pumps which Compress Air. The pumps considered in the preceding
Sections have their widest application in agricultural districts,
where by means of them water is raised to the surface of the earth or
is pumped into elevated tanks. From a commercial and industrial
standpoint a most important class of pump is that known as the
compression type; in these, air or any other gas is compressed rather
than rarefied.

Air brakes and self-opening and self-closing doors on cars are
operated by means of compression pumps. The laying of bridge and pier
foundations, in fact all work which must be done under water, is
possible only through the agency of compression pumps. Those who have
visited mines, and have gone into the heart of the underground
labyrinth, know how difficult it is for fresh air to make its way to
the miners. Compression pumps have eliminated this difficulty, and
to-day fresh air is constantly pumped into the mines to supply the
laborers there. Agricultural methods also have been modified by the
compression pump. The spraying of trees (Fig. 143), formerly done
slowly and laboriously, is now a relatively simple matter.

[Illustration: FIG. 143.--Spraying trees by means of a compression
pump.]

190. The Bicycle Pump. The bicycle pump is the best known of all
compression pumps. Here, as in other pumps of its type, the valves
open inward rather than outward. When the piston is lowered,
compressed air is driven through the rubber tubing, pushes open an
inward-opening valve in the tire, and thus enters the tire. When the
piston is raised, the lower valve closes, the upper valve is opened
by atmospheric pressure, and air from outside enters the cylinder; the
next stroke of the piston drives a fresh supply of air into the tire,
which thus in time becomes inflated. In most cheap bicycle pumps, the
piston valve is replaced by a soft piece of leather so attached to the
piston that it allows air to slip around it and into the cylinder, but
prevents its escape from the cylinder (Fig. 144).

[Illustration: FIG. 144.--The bicycle foot pump.]

191. How a Man works under Water. Place one end of a piece of glass
tube in a vessel of water and notice that the water rises in the tube
(Fig. 145). Blow into the tube and see whether you can force the water
wholly or partially down the tube. If the tube is connected to a small
compression pump, sufficient air can be sent into the tube to cause
the water to sink and to keep the tube permanently clear of water.
This is, in brief, the principle employed for work under water. A
compression pump forces air through a tube into the chamber in which
men are to work (Fig. 146). The air thus furnished from above supplies
the workmen with oxygen, and by its pressure prevents water from
entering the chamber. When the task has been completed, the chamber is
raised and later lowered to a new position.

[Illustration: FIG. 145.--Water does not enter the tube as long as we
blow into it.]

Figure 147 shows men at work on a bridge foundation. Workmen, tools,
and supplies are lowered in baskets through a central tube _BC_
provided with an air chamber _L_, having air-tight gates at _A_ and
_A'_. The gate _A_ is opened and workmen enter the air chamber. The
gate _A_ is then closed and the gate _A'_ is opened slowly to give the
men time to get accustomed to the high pressure in _B_, and then the
men are lowered to the bottom. Excavated earth is removed in a similar
manner. Air is supplied through a tube _DD_. Such an arrangement for
work under water is called a caisson. It is held in position by a mass
of concrete _EE_.

[Illustration: FIG. 146--The principle of work under water.]

[Illustration: FIG. 147--Showing how men can work under water.]

In many cases men work in diving suits rather than in caissons; these
suits are made of rubber except for the head piece, which is of metal
provided with transparent eyepieces. Air is supplied through a
flexible tube by a compression pump. The diver sometimes carries on
his back a tank of compressed air, from which the air escapes through
a tube to the space between the body and the suit. When the air has
become foul, the diver opens a valve in his suit and allows it to pass
into the water, at the same time admitting a fresh supply from the
tank. The valve opens outward from the body, and hence will allow of
the exit of air but not of the entrance of water. When the diver
ceases work and desires to rise to the surface, he signals and is
drawn up by a rope attached to the suit.

192. Combination of Pumps. In many cases the combined use of both
exhaust and compression pumps is necessary to secure the desired
result; as, for example, in pneumatic dispatch tubes. These are
employed in the transportation of letters and small packages from
building to building or between parts of the same building. A pump
removes air from the part of the tube ahead of the package, and thus
reduces the resistance, while a compression pump forces air into the
tube behind the package and thus drives it forward with great speed.



CHAPTER XIX

THE WATER PROBLEM OF A LARGE CITY


193. It is by no means unusual for the residents of a large city or
town to receive through the newspapers a notification that the city
water supply is running low and that economy should be exercised in
its use. The problem of supplying a large city with an abundance of
pure water is among the most difficult tasks which city officials have
to perform, and is one little understood and appreciated by the
average citizen.

Intense interest in personal and domestic affairs is natural, but
every citizen, rich or poor, should have an interest in civic affairs
as well, and there is no better or more important place to begin than
with the water supply. One of the most stirring questions in New York
to-day has to do with the construction of huge aqueducts designed to
convey to the residents of the city, water from the distant Catskill
Mountains. The growth of the population has been so phenomenally rapid
that the combined output of all available near-by sources does not
suffice to meet the increasing consumption.

Where does your city obtain its water? Does it bring it to its
reservoirs in the most economic way possible, and is there any
legitimate excuse for the scarcity of water which many communities
face in dry seasons?

194. Two Possibilities. Sometimes a city is fortunate enough to be
situated near hills and mountains through which streams flow, and in
that case the water problem is simple. In such a case all that is
necessary is to run pipes, usually underground, from the elevated
lakes or streams to the individual houses, or to common reservoirs
from which it is distributed to the various buildings.

[Illustration: FIG. 148.--The elevated mountain lake serves as a
source of water.]

Figure 148 illustrates in a simple way the manner in which a mountain
lake may serve to supply the inhabitants of a valley. The city of
Denver, for example, is surrounded by mountains abounding in streams
of pure, clear water; pipes convey the water from these heights to the
city, and thus a cheap and adequate flow is obtained. Such a system is
known as the gravity system. The nearer and steeper the elevation, the
greater the force with which the water flows through the valley pipes,
and hence the stronger the discharge from the faucets.

Relatively few cities and towns are so favorably situated as regards
water; more often the mountains are too distant, or the elevation is
too slight, to be of practical value. Cities situated in plains and
remote from mountains are obliged to utilize the water of such streams
as flow through the land, forcing it to the necessary height by means
of pumps. Streams which flow through populated regions are apt to be
contaminated, and hence water from them requires public filtration.
Cities using such a water supply thus have the double expense of
pumping and filtration.

195. The Pressure of Water. No practical business man would erect a
turbine or paddle wheel without calculating in advance the value of
his water power. The paddle wheel might be so heavy that the stream
could not turn it, or so frail in comparison with the water force that
the stream would destroy it. In just as careful a manner, the size and
the strength of municipal reservoirs and pumps must be calculated. The
greater the quantity of water to be held in the reservoir, the heavier
are the walls required; the greater the elevation of the houses, the
stronger must be the pumps and the engines which run them.

In order to understand how these calculations are made, we must study
the physical characteristics of water just as we studied the physical
characteristics of air.

When we measure water, we find that 1 cubic foot of it weighs about
62.5 pounds; this is equivalent to saying that water 1 foot deep
presses on the bottom of the containing vessel with a force of 62.5
pounds to the square foot. If the water is 2 feet deep, the load
supported by the vessel is doubled, and the pressure on each square
foot of the bottom of the vessel will be 125 pounds, and if the water
is 10 feet deep, the load borne by each square foot will be 625
pounds. The deeper the water, the greater will be the weight sustained
by the confining vessel and the greater the pressure exerted by the
water.

[Illustration: FIG. 149.--Water 1 foot deep exerts a pressure of 62.5
pounds a square foot.]

Since the pressure borne by 1 square foot of surface is 62.5 pounds,
the pressure supported by 1 square inch of surface is 1/144 of 62.5
pounds, or .43 pound, nearly 1/2 pound. Suppose a vessel held water to
the depth of 10 feet, then upon every square inch of the bottom of
that vessel there would be a pressure of 4.34 pounds. If a one-inch
tap were inserted in the bottom of the vessel so that the water flowed
out, it would gush forth with a force of 4.34 pounds. If the water
were 20 feet deep, the force of the outflowing water would be twice as
strong, because the pressure would be doubled. But the flow would not
remain constant, because as the water leaves the outlet, less and less
of it remains in the vessel, and hence the pressure gradually sinks
and the flow drops correspondingly.

In seasons of prolonged drought, the streams which feed a city
reservoir are apt to contain less than the usual amount of water,
hence the level of the water supply sinks, the pressure at the outlet
falls, and the force of the outflowing water is lessened (Fig. 150).

[Illustration: FIG. 150.--The pressure at an outlet decreases as the
level of the water supply sinks.]

196. Why the Water Supply is not uniform in All Parts of the City.
In the preceding Section, we saw that the flow from a faucet depends
upon the height of the reserve water above the tap. Houses on a level
with the main supply pipes (Figs. 148 and 151) have a strong flow
because the water is under the pressure of a column _A_; houses
situated on elevation _B_ have less flow, because the water is under
the pressure of a shorter column _B_; and houses at a considerable
elevation _C_ have a less rapid flow corresponding to the diminished
depth _(C)_.

Not only does the flow vary with the elevation of the house, but it
varies with the location of the faucet within the house. Unless the
reservoir is very high, or the pumps very powerful, the flow on the
upper floors is noticeably less than that in the cellar, and in the
upper stories of some high building the flow is scarcely more than a
feeble trickle.

[Illustration: FIG. 151.--Water pressure varies in different parts of
a water system.]

When the respective flows at _A_, _B_, and _C_ (Fig. 151) are measured,
they are found to be far lower than the pressures which columns of
water of the heights _A_, _B_, and _C_ have been shown by actual
demonstration to exert. This is because water, in flowing from place
to place, expends force in overcoming the friction of the pipes and
the resistance of the air. The greater the distance traversed by the
water in its journey from reservoir to faucet, the greater the waste
force and the less the final flow.

In practice, large mains lead from the reservoir to the city, smaller
mains convey the water to the various sections of the city, and
service pipes lead to the individual house taps. During this long
journey, considerable force is expended against friction, and hence
the flow at a distance from the reservoir falls to but a fraction of
its original strength. For this reason, buildings situated near the
main supply have a much stronger flow (Fig. 152) than those on the
same level but remote from the supply. Artificial reservoirs are
usually constructed on the near outskirts of a town in order that the
frictional force lost in transmission may be reduced to a minimum.

[Illustration: FIG. 152.--The more distant the fountain, the weaker
the flow.]

In the case of a natural reservoir, such as an elevated lake or
stream, the distance cannot be planned or controlled. New York, for
example, will secure an abundance of pure water from the Catskill
Mountains, but it will lose force in transmission. Los Angeles is
undertaking one of the greatest municipal projects of the day. Huge
aqueducts are being built which will convey pure mountain water a
distance of 250 miles, and in quantities sufficient to supply two
million people. According to calculations, the force of the water will
be so great that pumps will not be needed.

197. Why Water does not always flow from a Faucet. Most of us have
at times been annoyed by the inability to secure water on an upper
story, because of the drawing off of a supply on a lower floor.
During the working hours of the day, immense quantities of water are
drawn off from innumerable faucets, and hence the quantity in the
pipes decreases considerably unless the supply station is able to
drive water through the vast network of pipes as fast as it is drawn
off. Buildings at a distance from the reservoir suffer under such
circumstances, because while the diminished pressure is ordinarily
powerful enough to supply the lower floors, it is frequently too weak
to force a continuous stream to high levels. At night, however, and
out of working hours, few faucets are open, less water is drawn off at
any one time, and the intricate pipes are constantly full of water
under high pressure. At such times, a good flow is obtainable even on
the uppermost floors.

In order to overcome the disadvantage of a decrease in flow during the
day, standpipes (Fig. 153) are sometimes placed in various sections.
These are practically small steel reservoirs full of water and
connecting with the city pipes. During "rush" hours, water passes from
these into the communicating pipes and increases the available supply,
while during the night, when the faucets are turned off, water
accumulates in the standpipe against the next emergency (Figs. 151 and
154). The service rendered by the standpipe is similar to that of the
air cushion discussed in Section 184.

[Illustration: FIG. 153.--A standpipe.]

198. The Cost of Water. In the gravity system, where an elevated
lake or stream serves as a natural reservoir, the cost of the city's
waterworks is practically limited to the laying of pipes. But when the
source of the supply is more or less on a level with the surrounding
land, the cost is great, because the supply for the entire city must
either be pumped into an artificial reservoir, from which it can be
distributed, or else must be driven directly through the mains (Fig.
154).

[Illustration: FIG. 154.--Water must be got to the houses by means of
pumps.]

A gallon of water weighs approximately 8.3 pounds, and hence the work
done by a pump in raising a gallon of water to the top of an average
house, an elevation of 50 feet, is 8.3 × 50, or 415 foot pounds. A
small manufacturing town uses at least 1,000,000 gallons daily, and
the work done by a pump in raising that amount to an elevation of 50
feet would be 8.3 × 1,000,000 × 50, or 415,000,000 foot pounds.

The total work done during the day by the pump, or the engine driving
the pump, is 415,000,000 foot pounds, and hence the work done during
one hour would be 1/24 of 415,000,000, or 17,291,666 foot pounds; the
work done in one minute would be 1/60 of 17,291,666, or 288,194 foot
pounds, and the work done each second would be 1/60 of 288,194, or
4803 foot pounds.

A 1-H.P. engine does 550 foot pounds of work each second, and
therefore if the pump is to be operated by an engine, the strength of
the latter would have to be 8.7 H.P. An 8.7-H.P. pumping engine
working at full speed every second of the day and night would be able
to supply the town with the necessary amount of water. When, however,
we consider the actual height to which the water is raised above the
pumping station, and the extra pumping which must be done in order to
balance the frictional loss, it is easy to understand that in actual
practice a much more powerful engine would be needed. The larger the
piston and the faster it works, the greater is the quantity of water
raised at each stroke, and the stronger must be the engine which
operates the pump.

In many large cities there is no one single pumping station from which
supplies run to all parts of the city, but several pumping stations
are scattered throughout the city, and each of them supplies a
restricted territory.

199. The Bursting of Dams and Reservoirs. The construction of a safe
reservoir is one of the most important problems of engineers. In
October, 1911, a town in Pennsylvania was virtually wiped out of
existence because of the bursting of a dam whose structure was of
insufficient strength to resist the strain of the vast quantity of
water held by it. A similar breakage was the cause of the fatal
Johnstown flood in 1889, which destroyed no less than seven towns, and
in which approximately 2000 persons are said to have lost their lives.

Water presses not only on the bottom of a vessel, but upon the sides
as well; a bucket leaks whether the hole is in its side or its bottom,
showing that water presses not only downward but outward. Usually a
leak in a dam or reservoir occurs near the bottom. Weak spots at the
top are rare and easily repaired, but a leak near the bottom is
usually fatal, and in the case of a large reservoir the outflowing
water carries death and destruction to everything in its path.

If the leak is near the surface, as at _a_ (Fig. 155), the water
issues as a feeble stream, because the pressure against the sides at
that level is due solely to the relatively small height of water
above _a_ (Section 195). If the leak is lower, as at _b_, the issuing
stream is stronger and swifter, because at that level the outward
pressure is much greater than at _a_, the increase being due to the
fact that the height of the water above _b_ is greater than that above
_a_. If the leak is quite low, as at _c_, the issuing stream has a
still greater speed and strength, and gushes forth with a force
determined by the height of the water above _c_.

[Illustration: FIG. 155.--The flow from an opening depends upon the
height of water above the opening.]

The dam at Johnstown was nearly 1/2 mile wide, and 40 feet high, and
so great was the force and speed of the escaping stream that within an
hour after the break had occurred, the water had traveled a distance
of 18 miles, and had destroyed property to the value of millions of
dollars.

If a reservoir has a depth of 100 feet, the pressure exerted upon each
square foot of its floor is 62.5 × 100, or 6250 pounds; the weight
therefore to be sustained by every square foot of the reservoir floor
is somewhat more than 3 tons, and hence strong foundations are
essential. The outward lateral pressure at a depth of 25 feet would be
only one fourth as great as that on the bottom--hence the strain on
the sides at that depth would be relatively slight, and a less
powerful construction would suffice. But at a depth of 50 feet the
pressure on the sides would be one half that of the floor pressure, or
1-1/2 tons. At a depth of 75 feet, the pressure on the sides would be
three quarters that on the bottom, or 2-1/4 tons. As the bottom of the
reservoir is approached, the pressure against the sides increases, and
more powerful construction becomes necessary.

Small elevated tanks, like those of the windmill, frequently have
heavy iron bands around their lower portion as a protection against
the extra strain.

Before erecting a dam or reservoir, the maximum pressure to be exerted
upon every square inch of surface should be accurately calculated, and
the structure should then be built in such a way that the varying
pressure of the water can be sustained. It is not sufficient that the
bottom be strong; the sides likewise must support their strain, and
hence must be increased in strength with depth. This strengthening of
the walls is seen clearly in the reservoir shown in Figure 152. The
bursting of dams and reservoirs has occasioned the loss of so many
lives, and the destruction of so much property, that some states are
considering the advisability of federal inspection of all such
structures.

[Illustration: FIG. 156.--The lock gates must be strong in order to
withstand the great pressure of the water against them.]

200. The Relation of Forests to the Water Supply. When heavy rains
fall on a bare slope, or when snow melts on a barren hillside, a small
amount of the water sinks into the ground, but by far the greater part
of it runs off quickly and swells brooks and streams, thus causing
floods and freshets.

When, however, rain falls on a wooded slope, the action is reversed; a
small portion runs off, while the greater portion sinks into the soft
earth. This is due partly to the fact that the roots of trees by their
constant growth keep the soil loose and open, and form channels, as it
were, along which the water can easily run. It is due also to the
presence on the ground of decaying leaves and twigs, or humus. The
decaying vegetable matter which covers the forest floor acts more or
less as a sponge, and quickly absorbs falling rain and melting snow.
The water which thus passes into the humus and the soil beneath does
not remain there, but slowly seeps downward, and finally after weeks
and months emerges at a lower level as a stream. Brooks and springs
formed in this way are constant feeders of rivers and lakes.

In regions where the land has been deforested, the rivers run low in
season of prolonged drought, because the water which should have
slowly seeped through the soil, and then supplied the rivers for weeks
and months, ran off from the barren slopes in a few days.

Forests not only lessen the danger of floods, but they conserve our
waterways, preventing a dangerous high-water mark in the season of
heavy rains and melting snows, and then preventing a shrinkage in dry
seasons when the only feeders of the rivers are the underground
sources. In the summer of 1911, prolonged drought in North Carolina
lowered the rivers to such an extent that towns dependent upon them
suffered greatly. The city of Charlotte was reduced for a time to a
practically empty reservoir; washing and bathing were eliminated,
machinery dependent upon water-power and steam stood idle, and every
glass of water drunk was carefully reckoned. Thousands of gallons of
water were brought in tanks from neighboring cities, and were emptied
into the empty reservoir from whence it trickled slowly through the
city mains. The lack of water caused not only personal inconvenience
and business paralysis, but it occasioned real danger of disease
through unflushed sewers and insufficiently drained pipes.

The conservation of the forest means the conservation of our
waterways, whether these be used for transportation or as sources of
drinking water.



CHAPTER XX

MAN'S CONQUEST OF SUBSTANCES


201. Chemistry. Man's mechanical inventions have been equaled by his
chemical researches and discoveries, and by the application he has
made of his new knowledge.

The plain cotton frock of our grandmothers had its death knell sounded
a few years ago, when John Mercer showed that cotton fabrics soaked in
caustic soda assumed under certain conditions a silky sheen, and when
dyed took on beautiful and varied hues. The demonstration of this
simple fact laid the foundation for the manufacture of a vast variety
of attractive dress materials known as mercerized cotton.

Possibly no industry has been more affected by chemical discovery than
that of dyeing. Those of us who have seen the old masterpieces in
painting, or reproductions of them, know the softness, the mellowness,
the richness of tints employed by the old masters. But if we look for
the brilliancy and variety of color seen in our own day, the search
will be fruitless, because these were unknown until a half century
ago. Up to that time, dyes were few in number and were extracted
solely from plants, principally from the indigo and madder plants. But
about the year 1856 it was discovered that dyes in much greater
variety and in purer form could be obtained from coal tar. This
chemical production of dyes has now largely supplanted the original
method, and the industry has grown so rapidly that a single firm
produced in one year from coal tar a quantity of indigo dye which
under the natural process of plant extraction would have required a
quarter million acres of indigo plant.

The abundance and cheapness of newspapers, coarse wrapping papers,
etc., is due to the fact that man has learned to substitute wood for
rags in the manufacture of paper. Investigation brought out the fact
that wood contained the substance which made rags valuable for paper
making. Since the supply of rags was far less than the demand, the
problem of the extraction from wood of the paper-forming substance was
a vital one. From repeated trials, it was found that caustic soda when
heated with wood chips destroyed everything in the wood except the
desired substance, cellulose; this could be removed, bleached, dried,
and pressed into paper. The substitution of wood for rags has made
possible the daily issue of newspapers, for the making of which
sufficient material would not otherwise have been available. When we
reflect that a daily paper of wide circulation consumes ten acres of
wood lot per day, we see that all the rags in the world would be
inadequate to meet this demand alone, to say nothing of periodicals,
books, tissue paper, etc.

Chemistry plays a part in every phase of life; in the arts, the
industries, the household, and in the body itself, where digestion,
excretion, etc., result from the action of the bodily fluids upon
food. The chemical substances of most interest to us are those which
affect us personally rather than industrially; for example, soap,
which cleanses our bodies, our clothing, our household possessions;
washing soda, which lightens laundry work; lye, which clears out the
drain pipe clogged with grease; benzine, which removes stains from
clothing; turpentine, which rids us of paint spots left by careless
workmen; and hydrogen peroxide, which disinfects wounds and sores.

In order to understand the action of several of these substances we
must study the properties of two groups of chemicals--known
respectively as acids and bases; the first of these may be represented
by vinegar, sulphuric acid, and oxalic acid; and the second, by
ammonia, lye, and limewater.

202. Acids. All of us know that vinegar and lemon juice have a sour
taste, and it is easy to show that most acids are characterized by a
sour taste. If a clean glass rod is dipped into very dilute acid, such
as acetic, sulphuric, or nitric acid, and then lightly touched to the
tongue, it will taste sour. But the best test of an acid is by sight
rather than by taste, because it has been found that an acid is able
to discolor a plant substance called litmus. If paper is soaked in a
litmus solution until it acquires the characteristic blue hue of the
plant substance, and is then dried thoroughly, it can be used to
detect acids, because if it comes in contact with even the minutest
trace of acid, it loses its blue color and assumes a red tint. Hence,
in order to detect the presence of acid in a substance, one has merely
to put some of the substance on blue litmus paper, and note whether or
not the latter changes color. This test shows that many of our common
foods contain some acid; for example, fruit, buttermilk, sour bread,
and vinegar.

The damage which can be done by strong acids is well known; if a jar
of sulphuric acid is overturned, and some of it falls on the skin, it
eats its way into the flesh and leaves an ugly sore; if it falls on
carpet or coat, it eats its way into the material and leaves an
unsightly hole. The evil results of an accident with acid can be
lessened if we know just what to do and do it quickly, but for this we
must have a knowledge of bases, the second group of chemicals.

203. Bases. Substances belonging to this group usually have a bitter
taste and a slimy, soapy feeling. For our present purposes, the most
important characteristic of a base is that it will neutralize an acid
and in some measure hinder the damage effected by the former. If, as
soon as an acid has been spilled on cloth, a base, such as ammonia, is
applied to the affected region, but little harm will be done. In your
laboratory experiments you may be unfortunate enough to spill acid on
your body or clothing; if so, quickly apply ammonia. If you delay, the
acid does its work, and there is no remedy. If soda (a base) touches
black material, it discolors it and leaves an ugly brown spot; but the
application of a little acid, such as vinegar or lemon juice, will
often restore the original color and counteract the bad effects of the
base. Limewater prescribed by physicians in cases of illness is a
well-known base. This liquid neutralizes the too abundant acids
present in a weak system and so quiets and tones the stomach.

The interaction of acids and bases may be observed in another way. If
blue litmus paper is put into an acid solution, its color changes to
red; if now the red litmus paper is dipped into a base solution,
caustic soda, for example, its original color is partially restored.
What the acid does, the base undoes, either wholly or in part. Bases
always turn red litmus paper blue.

Bases, like acids, are good or bad according to their use; if they
come in contact with cloth, they eat or discolor it, unless
neutralized by an acid. But this property of bases, harmful in one
way, is put to advantage in the home, where grease is removed from
drainpipe and sink by the application of lye, a strong base. If the
lye is too concentrated, it will not only eat the grease, but will
corrode the metal piping; it is easy, however, to dilute base
solutions to such a degree that they will not affect piping, but will
remove grease. Dilute ammonia is used in almost every home and is an
indispensable domestic servant; diluted sufficiently, it is
invaluable in the washing of delicate fabrics and in the removing of
stains, and in a more concentrated form it is helpful as a smelling
salt in cases of fainting.

Some concentrated bases are so powerful in their action on grease,
cloth, and metal that they have received the designation _caustic_,
and are ordinarily known as caustic soda, caustic potash (lye), and
caustic lime. These more active bases are generally called alkalies in
distinction from the less active ones.

204. Neutral Substances. To any acid solution add gradually a small
quantity of a base, and test the mixture from time to time with blue
litmus paper; at first the paper will turn red quickly, but as more
and more of the base is added to the solution, it has less and less
effect on the blue litmus paper, and finally a point is reached when a
fresh strip of blue paper will not be affected. Such a result
indicates infallibly the absence of any acid qualities in the
solution. If now red litmus paper is tested in the same solution, its
color also will remain unchanged; such a result indicates infallibly
the absence of any basic quality. The solution has the characteristic
property of neither acid nor base and is said to be neutral.

If to the neutral solution an extra portion of base is added, so that
there is an excess of base over acid, the neutralization is
overbalanced and the red paper turns blue. If to the neutral solution
an extra portion of acid is added, so that there is an excess of acid
over base, the neutralization is overbalanced in the opposite
direction, and the solution acquires acid characteristics.

Most acids and bases will eat and corrode and discolor, while neutral
substances will not; it is for this reason that soap, a slightly
alkaline substance, is the safest cleansing agent for laundry, bath,
and general work. Good soaps, being carefully made, are so nearly
neutral that they will not fade the color out of clothing; the cheap
soaps are less carefully prepared and are apt to have a strong excess
of the base ingredient; such soaps are not safe for delicate work.

205. Soap. If we gather together scrapings of lard, butter, bits of
tallow from burned-out candles, scraps of waste fat, or any other sort
of grease, and pour a strong solution of lye over the mass, a soft
soapy substance is formed. In colonial times, every family made its
own supply of soap, utilizing, for that purpose, household scraps
often regarded by the housekeeper of to-day as worthless. Grease and
fat were boiled with water and hardwood ashes, which are rich in lye,
and from the mixture came the soft soap used by our ancestors. In
practice, the wood ashes were boiled in water, which was then strained
off, and the resulting filtrate, or lye, was mixed with the fats for
soap making.

Most fats contain a substance of an acid nature, and are decomposed by
the action of bases such as caustic soda and caustic potash. The acid
component of the grease partially neutralizes the base, and a new
substance is formed, namely, soap.

With the advance of civilization the labor of soap making passed from
the home to the factory, very much as bread making has done in our own
day. Different varieties of soaps appeared, of which the hard soap was
the most popular, owing to the ease with which it could be
transported. Within the last few years liquid soaps have come into
favor, especially in schools, railroad stations, and other public
places, where a cake of soap would be handled by many persons. By
means of a simple device (Fig. 157), the soap escapes from a
receptacle when needed. The mass of the soap does not come in contact
with the skin, and hence the spread of contagious skin diseases is
lessened.

[Illustration: FIG. 157.--Liquid soap container.]

Commercial soaps are made from a great variety of substances, such as
tallow, lard, castor oil, coconut oil, olive oil, etc.; or in cheaper
soaps, from rosin, cottonseed oil, and waste grease. The fats which go
to waste in our garbage could be made a source of income, not only to
the housewife, but to the city. In Columbus, Ohio, garbage is used as
a source of revenue; the grease from the garbage being sold for soap
making, and the tankage (Section 188) for fertilizer.

206. Why Soap Cleans. The natural oil of the skin catches and
retains dust and dirt, and makes a greasy film over the body. This
cannot be removed by water alone, but if soap is used and a generous
lather is applied to the skin, the dirt is "cut" and passes from the
body into the water. Soap affects a grease film and water very much as
the white of an egg affects oil and water. These two liquids alone do
not mix, the oil remaining separate on the surface of the water; but
if a small quantity of white of egg is added, an emulsion is formed,
the oil separating into minute droplets which spread through the
water. In the same way, soap acts on a grease film, separating it into
minute droplets which leave the skin and spread through the water,
carrying with them the dust and dirt particles. The warmer the water,
the better will be the emulsion, and hence the more effective the
removal of dirt and grease. This explanation holds true for the
removal of grease from any surface, whether of the body, clothing,
furniture, or dishes.

207. Washing Powders. Sometimes soap refuses to form a lather and
instead cakes and floats as a scum on the top of the water; this is
not the fault of the soap but of the water. As water seeps through
the soil or flows over the land, it absorbs and retains various soil
constituents which modify its character and, in some cases, render it
almost useless for household purposes. Most of us are familiar with
the rain barrel of the country house, and know that the housewife
prefers rain water for laundry and general work. Rain water, coming as
it does from the clouds, is free from the chemicals gathered by ground
water, and is hence practically pure. While foreign substances do not
necessarily injure water for drinking purposes (Section 69), they are
often of such a nature as to prevent soap from forming an emulsion,
and hence from doing its work. Under such circumstances the water is
said to be hard, and soap used with it is wasted. Even if water is
only moderately hard, much soap is lost. The substances which make
water hard are calcium and magnesium salts. When soap is put into
water containing one or both of these, it combines with the salts to
form sticky insoluble scum. It is therefore not free to form an
emulsion and to remove grease. As a cleansing agent it is valueless.
The average city supply contains so little hardness that it is
satisfactory for toilet purposes; but in the laundry, where there is
need for the full effect of the soap, and where the slightest loss
would aggregate a great deal in the course of time, something must be
done to counteract the hardness. The addition of soda, or sodium
carbonate to the water will usually produce the desired effect.
Washing soda combines with calcium and magnesium and prevents them
from uniting with soap. The soap is thus free to form an emulsion,
just as in ordinary water. Washing powders are sometimes used instead
of washing soda. Most washing powders contain, in addition to a
softening agent, some alkali, and hence a double good is obtained from
their use; they not only soften the water and allow the soap to form
an emulsion, but they also, through their alkali content, cut the
grease and themselves act as cleansers. In some cities where the water
is very hard, as in Columbus, Ohio, it is softened and filtered at
public expense, before it leaves the reservoirs. But even under these
circumstances, a moderate use of washing powder is general in laundry
work.

If washing powder is put on clothes dry, or is thrown into a crowded
tub, it will eat the clothes before it has a chance to dissolve in the
water. The only safe method is to dissolve the powder before the
clothes are put into the tub. The trouble with our public laundries is
that many of them are careless about this very fact, and do not take
time to dissolve the powder before mixing it with the clothes.

The strongest washing powder is soda, and this cheap form is as good
as any of the more expensive preparations sold under fancy names.
Borax is a milder powder and is desirable for finer work.

One of the most disagreeable consequences of the use of hard water for
bathing is the unavoidable scum which forms on the sides of bathtub
and washbowl. The removal of the caked grease is difficult, and if
soap alone is used, the cleaning of the tub requires both patience and
hard scrubbing. The labor can be greatly lessened by moistening the
scrubbing cloth with turpentine and applying it to the greasy film,
which immediately dissolves and thus can be easily removed. The
presence of the scum can be largely avoided by adding a small amount
of liquid ammonia to the bath water. But many persons object to this;
hence it is well to have some other easy method of removing the
objectionable matter.

208. To remove Stains from Cloth. While soap is, generally speaking,
the best cleansing agent, there are occasions when other substances
can be used to better advantage. For example, grease spots on carpet
and non-washable dress goods are best removed by the application of
gasoline or benzine. These substances dissolve the grease, but do not
remove it from the clothing; for that purpose a woolen cloth should be
laid under the stain in readiness to absorb the benzine and the grease
dissolved in it. If the grease is not absorbed while in solution, it
remains in the clothing and after the evaporation of the benzine
reappears in full force.

Cleaners frequently clean suits by laying a blotter over a grease spot
and applying a hot iron; the grease, when melted by the heat, takes
the easiest way of spreading itself and passes from cloth to blotter.

209. Salts. A neutral liquid formed as in Section 204, by the action
of hydrochloric acid and the alkali solution of caustic soda, has a
brackish, salty taste, and is, in fact, a solution of salt. This can
be demonstrated by evaporating the neutral liquid to dryness and
examining the residue of solid matter, which proves to be common salt.

When an acid is mixed with a base, the result is a substance more or
less similar in its properties to common salt; for this reason all
compounds formed by the neutralization of an acid with a base are
called salts. If, instead of hydrochloric acid (HCl), we use an acid
solution of potassium tartrate, and if instead of caustic soda we use
bicarbonate of soda (baking soda), the result is a brackish liquid as
before, but the salt in the liquid is not common salt, but Rochelle
salt. Different combinations of acids and bases produce different
salts. Of all the vast group of salts, the most abundant as well as
the most important is common salt, known technically as sodium
chloride because of its two constituents, sodium and chlorine.

We are not dependent upon neutralization for the enormous quantities
of salt used in the home and in commerce. It is from the active,
restless seas of the present, and from the dead seas of the
prehistoric past that our vast stores of salt come. The waters of the
Mediterranean and of our own Great Salt Lake are led into shallow
basins, where, after evaporation by the heat of the sun, they leave a
residue of salt. By far the largest quantity of salt, however, comes
from the seas which no longer exist, but which in far remote ages
dried up and left behind them their burden of salt. Deposits of salt
formed in this way are found scattered throughout the world, and in
our own country are found in greatest abundance in New York. The
largest salt deposit known has a depth of one mile and exists in
Germany.

Salt is indispensable on our table and in our kitchen, but the amount
of salt used in this way is far too small to account for a yearly
consumption of 4,000,000 tons in the United States alone. The
manufacture of soap, glass, bleaching powders, baking powders, washing
soda, and other chemicals depends on salt, and it is for these that
the salt beds are mined.

210. Baking Soda. Salt is by all odds the most important sodium
compound. Next to it come the so-called carbonates: first, sodium
carbonate, which is already familiar to us as washing soda; and
second, sodium bicarbonate, which is an ingredient of baking powders.
These are both obtained from sodium chloride by relatively simple
means; that is, by treating salt with the base, ammonia, and with
carbon dioxide.

Washing soda has already been discussed. Since baking powders in some
form are used in almost all homes for the raising of cake and pastry
dough, it is essential that their helpful and harmful qualities be
clearly understood.

The raising of dough by means of baking soda--bicarbonate of soda--is
a very simple process. When soda is heated, it gives off carbon
dioxide gas; you can easily prove this for yourself by burning a
little soda in a test tube, and testing the escaping gas in a test
tube of limewater. When flour and water alone are kneaded and baked
in loaves, the result is a mass so compact and hard that human teeth
are almost powerless to crush and chew it. The problem is to separate
the mass of dough or, in other words, to cause it to rise and lighten.
This can be done by mixing a little soda in the flour, because the
heat of the oven causes the soda to give off bubbles of gas, and these
in expanding make the heavy mass slightly porous. Bread is never
lightened with soda because the amount of gas thus given off is too
small to convert heavy compact bread dough into a spongy mass; but
biscuit and cake, being by nature less compact and heavy, are
sufficiently lightened by the gas given off from soda.

But there is one great objection to the use of soda alone as a
leavening agent. After baking soda has lost its carbon dioxide gas, it
is no longer baking soda, but is transformed into its relative,
washing soda, which has a disagreeable taste and is by no means
desirable for the stomach.

Man's knowledge of chemicals and their effect on each other has
enabled him to overcome this difficulty and, at the same time, to
retain the leavening effect of the baking soda.

211. Baking Powders. If some cooking soda is put into lemon juice or
vinegar, or any acid, bubbles of gas immediately form and escape from
the liquid. After the effervescence has ceased, a taste of the liquid
will show you that the lemon juice has lost its acid nature, and has
acquired in exchange a salty taste. Baking soda, when treated with an
acid, is transformed into carbon dioxide and a salt. The various
baking powders on the market to-day consist of baking soda and some
acid substance, which acts upon the soda, forces it to give up its
gas, and at the same time unites with the residue to form a harmless
salt.

Cream of tartar contains sufficient acid to act on baking soda, and is
a convenient and safe ingredient for baking powder. When soda and
cream of tartar are mixed dry, they do not react on each other,
neither do they combine rapidly in _cold_ moist dough, but as soon as
the heat of the oven penetrates the doughy mass, the cream of tartar
combines with the soda and sets free the gas needed to raise the
dough. The gas expands with the heat of the oven, raising the dough
still more. Meanwhile, the dough itself is influenced by the heat and
is stiffened to such an extent that it retains its inflated shape and
spongy nature.

Many housewives look askance at ready-made baking powders and prefer
to bake with soda and sour milk, soda and buttermilk, or soda and
cream of tartar. Sour milk and buttermilk are quite as good as cream
of tartar, because the lactic acid which they contain combines with
the soda and liberates carbon dioxide, and forms a harmless residue in
the dough.

The desire of manufacturers to produce cheap baking powders led to the
use of cheap acids and alkalies, regardless of the character of the
resulting salt. Alum and soda were popular for some time; but careful
examination proved that the particular salt produced by this
combination was not readily absorbed by the stomach, and that its
retention there was injurious to health. For this reason, many states
have prohibited the use of alum in baking powders.

It is not only important to choose the ingredients carefully; it is
also necessary to calculate the respective quantities of each,
otherwise there will be an excess of acid or alkali for the stomach to
take care of. A standard powder contains twice as much cream of tartar
as of bicarbonate of soda, and the thrifty housewife who wishes to
economize, can make for herself, at small cost, as good a baking
powder as any on the market, by mixing tartar and soda in the above
proportions and adding a little corn starch to keep the mixture dry.

The self-raising flour, so widely advertised by grocers, is flour in
which these ingredients or their equivalent have been mixed by the
manufacturer.

212. Soda Mints. Bicarbonate of soda is practically the sole
ingredient of the soda mints popularly sold for indigestion. These
correct a tendency to sour stomach because they counteract the surplus
acid in the stomach, and form with it a safe neutral substance.

Seidlitz powder is a simple remedy consisting of two powders, one
containing bicarbonate of soda, and the other, some acid such as cream
of tartar. When these substances are dissolved in water and mixed,
effervescence occurs, carbon dioxide escapes, and a solution of
Rochelle salt remains.

212_a_. Source of Soda. An enormous quantity of sodium carbonate, or
soda, as it is usually called, is needed in the manufacture of glass,
soap, bleaching powders, and other commercial products. Formerly, the
supply of soda was very limited because man was dependent upon natural
deposits and upon ashes of sea plants for it. Common salt, sodium
chloride, is abundant, and in 1775 a prize was offered to any one who
would find a way to obtain soda from salt. As a result of this, soda
was soon manufactured from common salt. In the most recent methods of
manufacture, salt, water, ammonia, and carbon dioxide are made to
react. Baking soda is formed from the reaction. The baking soda is
then heated and decomposed into washing soda or the soda of commerce.



CHAPTER XXI

FERMENTATION


213. While baking powder is universally used for biscuits and cake,
it is seldom, if ever, used for bread, because it does not furnish
sufficient gas to lighten the tough heavy mass of bread dough. Then,
too, most people prefer the taste of yeast-raised bread. There is a
reason for this widespread preference, but to understand it, we must
go somewhat far afield, and must study not only the bread of to-day,
but the bread of antiquity, and the wines as well.

If grapes are crushed, they yield a liquid which tastes like the
grapes; but if the liquid is allowed to stand in a warm place, it
loses its original character, and begins to ferment, becoming, in the
course of a few weeks, a strongly intoxicating drink. This is true not
only of grape juice but also of the juice of all other sweet fruits;
apple juice ferments to cider, currant juice to currant wine, etc.
This phenomenon of fermentation is known to practically all races of
men, and there is scarcely a savage tribe without some kind of
fermented drink; in the tropics the fermented juice of the palm tree
serves for wine; in the desert regions, the fermented juice of the
century plant; and in still other regions, the root of the ginger
plant is pressed into service.

The fermentation which occurs in bread making is similar to that which
is responsible for the transformation of plant juices into
intoxicating drinks. The former process is not so old, however, since
the use of alcoholic beverages dates back to the very dawn of history,
and the authentic record of raised or leavened bread is but little
more than 3000 years old.

214. The Bread of Antiquity. The original method of bread making and
the method employed by savage tribes of to-day is to mix crushed grain
and water until a paste is formed, and then to bake this over a camp
fire. The result is a hard compact substance known as unleavened
bread. A considerable improvement over this tasteless mass is
self-raised bread. If dough is left standing in a warm place a number
of hours, it swells up with gas and becomes porous, and when baked, is
less compact and hard than the savage bread. Exposure to air and
warmth brings about changes in dough as well as in fruit juices, and
alters the character of the dough and the bread made from it. Bread
made in this way would not seem palatable to civilized man of the
present day, accustomed, as he is, to delicious bread made light and
porous by yeast; but to the ancients, the least softening and
lightening was welcome, and self-fermented bread, therefore,
supplanted the original unleavened bread.

Soon it was discovered that a pinch of this fermented dough acted as a
starter on a fresh batch of dough. Hence, a little of the fermented
dough was carefully saved from a batch, and when the next bread was
made, the fermented dough, or leaven, was worked into the fresh dough
and served to raise the mass more quickly and effectively than mere
exposure to air and warmth could do in the same length of time. This
use of leaven for raising bread has been practiced for ages.

Grape juice mixed with millet ferments quickly and strongly, and the
Romans learned to use this mixture for bread raising, kneading a very
small amount of it through the dough.

215. The Cause of Fermentation. Although alcoholic fermentation, and
the fermentation which goes on in raising dough, were known and
utilized for many years, the cause of the phenomenon was a sealed book
until the nineteenth century. About that time it was discovered,
through the use of the microscope, that fermenting liquids contain an
army of minute plant organisms which not only live there, but which
actually grow and multiply within the liquid. For growth and
multiplication, food is necessary, and this the tiny plants get in
abundance from the fruit juices; they feed upon the sugary matter and
as they feed, they ferment it, changing it into carbon dioxide and
alcohol. The carbon dioxide, in the form of small bubbles, passes off
from the fermenting mass, while the alcohol remains in the liquid,
giving the stimulating effect desired by imbibers of alcoholic drinks.
The unknown strange organisms were called yeast, and they were the
starting point of the yeast cakes and yeast brews manufactured to-day
on a large scale, not only for bread making but for the commercial
production of beer, ale, porter, and other intoxicating drinks.

The grains, rye, corn, rice, wheat, from which meal is made, contain
only a small quantity of sugar, but, on the other hand, they contain a
large quantity of starch which is easily convertible into sugar. Upon
this the tiny yeast plants in the dough feed, and, as in the case of
the wines, ferment the sugar, producing carbon dioxide and alcohol.
The dough is thick and sticky and the gas bubbles expand it into a
spongy mass. The tiny yeast plants multiply and continue to make
alcohol and gas, and in consequence, the dough becomes lighter and
lighter. When it has risen sufficiently, it is kneaded and placed in
an oven; the heat of the oven soon kills the yeast plants and drives
the alcohol out of the bread; at the same time it expands the
imprisoned gas bubbles and causes them to lighten and swell the bread
still more. Meanwhile, the dough has become stiff enough to support
itself. The result of the fermentation is a light, spongy loaf.

216. Where does Yeast come From? The microscopic plants which we
call yeast are widely distributed in the air, and float around there
until chance brings them in contact with a substance favorable to
their growth, such as fruit juices and moist warm batter. Under the
favorable conditions of abundant moisture, heat, and food, they grow
and multiply rapidly, and cause the phenomenon of fermentation. Wild
yeast settles on the skin of grapes and apples, but since it does not
have access to the fruit juices within, it remains inactive very much
as a seed does before it is planted. But when the fruit is crushed,
the yeast plants get into the juice, and feeding on it, grow and
multiply. The stray yeast plants which get into the sirup are
relatively few, and hence fermentation is slow; it requires several
weeks for currant wine to ferment, and several months for the juice of
grapes to be converted into wine.

Stray yeast finds a favorable soil for growth in the warmth and
moisture of a batter; but although the number of these stray plants is
very large, it is insufficient to cause rapid fermentation, and if we
depended upon wild yeast for bread raising, the result would not be to
our liking.

When our remote ancestors saved a pinch of dough as leaven for the
next baking, they were actually cultivating yeast, although they did
not know it. The reserved portion served as a favorable breeding place
to the yeast plants within it; they grew and reproduced amazingly, and
became so numerous, that the small mass of old dough in which they
were gathered served to leaven the entire batch at the next baking.

As soon as man learned that yeast plants caused fermentation in
liquors and bread, he realized that it would be to his advantage to
cultivate yeast and to add it to bread and to plant juices rather than
to depend upon accidental and slow fermentation from wild yeast.
Shortly after the discovery of yeast in the nineteenth century, man
commenced his attempt to cultivate the tiny organisms. Their
microscopic size added greatly to his trouble, and it was only after
years of careful and tedious investigation that he was able to perfect
the commercial yeast cakes and yeast brews universally used by bakers
and brewers. The well-known compressed yeast cake is simply a mass of
live and vigorous yeast plants, embedded in a soft, soggy material,
and ready to grow and multiply as soon as they are placed under proper
conditions of heat, moisture, and food. Seeds which remain on our
shelves do not germinate, but those which are planted in the soil do;
so it is with the yeast plants. While in the cake they are as lifeless
as the seed; when placed in dough, or fruit juice, or grain water,
they grow and multiply and cause fermentation.



CHAPTER XXII

BLEACHING


217. The beauty and the commercial value of uncolored fabrics depend
upon the purity and perfection of their whiteness; a man's white
collar and a woman's white waist must be pure white, without the
slightest tinge of color. But all natural fabrics, whether they come
from plants, like cotton and linen, or from animals, like wool and
silk, contain more or less coloring matter, which impairs the
whiteness. This coloring not only detracts from the appearance of
fabrics which are to be worn uncolored, but it seriously interferes
with the action of dyes, and at times plays the dyer strange tricks.

Natural fibers, moreover, are difficult to spin and weave unless some
softening material such as wax or resin is rubbed lightly over them.
The matter added to facilitate spinning and weaving generally detracts
from the appearance of the uncolored fabric, and also interferes with
successful dyeing. Thus it is easy to see that the natural coloring
matter and the added foreign matter must be entirely removed from
fabrics destined for commercial use. Exceptions to this general fact
are sometimes made, because unbleached material is cheaper and more
durable than the bleached product, and for some purposes is entirely
satisfactory; unbleached cheesecloth and sheeting are frequently
purchased in place of the more expensive bleached material. Formerly,
the only bleaching agent known was the sun's rays, and linen and
cotton were put out to sun for a week; that is, the unbleached
fabrics were spread on the grass and exposed to the bleaching action
of sun and dew.

[Illustration: FIG. 158.--Preparing chlorine from hydrochloric acid
and manganese dioxide.]

218. An Artificial Bleaching Agent. While the sun's rays are
effective as a bleaching agent, the process is slow; moreover, it
would be impossible to expose to the sun's rays the vast quantity of
fabrics used in the civilized world of to-day, and the huge and
numerous bolts of material which daily come from our looms and
factories must therefore be whitened by artificial means. The
substance almost universally used as a rapid artificial bleaching
agent is chlorine, best known to us as a constituent of common salt.
Chlorine is never free in nature, but is found in combination with
other substances, as, for example, in combination with sodium in salt,
or with hydrogen in hydrochloric acid.

The best laboratory method of securing free chlorine is to heat in a
water bath a mixture of hydrochloric acid and manganese dioxide, a
compound containing one part of manganese and two parts of oxygen. The
heat causes the manganese dioxide to give up its oxygen, which
immediately combines with the hydrogen of the hydrochloric acid and
forms water. The manganese itself combines with part of the chlorine
originally in the acid, but not with all. There is thus some free
chlorine left over from the acid, and this passes off as a gas and can
be collected, as in Figure 158. Free chlorine is heavier than air, and
hence when it leaves the exit tube it settles at the bottom of the
jar, displacing the air, and finally filling the bottle.

Chlorine is a very active substance and combines readily with most
substances, but especially with hydrogen; if chlorine comes in contact
with steam, it abstracts the hydrogen and unites with it to form
hydrochloric acid, but it leaves the oxygen free and uncombined. This
tendency of chlorine to combine with hydrogen makes it valuable as a
bleaching agent. In order to test the efficiency of chlorine as a
bleaching agent, drop a wet piece of colored gingham or calico into
the bottle of chlorine, and notice the rapid disappearance of color
from the sample. If unbleached muslin is used, the moist strip loses
its natural yellowish hue and becomes a clear, pure white. The
explanation of the bleaching power of chlorine is that the chlorine
combines with the hydrogen of the water and sets oxygen free; the
uncombined free oxygen oxidizes the coloring matter in the cloth and
destroys it.

Chlorine has no effect on dry material, as may be seen if we put dry
gingham into the jar; in this case there is no water to furnish
hydrogen for combination with the chlorine, and no oxygen to be set
free.

219. Bleaching Powder. Chlorine gas has a very injurious effect on
the human body, and hence cannot be used directly as a bleaching
agent. It attacks the mucous membrane of the nose and lungs, and
produces the effect of a severe cold or catarrh, and when inhaled,
causes death. But certain compounds of chlorine are harmless, and can
be used instead of chlorine for destroying either natural or
artificial dyes. One of these compounds, namely, chloride of lime, is
the almost universal bleaching agent of commerce. It comes in the form
of powder, which can be dissolved in water to form the bleaching
solution in which the colored fabrics are immersed. But fabrics
immersed in a bleaching powder solution do not lose their color as
would naturally be expected. The reason for this is that the chlorine
gas is not free to do its work, but is restricted by its combination
with the other substances. By experiment it has been found that the
addition to the bleaching solution of an acid, such as vinegar or
lemon juice or sulphuric acid, causes the liberation of the chlorine.
The chlorine thus set free reacts with the water and liberates oxygen;
this in turn destroys the coloring matter in the fibers, and
transforms the material into a bleached product.

The acid used to liberate the chlorine from the bleaching powder, and
the chlorine also, rot materials with which they remain in contact for
any length of time. For this reason, fabrics should be removed from
the bleaching solution as soon as possible, and should then be rinsed
in some solution, such as ammonia, which is capable of neutralizing
the harmful substances; finally the fabric should be thoroughly rinsed
in water in order that all foreign matter may be removed. The reason
home bleaching is so seldom satisfactory is that most amateurs fail to
realize the necessity of immediate neutralization and rinsing, and
allow the fabric to remain too long in the bleaching solution, and
allow it to dry with traces of the bleaching substances present in the
fibers. Material treated in this way is thoroughly bleached, but is at
the same time rotten and worthless. Chloride of lime is frequently
used in laundry work; the clothes are whiter than when cleaned with
soap and simple washing powders, but they soon wear out unless the
precaution has been taken to add an "antichlor" or neutralizer to the
bleaching solution.

220. Commercial Bleaching. In commercial bleaching the material to
be bleached is first moistened with a very weak solution of sulphuric
acid or hydrochloric acid, and is then immersed in the bleaching
powder solution. As the moist material is drawn through the bleaching
solution, the acid on the fabric acts upon the solution and releases
chlorine. The chlorine liberates oxygen from the water. The oxygen in
turn attacks the coloring matter and destroys it.

[Illustration: FIG. 159.--The material to be bleached is drawn through
an acid _a_, then through a bleaching solution _b_, and finally
through a neutralizing solution _c_.]

The bleached material is then immersed in a neutralizing bath and is
finally rinsed thoroughly in water. Strips of cotton or linen many
miles long are drawn by machinery into and out of the various
solutions (Fig. 159), are then passed over pressing rollers, and
emerge snow white, ready to be dyed or to be used as white fabric.

221. Wool and Silk Bleaching. Animal fibers like silk, wool, and
feathers, and some vegetable fibers like straw, cannot be bleached by
means of chlorine, because it attacks not only the coloring matter but
the fiber itself, and leaves it shrunken and inferior. Cotton and
linen fibers, apart from the small amount of coloring matter present
in them, contain nothing but carbon, oxygen, and hydrogen, while
animal fibers contain in addition to these elements some compounds of
nitrogen. The presence of these nitrogen compounds influences the
action of the chlorine and produces unsatisfactory results. For animal
fibers it is therefore necessary to discard chlorine as a bleaching
agent, and to substitute a substance which will have a less disastrous
action upon the fibers. Such a substance is to be had in sulphurous
acid. When sulphur burns, as in a match, it gives off disagreeable
fumes, and if these are made to bubble into a vessel containing water,
they dissolve and form with the water a substance known as sulphurous
acid. That this solution has bleaching properties is shown by the fact
that a colored cloth dipped into it loses its color, and unbleached
fabrics immersed in it are whitened. The harmless nature of sulphurous
acid makes it very desirable as a bleaching agent, especially in the
home.

Silk, lace, and wool when bleached with chlorine become hard and
brittle, but when whitened with sulphurous acid, they retain their
natural characteristics.

This mild form of a bleaching substance has been put to uses which are
now prohibited by the pure food laws. In some canneries common corn is
whitened with sulphurous acid, and is then sold under false
representations. Cherries are sometimes bleached and then colored with
the bright shades which under natural conditions indicate freshness.

Bleaching with chlorine is permanent, the dyestuff being destroyed by
the chlorine; but bleaching with sulphurous acid is temporary, because
the milder bleach does not actually destroy the dyestuff, but merely
modifies it, and in time the natural yellow color of straw, cotton,
and linen reappears. The yellowing of straw hats during the summer is
familiar to everyone; the straw is merely resuming its natural color
which had been modified by the sulphurous acid solution applied to the
straw when woven.

222. Why the Color Returns. Some of the compounds formed by the
sulphurous acid bleaching process are gradually decomposed by
sunlight, and in consequence the original color is in time partially
restored. The portion of a hat protected by the band retains its
fresh appearance because the light has not had access to it. Silks and
other fine fabrics bleached in this way fade with age, and assume an
unnatural color. One reason for this is that the dye used to color the
fabric requires a clear white background, and loses its characteristic
hues when its foundation is yellow instead of white. Then, too,
dyestuffs are themselves more or less affected by light, and fade
slowly under a strong illumination.

Materials which are not exposed directly to an intense and prolonged
illumination retain their whiteness for a long time, and hence dress
materials and hats which have been bleached with sulphurous acid
should be protected from the sun's glare when not in use.

223. The Removal of Stains. Bleaching powder is very useful in the
removal of stains from white fabrics. Ink spots rubbed with lemon
juice and dipped in bleaching solution fade away and leave on the
cloth no trace of discoloration. Sometimes these stains can be removed
by soaking in milk, and where this is possible, it is the better
method.

Bleaching solution, however, while valuable in the removal of some
stains, is unable to remove paint stains, because paints owe their
color to mineral matter, and on this chlorine is powerless to act.
Paint stains are best removed by the application of gasoline followed
by soap and water.



CHAPTER XXIII

DYEING


224. Dyes. One of the most important and lucrative industrial
processes of the world to-day is that of staining and dyeing. Whether
we consider the innumerable shades of leather used in shoes and
harnesses and upholstery; the multitude of colors in the paper which
covers our walls and reflects light ranging from the somber to the
gay, and from the delicate to the gorgeous; the artificial scenery
which adorns the stage and by its imitation of trees and flowers and
sky translates us to the Forest of Arden; or whether we consider the
uncounted varieties of color in dress materials, in carpets, and in
hangings, we are dealing with substances which owe their beauty to
dyes and dyestuffs.

The coloring of textile fabrics, such as cotton, wool, and silk, far
outranks in amount and importance that of leather, paper, etc., and
hence the former only will be considered here; but the theories and
facts relative to textile dyeing are applicable in a general way to
all other forms as well.

225. Plants as a Source of Dyes. Among the most beautiful examples
of man's handiwork are the baskets and blankets of the North American
Indians, woven with a skill which cannot be equaled by manufacturers,
and dyed in mellow colors with a few simple dyes extracted from local
plants. The magnificent rugs and tapestries of Persia and Turkey, and
the silks of India and Japan, give evidence that a knowledge of dyes
is widespread and ancient. Until recently, the vegetable world was
the source of practically all coloring matter, the pulverized root of
the madder plant yielding the reds, the leaves and stems of the indigo
plant the blues, the heartwood of the tropical logwood tree the blacks
and grays, and the fruit of certain palm and locust trees yielding the
soft browns. So great was the commercial demand for dyestuffs that
large areas of land were given over to the exclusive cultivation of
the more important dye plants. Vegetable dyes are now, however, rarely
used because about the year 1856 it was discovered that dyes could be
obtained from coal tar, the thick sticky liquid formed as a by-product
in the manufacture of coal gas. These artificial coal-tar, or aniline,
dyes have practically undisputed sway to-day, and the vast areas of
land formerly used for the cultivation of vegetable dyes are now free
for other purposes.

226. Wool and Cotton Dyeing. If a piece of wool is soaked in a
solution of a coal-tar dye, such as magenta, the fiber of the cloth
draws some of the dye out of the solution and absorbs it, becoming in
consequence beautifully colored. The coloring matter becomes "part and
parcel," as it were, of the wool fiber, because repeated washing of
the fabric fails to remove the newly acquired color; the magenta
coloring matter unites chemically with the fiber of the wool, and
forms with it a compound insoluble in water, and hence fast to
washing.

But if cotton is used instead of wool, the acquired color is very
faint, and washes off readily. This is because cotton fibers possess
no chemical substance capable of uniting with the coloring matter to
form a compound insoluble in water.

If magenta is replaced by other artificial dyes,--for example,
scarlets,--the result is similar; in general, wool material absorbs
dye readily, and uniting with it is permanently dyed. Cotton material,
on the other hand, does not combine chemically with coloring matter
and therefore is only faintly tinged with color, and loses this when
washed. When silk and linen are tested, it is found that the former
behaves in a general way as did wool, while the linen has more
similarity to the cotton. That vegetable fibers, such as cotton and
linen, should act differently toward coloring matter from animal
fibers, such as silk and wool, is not surprising when we consider that
the chemical nature of the two groups is very different; vegetable
fibers contain only oxygen, carbon, and hydrogen, while animal fibers
always contain nitrogen in addition, and in many cases sulphur as
well.

227. The Selection of Dyes. When silk and wool, cotton and linen,
are tested in various dye solutions, it is found that the former have,
in general, a great affinity for coloring matter and acquire a
permanent color, but that cotton and linen, on the other hand, have
little affinity for dyestuffs. The color acquired by vegetable fibers
is, therefore, usually faint.

There are, of course, many exceptions to the general statement that
animal fibers dye readily and vegetable fibers poorly, because certain
dyes fail utterly with woolen and silk material and yet are fairly
satisfactory when applied to cotton and linen fabrics. Then, too, a
dye which will color silk may not have any effect on wool in spite of
the fact that wool, like silk, is an animal fiber; and certain
dyestuffs to which cotton responds most beautifully are absolutely
without effect on linen.

The nature of the material to be dyed determines the coloring matter
to be used; in dyeing establishments a careful examination is made of
all textiles received for dyeing, and the particular dyestuffs are
then applied which long experience has shown to be best suited to the
material in question. Where "mixed goods," such as silk and wool, or
cotton and wool, are concerned, the problem is a difficult one, and
the countless varieties of gorgeously colored mixed materials give
evidence of high perfection in the art of dyeing and weaving.

Housewives who wish to do successful home dyeing should therefore not
purchase dyes indiscriminately, but should select the kind best suited
to the material, because the coloring principle which will remake a
silk waist may utterly ruin a woolen skirt or a linen suit. Powders
designed for special purposes may be purchased from druggists.

228. Indirect Dyeing. We have seen that it is practically impossible
to color cotton and linen in a simple manner with any degree of
permanency, because of the lack of chemical action between vegetable
fibers and coloring matter. But the varied uses to which dyed articles
are put make fastness of color absolutely necessary. A shirt, for
example, must not be discolored by perspiration, nor a waist faded by
washing, nor a carpet dulled by sweeping with a dampened broom. In
order to insure permanency of dyes, an indirect method was originated
which consisted of adding to the fibers a chemical capable of acting
upon the dye and forming with it a colored compound insoluble in
water, and hence "safe." For example, cotton material dyed directly in
logwood solution has almost no value, but if it is soaked in a
solution of oxalic acid and alum until it becomes saturated with the
chemicals, and is then transferred to a logwood bath, the color
acquired is fast and beautiful.

This method of indirect dyeing is known as the mordanting process; it
consists of saturating the fabric to be dyed with chemicals which will
unite with the coloring matter to form compounds unaffected by water.
The chemicals are called mordants.

229. How Variety of Color is Secured. The color which is fixed on
the fabric as a result of chemical action between mordant and dye is
frequently very different from that of the dye itself. Logwood dye
when used alone produces a reddish brown color of no value either for
beauty or permanence; but if the fabric to be dyed is first mordanted
with a solution of alum and oxalic acid and is then immersed in a
logwood bath, it acquires a beautiful blue color.

Moreover, since the color acquired depends upon the mordant as well as
upon the dye, it is often possible to obtain a wide range of colors by
varying the mordant used, the dye remaining the same. For example,
with alum and oxalic acid as a mordant and logwood as a dye, blue is
obtained; but with a mordant of ferric sulphate and a dye of logwood,
blacks and grays result. Fabrics immersed directly in alizarin acquire
a reddish yellow tint; when, however, they are mordanted with certain
aluminium compounds they acquire a brilliant Turkey red, when
mordanted with chromium compounds, a maroon, and when mordanted with
iron compounds, the various shades of purple, lilac, and violet
result.

230. Color Designs in Cloth. It is thought that the earliest
attempts at making "fancy materials" consisted in painting designs on
a fabric by means of a brush. In more recent times the design was cut
in relief on hard wood, the relief being then daubed with coloring
matter and applied by hand to successive portions of the cloth. The
most modern method of design-making is that of machine or roller
printing. In this, the relief blocks are replaced by engraved copper
rolls which rotate continuously and in the course of their rotation
automatically receive coloring matter on the engraved portion. The
cloth is to be printed is then drawn uniformly over the rotating roll,
receiving color from the engraved design; in this way, the color
pattern is automatically printed on the cloth with perfect regularity.
In cases where the fabrics do not unite directly with the coloring
matter, the design is supplied with a mordant and the impression made
on the fabric is that of the mordant; when the fabric is later
transferred to a dye bath, the mordanted portions, represented by the
design, unite with the coloring matter and thus form the desired color
patterns.

Unless the printing is well done, the coloring matter does not
thoroughly penetrate the material, and only a faint blurred design
appears on the back of the cloth; the gaudy designs of cheap calicoes
and ginghams often do not show at all on the under side. Such
carelessly made prints are not fast to washing or light, and soon
fade. But in the better grades of material the printing is well done,
and the color designs are fairly fast, and a little care in the
laundry suffices to eliminate any danger of fading.

Color designs of the greatest durability are produced by the weaving
together of colored yarns. When yarn is dyed, the coloring matter
penetrates to every part of the fiber, and hence the patterns formed
by the weaving together of well-dyed yarns are very fast to light and
water.

If the color designs to be woven in the cloth are intricate, complex
machinery is necessary and skillful handwork; hence, patterns formed
by the weaving of colored yarns are expensive and less common than
printed fabrics.



CHAPTER XXIV

CHEMICALS AS DISINFECTANTS AND PRESERVATIVES


231. The prevention of disease epidemics is one of the most striking
achievements of modern science. Food, clothing, furniture, and other
objects contaminated in any way by disease germs may be disinfected by
chemicals or by heat, and widespread infection from persons suffering
with a contagious disease may be prevented.

[Illustration: FIG. 160.--Pasteurizing apparatus, an arrangement by
which milk is conveniently heated to destroy disease germs.]

When disease germs are within the body, the problem is far from
simple, because chemicals which would effectively destroy the germs
would be fatal to life itself. But when germs are outside the body, as
in water or milk, or on clothing, dishes, or furniture, they can be
easily killed. One of the best methods of destroying germs is to
subject them to intense heat. Contaminated water is made safe by
boiling for a few minutes, because the strong heat destroys the
disease-producing germs. Scalded or Pasteurized milk saves the lives
of scores of babies, because the germs of summer complaint which lurk
in poor milk are killed and rendered harmless in the process of
scalding. Dishes used by consumptives, and persons suffering from
contagious diseases, can be made harmless by thorough washing in thick
suds of almost boiling water.

The bedding and clothing of persons suffering with diphtheria,
tuberculosis, and other germ diseases should always be boiled and hung
to dry in the bright sunlight. Heat and sunshine are two of the best
disinfectants.

232. Chemicals. Objects, such as furniture, which cannot be boiled,
are disinfected by the use of any one of several chemicals, such as
sulphur, carbolic acid, chloride of lime, corrosive sublimate, etc.

One of the simplest methods of disinfecting consists in burning
sulphur in a room whose doors, windows, and keyholes have been closed,
so that the burning fumes cannot escape, but remain in the room long
enough to destroy disease germs. This is probably the most common
means of fumigation.

For general purposes, carbolic acid is one of the very best
disinfectants, but must be used with caution, as it is a deadly poison
except when very dilute.

Chloride of lime when exposed to the air and moisture slowly gives off
chlorine, and can be used as a disinfectant because the gas thus set
free attacks germs and destroys them. For this reason chloride of lime
is an excellent disinfectant of drainpipes. Certain bowel troubles,
such as diarrhoea, are due to microbes, and if the waste matter of a
person suffering from this or similar diseases is allowed passage
through the drainage system, much damage may be done. But a small
amount of chloride of lime in the closet bowl will insure
disinfection.

233. Personal Disinfection. The hands may gather germs from any
substances or objects with which they come in contact; hence the hands
should be washed with soap and water, and especially before eating.
Physicians who perform operations wash not only their hands, but their
instruments, sterilizing the latter by placing them in boiling water
for several minutes.

Cuts and wounds allow easy access to the body; a small cut has been
known to cause death because of the bacteria which found their way
into the open wound and produced disease. In order to destroy any
germs which may have entered into the cut from the instrument, it is
well to wash out the wound with some mild disinfectant, such as very
dilute carbolic acid or hydrogen peroxide, and then to bind the wound
with a clean cloth, to prevent later entrance of germs.

234. Chemicals as Food Preservatives. The spoiling of meats and
soups, and the souring of milk and preserves, are due to germs which,
like those producing disease, can be destroyed by heat and by
chemicals.

Milk heated to the boiling point does not sour readily, and successful
canning consists in cooking fruits and vegetables until all the germs
are killed, and then sealing the cans so that germs from outside
cannot find entrance and undo the work of the canner.

Some dealers and manufacturers have learned that certain chemicals
will act as food preservatives, and hence they have replaced the safe
method of careful canning by the quicker and simpler plan of adding
chemicals to food. Catchup, sauces, and jellies are now frequently
preserved in this way. But the chemicals which destroy bacteria
frequently injure the consumer as well. And so much harm has been done
by food preservatives that the pure food laws require that cans and
bottles contain a labeled statement of the kind and quantity of
chemicals used.

Even milk is not exempt, but is doctored to prevent souring, the
preservative most generally used by milk dealers being formaldehyde.
The vast quantity of milk consumed by young and old, sick and well,
makes the use of formaldehyde a serious menace to health, because no
constitution can endure the injury done by the constant use of
preservatives.

The most popular and widely used preservatives of meats are borax and
boric acid. These chemicals not only arrest decay, but partially
restore to old and bad meat the appearance of freshness; in this way
unscrupulous dealers are able to sell to the public in one form or
other meats which may have undergone partial decomposition; sausage
frequently contains partially decomposed meat, restored as it were by
chemicals.

In jams and catchups there is abundant opportunity for preservatives;
badly or partially decayed fruits are sometimes disinfected and used
as the basis of foods sold by so-called good dealers. Benzoate of
soda, and salicylic acid are the chemicals most widely employed for
this purpose, with coal-tar dyes to simulate the natural color of the
fruit.

Many of the cheap candies sold by street venders are not fit for
consumption, since they are not only made of bad material, but are
frequently in addition given a light dipping in varnish as a
protection against the decaying influences of the atmosphere.

The only wise preservatives are those long known and employed by our
ancestors; salt, vinegar, and spices are all food preservatives, but
they are at the same time substances which in small amounts are not
injurious to the body. Smoked herring and salted mackerel are
chemically preserved foods, but they are none the less safe and
digestible.

235. The Preservation of Wood and Metal. The decaying of wood and
the rusting of metal are due to the action of air and moisture. When
wood and metal are surrounded with a covering which neither air nor
moisture can penetrate, decay and rust are prevented. Paint affords
such a protective covering. The main constituent of paint is a
compound of white lead or other metallic substance; this is mixed with
linseed oil or its equivalent in order that it may be spread over wood
and metal in a thin, even coating. After the mixture has been applied,
it hardens and forms a tough skin fairly impervious to weathering. For
the sake of ornamentation, various colored pigments are added to the
paint and give variety of effect.

Railroad ties and street paving blocks are ordinarily protected by oil
rather than paint. Wood is soaked in creosote oil until it becomes
thoroughly saturated with the oily substance. The pores of the wood
are thus closed to the entrance of air and moisture, and decay is
avoided. Wood treated in this way is very durable. Creosote is
poisonous to insects and many small animals, and thus acts as a
preservation not only against the elements but against animal life as
well.



CHAPTER XXV

DRUGS AND PATENT MEDICINES


236. Stimulants and Narcotics. Man has learned not only the action
of substances upon each other, such as bleaching solution upon
coloring matter, washing soda upon grease, acids upon bases, but also
the effect which certain chemicals have upon the human body.

Drugs and their varying effects upon the human system have been known
to mankind from remote ages; in the early days, familiar leaves,
roots, and twigs were steeped in water to form medicines which served
for the treatment of all ailments. In more recent times, however,
these simple herb teas have been supplanted by complex drugs, and now
medicines are compounded not only from innumerable plant products, but
from animal and mineral matter as well. Quinine, rhubarb, and arnica
are examples of purely vegetable products; iron, mercury, and arsenic
are equally well known as distinctly mineral products, while cod-liver
oil is the most familiar illustration of an animal remedy. Ordinarily
a combination of products best serves the ends of the physician.

Substances which, like cod-liver oil, serve as food to a worn-out
body, or, like iron, tend to enrich the blood, or, like quinine, aid
in bringing an abnormal system to a healthy condition, are valuable
servants and cannot be entirely dispensed with so long as man is
subject to disease.

But substances which, like opium, laudanum, and alcohol, are not
required by the body as food, or as a systematic, intelligent aid to
recovery, but are taken solely for the stimulus aroused or for the
insensibility induced, are harmful to man, and cannot be indulged in
by him without ultimate mental, moral, and physical loss. Substances
of the latter class are known as narcotics and stimulants.

237. The Cost of Health. In the physical as in the financial world,
nothing is to be had without a price. Vigor, endurance, and mental
alertness are bought by hygienic living; that is, by proper food,
fresh air, exercise, cleanliness, and reasonable hours. Some people
wish vigor, endurance, etc., but are unwilling to live the life which
will develop these qualities. Plenty of sleep, exercise, and simple
food all tend to lay the foundations of health. Many, however, are not
willing to take the care necessary for healthful living, because it
would force them to sacrifice some of the hours of pleasure. Sooner or
later, these pleasure-seekers begin to feel tired and worn, and some
of them turn to drugs and narcotics for artificial strength. At first
the drugs seem to restore the lost energy, and without harm; however,
the cost soon proves to be one of the highest Nature ever demands.

238. The Uncounted Cost. The first and most obvious effect of opium,
for example, is to deaden pain and to arouse pleasure; but while the
drug is producing these soothing sensations, it interferes with bodily
functions. Secretion, digestion, absorption of food, and the removal
of waste matters are hindered. Continued use of the drug leads to
headache, exhaustion, nervous depression, and heart weakness. There is
thus a heavy toll reckoned against the user, and the creditor is
relentless in demanding payment.

Moreover, the respite allowed by a narcotic is exceedingly brief, and
a depression which is long and deep inevitably follows. In order to
overcome this depression, recourse is usually had to a further dose,
and as time goes on, the intervals of depression become more frequent
and lasting, and the necessity to overcome them increases. Thus
without intention one finds one's self bound to the drug, its fast
victim. The sanatoria of our country are crowded with people who are
trying to free themselves of a drug habit into which they have drifted
unintentionally if not altogether unknowingly. What is true of opium
is equally applicable to other narcotics.

239. The Right Use of Narcotics. In the hands of the physician,
narcotics are a great blessing. In some cases, by relieving pain, they
give the system the rest necessary for overcoming the cause of the
pain. Only those who know of the suffering endured in former times can
fully appreciate the decrease in pain brought about by the proper use
of narcotics.

240. Patent Medicines, Cough Sirups. A reputable physician is
solicitous regarding the permanent welfare of his patient and
administers carefully chosen and harmless drugs. Mere medicine
venders, however, ignore the good of mankind, and flood the market
with cheap patent preparations which delude and injure those who
purchase, but bring millions of dollars to those who manufacture.

Practically all of these patent, or proprietary, preparations contain
a large proportion of narcotics or stimulants, and hence the benefit
which they seem to afford the user is by no means genuine; examination
shows that the relief brought by them is due either to a temporary
deadening of sensibilities by narcotics or to a fleeting stimulation
by alcohol and kindred substances.

Among the most common ailments of both young and old are coughs and
colds; hence many patent cough mixtures have been manufactured and
placed on the market for the consumption of a credulous public. Such
"quick cures" almost invariably contain one or more narcotic drugs,
and not only do not relieve the cold permanently, but occasion
subsequent disorders. Even lozenges and pastilles are not free from
fraud, but have a goodly proportion of narcotics, containing in some
cases chloroform, morphine, and ether.

The widespread use of patent cough medicines is due largely to the
fact that many persons avoid consulting a physician about so trivial
an ailment as an ordinary cold, or are reluctant to pay a medical fee
for what seems a slight indisposition and hence attempt to doctor
themselves.

Catarrh is a very prevalent disease in America, and consequently
numerous catarrh remedies have been devised, most of which contain in
a disguised form the pernicious drug, cocaine. Laws have been enacted
which require on the labels a declaration of the contents of the
preparation, both as to the kind of drug used and the amount, and the
choice of accepting or refusing such mixtures is left to the
individual. But the great mass of people are ignorant of the harmful
nature of drugs in general, and hence do not even read the
self-accusing label, or if they do glance at it, fail to comprehend
the dangerous nature of the drugs specified there. In order to
safeguard the uninformed purchaser and to restrict the manufacture of
harmful patent remedies, some states limit the sale of all
preparations containing narcotics and thus give free rein to neither
consumer nor producer.

241. Soothing Sirups; Soft Drinks. The development of a race is
limited by the mental and physical growth of its children, and yet
thousands of its children are annually stunted and weakened by drugs,
because most colic cures, teething concoctions, and soothing syrups
are merely agreeably flavored drug mixtures. Those who have used such
preparations freely, know that a child usually becomes fretful and
irritable between doses, and can be quieted only by larger and more
frequent supplies. A habit formed in this way is difficult to
overcome, and many a child when scarcely over its babyhood had a
craving which in later years may lead to systematic drug taking. And
even though the pernicious drug craving is not created, considerable
harm is done to the child, because its body is left weak and
non-resistant to diseases of infancy and childhood.

Many of our soft drinks contain narcotics. The use of the coca leaf
and the kola nut for such preparations has increased very greatly
within the last few years, and doubtless legislation will soon be
instituted against the indiscriminate sale of soft drinks.

242. Headache Powders. The stress and strain of modern life has
opened wide the door to a multitude of bodily ills, among which may be
mentioned headache. Work must be done and business attended to, and
the average sufferer does not take time from his vocation to
investigate the cause of the headache, but unthinkingly grasps at any
remedy which will remove the immediate pain, and utterly disregards
later injury. The relief afforded by most headache mixtures is due to
the presence of antipyrin or acetanilid, and it has been shown
conclusively that these drugs weaken heart action, diminish
circulation, reduce the number of red corpuscles in the blood, and
bring on a condition of chronic anemia. Pallid cheeks and blue lips
are visible evidence of the too frequent use of headache powders.

The labels required by law are often deceptive and convey no adequate
idea of the amount of drug consumed; for example, 240 grains of
acetanilid to an ounce seems a small quantity of drug for a powder,
but when one considers that there are only 480 grains in an ounce, it
will be seen that each powder is one half acetanilid.

Powders taken in small quantities and at rare intervals are apparently
harmless; but they never remove the cause of the trouble, and hence
the discomfort soon returns with renewed force. Ordinarily, hygienic
living will eliminate the source of the trouble, but if it does not, a
physician should be consulted and medicine should be procured from him
which will restore the deranged system to its normal healthy
condition.

243. Other Deceptions. Nearly all patent medicines contain some
alcohol, and in many, the quantity of alcohol is far in excess of that
found in the strongest wines. Tonics and bitters advertised as a cure
for spring fever and a worn-out system are scarcely more than cheap
cocktails, as one writer has derisively called them, and the amount of
alcohol in some widely advertised patent remedies is alarmingly large
and almost equal to that of strong whisky.

[Illustration: FIG. 161.--Diagram showing the amount of alcohol in
some alcoholic drinks and in one much used patent medicine.]

Some conscientious persons who would not touch beer, wine, whisky, or
any other intoxicating drink consume patent remedies containing large
quantities of alcohol and thus unintentionally expose themselves to
mental and physical danger. In all cases of bodily disorder, the only
safe course is to consult a physician who has devoted himself to the
study of the body and the methods by which a disordered system may be
restored to health.



CHAPTER XXVI

NITROGEN AND ITS RELATION TO PLANTS


244. Nitrogen. A substance which plays an important part in animal
and plant life is nitrogen. Soil and the fertilizers which enrich it,
the plants which grow on it, and the animals which feed on these, all
contain nitrogen or nitrogenous compounds. The atmosphere, which we
ordinarily think of as a storehouse of oxygen, contains far more
nitrogen than oxygen, since four fifths of its whole weight is made up
of this element.

Nitrogen is colorless, odorless, and tasteless. Air is composed
chiefly of oxygen and nitrogen; if, therefore, the oxygen in a vessel
filled with air can be made to unite with some other substance or can
be removed, there will be a residue of nitrogen. This can be done by
floating on water a light dish containing phosphorus, then igniting
the phosphorus, and placing an inverted jar over the burning
substance. The phosphorus in burning unites with the oxygen of the air
and hence the gas that remains in the jar is chiefly nitrogen. It has
the characteristics mentioned above and, in addition, does not combine
readily with other substances.

245. Plant Food. Food is the course of energy in every living thing
and is essential to both animal and plant life. Plants get their food
from the lifeless matter which exists in the air and in the soil;
while animals get their food from plants. It is true that man and many
other animals eat fleshy foods and depend upon them for partial
sustenance, but the ultimate source of all animal food is plant life,
since meat-producing animals live upon plant growth.

Plants get their food from the air, the soil, and moisture. From the
air, the leaves take carbon dioxide and water and transform them into
starchy food; from the soil, the roots take water rich in mineral
matters dissolved from the soil. From the substances thus gathered,
the plant lives and builds up its structure.

A food substance necessary to plant life and growth is nitrogen. Since
a vast store of nitrogen exists in the air, it would seem that plants
should never lack for this food, but most plants are unable to make
use of the boundless store of atmospheric nitrogen, because they do
not possess the power of abstracting nitrogen from the air. For this
reason, they have to depend solely upon nitrogenous compounds which
are present in the soil and are soluble in water. The soluble
nitrogenous soil compounds are absorbed by roots and are utilized by
plants for food.

246. The Poverty of the Soil. Plant roots are constantly taking
nitrogen and its compounds from the soil. If crops which grow from the
soil are removed year after year, the soil becomes poorer in nitrogen,
and finally possesses too little of it to support vigorous and healthy
plant life. The nitrogen of the soil can be restored if we add to it a
fertilizer containing nitrogen compounds which are soluble in water.
Decayed vegetable matter contains large quantities of nitrogen
compounds, and hence if decayed vegetation is placed upon soil or is
plowed into soil, it acts as a fertilizer, returning to the soil what
was taken from it. Since man and all other animals subsist upon
plants, their bodies likewise contain nitrogenous substances, and
hence manure and waste animal matter is valuable as a fertilizer or
soil restorer.

247. Bacteria as Nitrogen Gatherers. Soil from which crops are removed
year after year usually becomes less fertile, but the soil from which
crops of clover, peas, beans, or alfalfa have been removed is richer in
nitrogen rather than poorer. This is because the roots of these plants
often have on them tiny swellings, or tubercles, in which millions of
certain bacteria live and multiply. These bacteria have the remarkable
power of taking free nitrogen from the air in the soil and of combining
it with other substances to form compounds which plants can use. The
bacteria-made compounds dissolve in the soil water and are absorbed into
the plant by the roots. So much nitrogen-containing material is made by
the root bacteria of plants of the pea family that the soil in which
they grow becomes somewhat richer in nitrogen, and if plants which
cannot make nitrogen are subsequently planted in such a soil, they find
there a store of nitrogen. A crop of peas, beans, or clover is
equivalent to nitrogenous fertilizer and helps to make ready the soil
for other crops.

[Illustration: FIG. 162.--Roots of soy bean having tubercle-bearing
bacteria.]

248. Artificial Fertilizers. Plants need other foods besides
nitrogen, and they exhaust the soil not only of nitrogen, but also of
phosphorus and potash, since large quantities of these are necessary
for plant life. There are many other substances absorbed from the soil
by the plant, namely, iron, sodium, calcium, magnesium, but these are
used in smaller quantities and the supply in the soil does not readily
become exhausted.

Commercial fertilizers generally contain nitrogen, phosphorus, and
potash in amounts varying with the requirements of the soil. Wheat
requires a large amount of phosphorus and quickly exhausts the ground
of that food stuff; a field which has supported a crop of wheat is
particularly poor in phosphorus, and a satisfactory fertilizer for
that land would necessarily contain a large percentage of phosphorus.
The fertilizer to be used in a soil depends upon the character of the
soil and upon the crops previously grown on it.

[Illustration: FIG. 163.--Water cultures of buckwheat: 1, with all the
food elements; 2, without potash; 3, without nitrates.]

The quantity of fertilizer needed by the farmers of the world is
enormous, and the problem of securing the necessary substances in
quantities sufficient to satisfy the demand bids fair to be serious.
But modern chemistry is at work on the problem, and already it is
possible to make some nitrogen compounds on a commercial scale. When
nitrogen gas is in contact with heated calcium carbide, a reaction
takes place which results in the formation of calcium nitride, a
compound suitable for enriching the soil. There are other commercial
methods for obtaining nitrogen compounds which are suitable for
absorption by plant roots.

Phosphorus is obtained from bone ash and from phosphate rock which is
widely distributed over the surface of the earth. Bone ash and
thousands of tons of phosphate rock are treated with sulphuric acid to
form a phosphorus compound which is soluble in soil water and which,
when added to soil, will be usable by the plants growing there.

The other important ingredient of most fertilizers is potash. Wood
ashes are rich in potash and are a valuable addition to the soil. But
the amount of potash thus obtained is far too limited to supply the
needs of agriculture; and to-day the main sources of potash are the
vast deposits of potassium salts found in Prussia.

Although Germany now furnishes the American farmer with the bulk of
his potash, she may not do so much longer. In 1911 an indirect potash
tax was levied by Germany on her best customer, the United States, to
whom 15 million dollars' worth of potash had been sold the preceding
year. This led Americans to inquire whether potash could not be
obtained at home.

Geologists say that long ages ago Germany was submerged, that the
waters slowly evaporated and that the various substances in the sea
water were deposited in thick layers. The deposits thus left by the
evaporation of the sea water gradually became hidden by sediment and
soil, and lost to sight. From such deposits, potash is obtained.
Geologists tell us that our own Western States were once submerged,
and that the waters evaporated and disappeared from our land very much
as they did from Germany. The Great Salt Lake of Utah is a relic of a
great body of water. If it be true that waters once covered our
Western States, there may be buried deposits of potash there, and
to-day the search for the hidden treasure is going on with the energy
and enthusiasm characteristic of America.

Another probable source of potash is seaweed. The sea is a vast
reservoir of potash, and seaweed, especially the giant kelp, absorbs
large quantities of this potash. A ton of dried kelp (dried by sun and
wind) contains about 500 pounds of pure potash. The kelps are
abundant, covering thousands of square miles in the Pacific Ocean,
from Mexico to the Arctic Ocean.



CHAPTER XXVII

SOUND


249. The Senses. All the information which we possess of the world
around us comes to us through the use of the senses of sight, hearing,
taste, touch, and smell. Of the five senses, sight and hearing are
generally considered the most valuable. In preceding Chapters we
studied the important facts relative to light and the power of vision;
it remains for us to study Sound as we studied Light, and to learn
what we can of sound and the power to hear.

250. How Sound is Produced. If one investigates the source of any
sound, he will always find that it is due to motion of some kind. A
sudden noise is traced to the fall of an object, or to an explosion,
or to a collision; in fact, is due to the motion of matter. A piano
gives out sound whenever a player strikes the keys and sets in motion
the various wires within the piano; speech and song are caused by the
motion of chest, vocal cords, and lips.

[Illustration: FIG. 164.--Sprays of water show that the fork is in
motion.]

If a large dinner bell is rung, its motion or vibration may be felt on
touching it with the finger. If a tuning fork is made to give forth
sound by striking it against the knee, or hitting it with a rubber
hammer, and is then touched to the surface of water, small sprays of
water will be thrown out, showing that the prongs of the fork are in
rapid motion. (A rubber hammer is made by putting a piece of glass
tubing through a rubber cork.)

If a light cork ball on the end of a thread is brought in contact with
a sounding fork, the ball does not remain at rest, but vibrates back
and forth, being driven by the moving prongs.

[Illustration: FIG. 165.--The ball does not remain at rest]

These simple facts lead us to conclude that all sound is due to the
motion of matter, and that a sounding body of any kind is in rapid
motion.

251. Sound is carried by Matter. In most cases sound reaches the ear
through the air; but air is not the only medium through which sound is
carried. A loud noise will startle fish, and cause them to dart away,
so we conclude that the sound must have reached them through the
water. An Indian puts his ear to the ground in order to detect distant
footsteps, because sounds too faint to be heard through the air are
comparatively clear when transmitted through the earth. A gentle
tapping at one end of a long table can be distinctly heard at the
opposite end if the ear is pressed against the table; if the ear is
removed from the wood, the sound of tapping is much fainter, showing
that wood transmits sound more readily than air. We see therefore that
sound can be transmitted to the ear by solids, liquids, or gases.

Matter of any kind can transmit sound to the ear. The following
experiments will show that matter is necessary for transmission.
Attach a small toy bell to a glass rod (Fig. 166) by means of a rubber
tube and pass the rod through one of two openings in a rubber cork.
Insert the cork in a strong flask containing a small quantity of water
and shake the bell, noting the sound produced. Then heat the flask,
allowing the water to boil briskly, and after the boiling has
continued for a few minutes remove the flame and instantly close up
the second opening by inserting a glass stopper. Now shake the flask
and note that the sound is very much fainter than at first. As the
flask was warmed, air was rapidly expelled; so that when the flask was
shaken the second time, less air was present to transmit the sound. If
the glass stopper is removed and the air is allowed to reenter the
flask, the loudness of the sound immediately increases.

[Illustration: FIG. 166.--Sound is carried by the air.]

Since the sound of the bell grows fainter as air is removed, we infer
that there would be no sound if all the air were removed from the
flask; that is to say, sound cannot be transmitted through empty space
or a vacuum. If sound is to reach our ears, it must be through the
agency of matter, such as wood, water, or air, etc.

252. How Sound is transmitted through Air. We saw in Section 250
that sound can always be traced to the motion or vibration of matter.
It is impossible to conceive of an object being set into sudden and
continued motion without disturbing the air immediately surrounding
it. A sounding body always disturbs and throws into vibration the air
around it, and the air particles which receive motion from a sounding
body transmit their motion to neighboring particles, these in turn to
the next adjacent particles, and so on until the motion has traveled
to very great distances. The manner in which vibratory motion is
transmitted by the atmosphere must be unusual in character, since no
motion of the air is apparent, and since in the stillness of night
when "not a breath of air" is stirring, the shriek of a railroad
whistle miles distant may be heard with perfect clearness. Moreover,
the most delicate notes of a violin can be heard in the remotest
corners of a concert hall, when not the slightest motion of the air
can be seen or felt.

In our study of the atmosphere we saw that air can be compressed and
rarefied; in other words, we saw that air is very elastic. It can be
shown experimentally that whenever an elastic body in motion comes in
contact with a body at rest, the moving body transfers its motion to
the second body and then comes to rest itself. Let two billiard balls
be suspended in the manner indicated in Figure 167. If one of the
balls is drawn aside and is then allowed to fall against the other,
the second ball is driven outward to practically the height from which
the first ball fell and the first ball comes to rest.

[Illustration: FIG. 167.--Elastic balls.]

[Illustration: FIG. 168.--Suspended billiard balls.]

If a number of balls are arranged in line as in Figure 168 or Figure
169, and the end ball is raised and then allowed to fall, or if _A_ is
pushed against _C_, the last ball _B_ will move outward alone, with a
force nearly equal to that originally possessed by _A_ and to a
distance nearly equal to that through which _A_ moved. But there will
be no _visible_ motion of the intervening balls. The force of the
moving ball _A_ is given to the second ball, and the second ball in
turn gives the motion to the third, and so on throughout the entire
number, until _B_ is reached. But _B_ has no ball to give its motion
to, hence _B_ itself moves outward, and moves with a force nearly
equal to that originally imparted by _A_ and to a distance nearly
equal to that through which _A_ fell. Motion at _A_ is transmitted to
_B_ without any perceptible motion of the balls lying between these
points. Similarly the particles of air set into motion by a sounding
body impart their motion to each other, the motion being transmitted
onward without any perceptible motion of the air itself. When this
motion reaches the ear, it sets the drum of the ear into vibration,
and these vibrations are in turn transmitted to the auditory nerves,
which interpret the motion as sound.

[Illustration: FIG. 169.--Elastic balls transmit motion.]

[Illustration: FIG. 170.--When a ball meets more than one ball, it
divides its motion.]

253. Why Sound dies away with Distance. Since the last ball _B_ is
driven outward with a force nearly equal to that possessed by _A_, it
would seem that the effect on the ear drum should be independent of
distance and that a sound should be heard as distinctly when remote as
when near. But we know from experience that this is not true, because
the more distant the source of sound, the fainter the impression; and
finally, if the distance between the source of sound and the hearer
becomes too great, the sound disappears entirely and nothing is heard.
The explanation of this well-known fact is found in a further study of
the elastic balls (Fig. 170). If _A_ hits two balls instead of one,
the energy possessed by _A_ is given in part to one ball, and in part
to the other, so that neither obtains the full amount. These balls,
having each received less than the original energy, have less to
transmit; each of these balls in turn meets with others, and hence the
motion becomes more and more distributed, and distant balls receive
less and less impetus. The energy finally given becomes too slight to
affect neighboring balls, and the system comes to rest. This is what
occurs in the atmosphere; a moving air particle meets not one but many
adjacent air particles, and each of these receives a portion of the
original energy and transmits a portion. When the original disturbance
becomes scattered over a large number of air particles, the energy
given to any one air particle becomes correspondingly small, and
finally the energy becomes so small that further particles are not
affected; beyond this limit the sound cannot be heard.

If an air particle transmitted motion only to those air particles
directly in line with it, we should not be able to detect sound unless
the ear were in direct line with the source. The fact that an air
particle divides its motion among all particles which it touches, that
is, among those on the sides as well as those in front, makes it
possible to hear sound in all directions. A good speaker is heard not
only by those directly in front of him, but by those on the side, and
even behind him.

254. Velocity of Sound. The transmission of motion from particle to
particle does not occur instantaneously, but requires time. If the
distance is short, so that few air particles are involved, the time
required for transmission is very brief, and the sound is heard at
practically the instant it is made. Ordinarily we are not conscious
that it requires time for sound to travel from its source to our ears,
because the distance involved is too short. At other times we
recognize that there is a delay; for example, thunder reaches our ears
after the lightning which caused the thunder has completely
disappeared. If the storm is near, the interval of time between the
lightning and the thunder is brief, because the sound does not have
far to travel; if the storm is distant, the interval is much longer,
corresponding to the greater distance through which the sound travels.
Sound does not move instantaneously, but requires time for its
transmission. The report of a distant cannon is heard after the flash
and smoke are seen; the report of a near cannon is heard the instant
the flash is seen.

The speed with which sounds travels through the air, or its velocity,
was first measured by noting the interval (54.6 seconds) which elapsed
between the flash of a cannon and the sound of the report. The
distance of the cannon from the observer was measured and found to be
61,045 feet, and by dividing this distance by the number of seconds,
we find that the distance traveled by sound in one second is
approximately 1118 feet.

High notes and low notes, soft notes and shrill notes, all travel at
the same rate. If bass notes traveled faster or slower than soprano
notes, or if the delicate tones of the violin traveled faster or
slower than the tones of a drum, music would be practically
impossible, because at a distance from the source of sound the various
tones which should be in unison would be out of time--some arriving
late, some early.

255. Sound Waves. Practically everyone knows that a hammock hung
with long ropes swings or vibrates more slowly than one hung with
short ropes, and that a stone suspended by a long string swings more
slowly than one suspended by a short string. No two rocking chairs
vibrate in the same way unless they are exactly alike in shape, size,
and material. An object when disturbed vibrates in a manner peculiar
to itself, the vibration being slow, as in the case of the long-roped
swing, or quick, as in the case of the short-roped swing. The time
required for a single swing or vibration is called the _period_ of the
body, and everything that can vibrate has a characteristic period.
Size and shape determine to a large degree the period of a body; for
example, a short, thick tuning fork vibrates more rapidly than a tall
slender fork.

[Illustration: FIG. 171.--The two hammocks swing differently.]

Some tuning forks when struck vibrate so rapidly that the prongs move
back and forth more than 5000 times per second, while other tuning
forks vibrate so slowly that the vibrations do not exceed 50 per
second. In either case the distance through which the prongs move is
very small and the period is very short, so that the eye can seldom
detect the movement itself. That the prongs are in motion, however, is
seen by the action of a pith ball when brought in contact with the
prongs (see Section 250).

[Illustration: FIG. 172.--The pitch given out by a fork depends upon
its shape.]

The disturbance created by a vibrating body is called a wave.

256. Waves. While the disturbance which travels out from a sounding
body is commonly called a wave, it is by no means like the type of
wave best known to us, namely, the water wave.

If a closely coiled heavy wire is suspended as in Figure 173 and the
weight is drawn down and then released, the coil will assume the
appearance shown; there is clearly an overcrowding or condensation in
some places, and a spreading out or rarefaction in other places. The
pulse of condensation and rarefaction which travels the length of the
wire is called a wave, although it bears little or no resemblance to
the familiar water wave. Sound waves are similar to the waves formed
in the stretched coil.

[Illustration: FIG. 173.--Waves in a coiled wire.]

Sound waves may be said to consist of a series of condensations and
rarefactions, and the distance between two consecutive condensations
and rarefactions may be defined as the wave length.

257. How One Sounding Body produces Sound in Another Body. In
Section 255 we saw that any object when disturbed vibrates in a manner
peculiar to itself,--its natural period,--a long-roped hammock
vibrating slowly and a short-roped hammock vibrating rapidly. From
observation we learn that it requires but little force to cause a body
to vibrate in its natural period. If a sounding body is near a body
which has the same period as itself, the pulses of air produced by the
sounding body will, although very small, set the second body into
motion and cause it to make a faint sound. When a piano is being
played, we are often startled to find that a window pane or an
ornament responds to some note of the piano. If two tuning forks of
exactly identical periods (that is, of the same frequency) are placed
on a table as in Figure 174, and one is struck so as to give forth a
clear sound, the second fork will likewise vibrate, even though the
two forks may be separated by several feet of air. We can readily see
that the second fork is in motion, although it has not been struck,
because it will set in motion a pith ball suspended beside it; at
first the pith ball does not move, then it moves slightly, and finally
bounces rapidly back and forth. If the periods of the two forks are
not identical, but differ in the slightest degree, the second fork
will not respond to the first fork, no matter how long or how loud the
sound of the first fork. If we suppose that the fork vibrates 256
times each second, then 256 gentle pulses of air are produced each
second, and these, traveling outward through the air, reach the silent
fork and tend to set it in motion. A single pulse of air could not
move the solid, heavy prongs, but the accumulated action of 256
vibrations per second soon makes itself felt, and the second fork
begins to vibrate, at first gently, then gradually stronger, and
finally an audible tone is given forth.

[Illustration: FIG. 174.--When the first fork vibrates, the second
responds.]

The cumulative power of feeble forces acting frequently at definite
intervals is seen in many ways in everyday life. A small boy can
easily swing a much larger boy, provided he gives the swing a gentle
push in the right direction every time it passes him. But he must be
careful to push at the proper instant, since otherwise his effort does
not count for much; if he pushes forward when the swing is moving
backward, he really hinders the motion; if he waits until the swing
has moved considerably forward, his push counts for little. He must
push at the proper instant; that is, the way in which his hand moves
in giving the push must correspond exactly with the way in which the
swing would naturally vibrate. A very striking experiment can be made
by suspending from the ceiling a heavy weight and striking this weight
gently at regular, properly timed intervals with a small cork hammer.
Soon the pendulum, or weight, will be set swinging.

[Illustration: FIG. 175.--The hollow wooden box reënforces the sound.]

258. Borrowed Sound. Picture frames and ornaments sometimes buzz and
give forth faint murmurs when a piano or organ is played. The waves
sent out by a sounding body fall upon all surrounding objects and by
their repeated action tend to throw these bodies into vibration. If
the period of any one of the objects corresponds with the period of
the sounding body, the gentle but frequent impulses affect the object,
which responds by emitting a sound. If, however, the periods do not
correspond, the action of the sound waves is not sufficiently powerful
to throw the object into vibration, and no sound is heard. Bodies
which respond in this way are said to be sympathetic and the response
produced is called _resonance_. Seashells when held to the ear seem to
contain the roar of the sea; this is because the air within the shell
is set into sympathetic vibrations by some external tone. If the
seashell were held to the ear in an absolutely quiet room, no sound
would be heard, because there would be no external forces to set into
vibration the air within the shell.

Tuning forks do not produce strong tones unless mounted on hollow
wooden boxes (Fig. 175), whose size and shape are so adjusted that
resonance occurs and strengthens the sound. When a human being talks
or sings, the air within the mouth cavity is thrown into sympathetic
vibration and strengthens the otherwise feeble tone of the speaker.

259. Echo. If one shouts in a forest, the sound is sometimes heard a
second time a second or two later. This is because sound is reflected
when it strikes a large obstructing surface. If the sound waves
resulting from the shout meet a cliff or a mountain, they are
reflected back, and on reaching the ear produce a later sensation of
sound.

By observation it has been found that the ear cannot distinguish
sounds which are less than one tenth of a second apart; that is, if
two sounds follow each other at an interval less than one tenth of a
second, the ear recognizes not two sounds, but one. This explains why
a speaker can be heard better indoors than in the open air. In the
average building, the walls are so close that the reflected waves have
but a short distance to travel, and hence reach the ear at practically
the same time as those which come directly from the speaker. In the
open, there are no reflecting walls or surfaces, and the original
sound has no reënforcement from reflection.

If the reflected waves reach the ear too late to blend with the
original sound, that is, come later than one tenth of a second after
the first impression, an echo is heard. What we call the rolling of
thunder is really the reflection and re-reflection of the original
thunder from cloud and cliff.

Some halls are so large that the reflected sounds cause a confusion of
echoes, but this difficulty can be lessened by hanging draperies,
which break the reflection.

260. Motion does not always produce Sound. While we know that all
sound can be traced to motion, we know equally well that motion does
not always produce sound. The hammock swinging in the breeze does not
give forth a sound; the flag floating in the air does not give forth a
sound unless blown violently by the wind; a card moved slowly through
the air does not produce sound, but if the card is moved rapidly back
and forth, a sound becomes audible.

Motion, in order to produce sound, must be rapid; a ball attached to a
string and moved slowly through the air produces no sound, but the
same ball, whirled rapidly, produces a distinct buzz, which becomes
stronger and stronger the faster the ball is whirled.

261. Noise and Music. When the rapid motions which produce sound are
irregular, we hear noise; when the motions are regular and definite,
we have a musical tone; the rattling of carriage wheels on stones, the
roar of waves, the rustling of leaves are noise, not music. In all
these illustrations we have rapid but irregular motion; no two stones
strike the wheel in exactly the same way, no two waves produce pulses
in the air of exactly the same character, no two leaves rustle in
precisely the same way. The disturbances which reach the ear from
carriage, waves, and leaves are irregular both in time and strength,
and irritate the ear, causing the sensation which we call noise.

The tuning fork is musical. Here we have rapid, regular motion;
vibrations follow each other at perfectly definite intervals, and the
air disturbance produced by one vibration is exactly like the
disturbance produced by a later vibration. The sound waves which reach
the ear are regular in time and kind and strength, and we call the
sensation music.

To produce noise a body must vibrate in such a way as to give short,
quick shocks to the air; to produce music a body must not only impart
short, quick shocks to the air, but must impart these shocks with
unerring regularity and strength. A flickering light irritates the
eye; a flickering sound or noise irritates the ear; both are painful
because of the sudden and abrupt changes in effect which they cause,
the former on the eye, the latter on the ear.

The only thing essential for the production of a musical sound is that
the waves which reach the ear shall be rapid and regular; it is
immaterial how these waves are produced. If a toothed wheel is mounted
and slowly rotated, and a stiff card is held against the teeth of the
wheel, a distinct tap is heard every time the card strikes the wheel.
But if the wheel is rotated rapidly, the ear ceases to hear the
various taps and recognizes a deep continuous musical tone. The
blending of the individual taps, occurring at regular intervals, has
produced a sustained musical tone. A similar result is obtained if a
card is drawn slowly and then rapidly over the teeth of a comb.

[Illustration: FIG. 176.--A rotating disk.]

That musical tones are due to a succession of regularly timed impulses
is shown most clearly by means of a rotating disk on which are cut two
sets of holes, the outer set equally spaced, and the inner set
unequally spaced (Fig. 176).

If, while the disk is rotating rapidly, a tube is held over the
outside row and air is blown through the tube, a sustained musical
tone will be heard. If, however, the tube is held, during the rotation
of the disk, over the inner row of unequally spaced holes, the musical
tone disappears, and a series of noises take its place. In the first
case, the separate puffs of air followed each other regularly and
blended into one tone; in the second case, the separate puffs of air
followed each other at uncertain and irregular intervals and the
result was noise.

Sound possesses a musical quality only when the waves or pulses follow
each other at absolutely regular intervals.

262. The Effect of the Rapidity of Motion on the Musical Tone
Produced. If the disk is rotated so slowly that less than about 16
puffs are produced in one second, only separate puffs are heard, and a
musical tone is lacking; if, on the other hand, the disk is rotated in
such a way that 16 puffs or more are produced in one second, the
separate puffs will blend together to produce a continuous musical
note of very low pitch. If the speed of the disk is increased so that
the puffs become more frequent, the pitch of the resulting note rises;
and at very high speeds the notes produced become so shrill and
piercing as to be disagreeable to the ear. If the speed of the disk is
lessened, the pitch falls correspondingly; and if the speed again
becomes so low that less than 16 puffs are formed per second, the
sustained sound disappears and a series of intermittent noises is
produced.

263. The Pitch of a Note. By means of an apparatus called the siren,
it is possible to calculate the number of vibrations producing any
given musical note, such, for example, as middle C on the piano. If
air is forced continuously against the disk as it rotates, a series of
puffs will be heard (Fig. 177).

If the disk turns fast enough, the puffs blend into a musical sound,
whose pitch rises higher and higher as the disk moves faster and
faster, and produces more and more puffs each second.

The instrument is so constructed that clockwork at the top registers
the number of revolutions made by the disk in one second. The number
of holes in the disk multiplied by the number of revolutions a second
gives the number of puffs of air produced in one second. If we wish to
find the number of vibrations which correspond to middle C on the
piano, we increase the speed of the disk until the note given forth by
the siren agrees with middle C as sounded on the piano, as nearly as
the ear can judge; we then calculate the number of puffs of air which
took place each second at that particular speed of the disk. In this
way we find that middle C is due to about 256 vibrations per second;
that is, a piano string must vibrate 256 times per second in order for
the resultant note to be of pitch middle C. In a similar manner we
determine the following frequencies:--

  |do  |re  |mi  |fa  |sol |la  |si  |do  |
  |C   |D   |E   |F   |G   |A   |B   |C'  |
  |256 |288 |320 |341 |384 |427 |480 |512 |

[Illustration: FIG. 177.--A siren.]

The pitch of pianos, from the lowest bass note to the very highest
treble, varies from 27 to about 3500 vibrations per second. No human
voice, however, has so great a range of tone; the highest soprano
notes of women correspond to but 1000 vibrations a second, and the
deepest bass of men falls but to 80 vibrations a second.

While the human voice is limited in its production of sound,--rarely
falling below 80 vibrations a second and rarely exceeding 1000
vibrations a second,--the ear is by no means limited to that range in
hearing. The chirrup of a sparrow, the shrill sound of a cricket, and
the piercing shrieks of a locomotive are due to far greater
frequencies, the number of vibrations at times equaling 38,000 per
second or more.

264. The Musical Scale. When we talk, the pitch of the voice changes
constantly and adds variety and beauty to conversation; a speaker
whose tone, or pitch, remains too constant is monotonous and dull, no
matter how brilliant his thoughts may be.

While the pitch of the voice changes constantly, the changes are
normally gradual and slight, and the different tones merge into each
other imperceptibly. In music, however, there is a well-defined
interval between even consecutive notes; for example, in the musical
scale, middle C (do) with 256 vibrations is followed by D (re) with
288 vibrations, and the interval between these notes is sharp and well
marked, even to an untrained ear. The interval between two notes is
defined as the ratio of the frequencies; hence, the interval between C
and D (do and re) is 288/256, or 9/8. Referring to Section 263, we see
that the interval between C and E is 320/256, or 5/4, and the interval
between C and C' is 512/256, or 2; the interval between any note and
its octave is 2.

The successive notes in one octave of the musical scale are related as
follows:--

  |Key of C          |C   |D   |E   |F   |G   |A    |B   |C'  |
  |No. of vibrations |    |    |    |    |    |     |    |    |
  |per sec.          |256 |288 |320 |341 |384 |427  |480 |512 |
  |Interval          |9/8 |5/4 |4/3 |3/2 |5/3 |15/8 |2   |    |

The intervals of F and A are not strictly 4/3 and 5/3, but are nearly
so; if F made 341.3 vibrations per second instead of 341; and if A
made 426.6 instead of 427, then the intervals would be exactly 4/3 and
5/3. Since the real difference is so slight, we can assume the simpler
ratios without appreciable error.

Any eight notes whose frequencies are in the ratio of 9/8, 5/4, etc.,
will when played in succession give the familiar musical scale; for
example, the deepest bass voice starts a musical scale whose notes
have the frequencies 80, 90, 100, 107, 120, 133, 150, 160, but the
intervals here are identical with those of a higher scale; the
interval between C and D, 80 and 90, is 9/8, just as it was before
when the frequencies were much greater; that is, 256 and 288. In
singing "Home, Sweet Home," for example, a bass voice may start with a
note vibrating only 132 times a second; while a tenor may start at a
higher pitch, with a note vibrating 198 times per second, and a
soprano would probably take a much higher range still, with an initial
frequency of 528 vibrations per second. But no matter where the voices
start, the intervals are always identical. The air as sung by the bass
voice would be represented by _A_. The air as sung by the tenor voice
would be represented by _B_. The air as sung by the soprano voice
would be represented by _C_.

[Illustration: FIG. 178.--A song as sung by three voices of different
pitch.]



CHAPTER XXVIII

MUSICAL INSTRUMENTS


265. Musical instruments maybe divided into three groups according
to the different ways in which their tones are produced:--

_First._ The stringed instruments in which sound is produced by the
vibration of stretched strings, as in the piano, violin, guitar,
mandolin.

_Second._ The wind instruments in which sound is produced by the
vibrations of definite columns of air, as in the organ, flute, cornet,
trombone.

_Third._ The percussion instruments, in which sound is produced by the
motion of stretched membranes, as in the drum, or by the motion of
metal disks, as in the tambourines and cymbals.

266. Stringed Instruments. If the lid of a piano is opened, numerous
wires are seen within; some long, some short, some coarse, some fine.
Beneath each wire is a small felt hammer connected with the keys in
such a way that when a key is pressed, a string is struck by a hammer
and is thrown into vibration, thereby producing a tone.

If we press the lowest key, that is, the key giving forth the lowest
pitch, we see that the longest wire is struck and set into vibration;
if we press the highest key, that is, the key giving the highest
pitch, we see that the shortest wire is struck. In addition, it is
seen that the short wires which produce the high tones are fine,
while the long wires which produce the low tones are coarse. The
shorter and finer the wire, the higher the pitch of the tone produced.
The longer and coarser the wire, the lower the pitch of the tone
produced.

[Illustration: FIG. 179.--Piano wires seen from the back.]

The constant striking of the hammers against the strings stretches and
loosens them and alters their pitch; for this reason each string is
fastened to a screw which can be turned so as to tighten the string or
to loosen it if necessary. The tuning of the piano is the adjustment
of the strings so that each shall produce a tone of the right pitch.
When the strings are tightened, the pitch rises; when the strings are
loosened, the pitch falls.

What has been said of the piano applies as well to the violin, guitar,
and mandolin. In the latter instruments the strings are few in number,
generally four, as against eighty-eight in the piano; the hammer of
the piano is replaced in the violin by the bow, and in the guitar by
the fingers; varying pitches on any one string are obtained by sliding
a finger of the left hand along the wire, and thus altering its
length.

Frequent tuning is necessary, because the fine adjustments are easily
disturbed. The piano is the best protected of all the stringed
instruments, being inclosed by a heavy framework, even when in use.

[Illustration: FIG. 180.--Front view of an open piano.]

267. Strings and their Tones. Fasten a violin string to a wooden
frame or box, as shown in Figure 181, stretching it by means of some
convenient weight; then lay a yardstick along the box in order that
the lengths may be determined accurately. If the stretched string is
plucked with the fingers or bowed with the violin bow, a clear musical
sound of definite pitch will be produced. Now divide the string into
two equal parts by inserting the bridge midway between the two ends;
and pluck either half as before. The note given forth is of a
decidedly higher pitch, and if by means of the siren we compare the
pitches in the two cases, we find that the note sounded by the half
wire is the octave of the note sounded by the entire wire; the
frequency has been doubled by halving the length. If now the bridge is
placed so that the string is divided into two unequal portions such as
1:3 and 2:3, and the shorter portion is plucked, the pitch will be
still higher; the shorter the length plucked, the higher the pitch
produced. This movable bridge corresponds to the finger of the
violinist; the finger slides back and forth along the string, thus
changing the length of the bowed portion and producing variations in
pitch.

[Illustration: FIG. 181.--The length of a string influences the
pitch.]

[Illustration: FIG. 182.--Only one half of the string is bowed, but
both halves vibrate.]

If there were but one string, only one pitch could be sounded at any
one time; the additional strings of the violin allow of the
simultaneous production of several tones.

268. The Freedom of a String. Some stringed instruments give forth
tones which are clear and sweet, but withal thin and lacking in
richness and fullness. The tones sounded by two different strings may
agree in pitch and loudness and yet produce quite different effects on
the ear, because in one case the tone may be much more pleasing than
in the other. The explanation of this is, that a string may vibrate in
a number of different ways.

Touch the middle of a wire with the finger or a pencil (Fig. 182),
thus separating it into two portions and draw a violin bow across the
center of either half. Only one half of the entire string is struck,
but the motion of this half is imparted to the other half and throws
it into similar motion, and if a tiny A-shaped piece of paper or rider
is placed upon the unbowed half, it is hurled off.

[Illustration: FIG. 183.--The string vibrates in three portions.]

If the wire is touched at a distance of one third its length and a bow
is drawn across the middle of the smaller portion, the string will
vibrate in three parts; we cannot always see these various motions in
different parts of the string, but we know of their existence through
the action of the riders.

Similarly, touching the wire one fourth of its length from an end
makes it vibrate in four segments; touching it one fifth of its length
makes it vibrate in five segments.

In the first case, the string vibrated as a whole string and also as
two strings of half the length; hence, three tones must have been
given out, one tone due to the entire string and two tones due to the
segments. But we saw in Section 267 that halving the length of a
string doubles the pitch of the resulting tone, and produces the
octave of the original tone; hence a string vibrating as in Figure 183
gives forth three tones, one of which is the fundamental tone of the
string, and two of which are the octave of the fundamental tone.
Hence, the vibrating string produces two sensations, that of the
fundamental note and of its octave.

[Illustration: FIG. 184.--When a string vibrates as a whole, it gives
out the fundamental note.]

When a string is plucked in the middle without being held, it vibrates
simply as a whole (Fig. 184), and gives forth but one note; this is
called the fundamental. If the string is made to vibrate in two parts,
it gives forth two notes, the fundamental, and a note one octave
higher than the fundamental; this is called the first overtone. When
the string is made to move as in Figure 183, three distinct motions
are called forth, the motion of the entire string, the motion of the
portion plucked, and the motion of the remaining unplucked portion of
the string. Here, naturally, different tones arise, corresponding to
the different modes of vibration. The note produced by the vibration
of one third of the original string is called the second overtone.

The above experiments show that a string is able to vibrate in a
number of different ways at the same time, and to emit simultaneously
a number of different tones; also that the resulting complex sound
consists of the fundamental and one or more overtones, and that the
number of overtones present depends upon how and where the string is
plucked.

[Illustration: FIG. 185.--A string can vibrate in a number of
different ways simultaneously, and can produce different notes
simultaneously.]

269. The Value of Overtones. The presence of overtones determines
the quality of the sound produced. If the string vibrates as a whole
merely, the tone given out is simple, and seems dull and
characterless. If, on the other hand, it vibrates in such a way that
overtones are present, the tone given forth is full and rich and the
sensation is pleasing. A tuning fork cannot vibrate in more than one
way, and hence has no overtones, and its tone, while clear and sweet,
is far less pleasing than the same note produced by a violin or piano.
The untrained ear is not conscious of overtones and recognizes only
the strong dominant fundamental. The overtones blend in with the
fundamental and are so inconspicuously present that we do not realize
their existence; it is only when they are absent that we become aware
of the beauty which they add to the music. A song played on tuning
forks instead of on strings would be lifeless and unsatisfying because
of the absence of overtones.

It is not necessary to hold finger or pencil at the points 1:3, 1:4,
etc., in order to cause the string to vibrate in various ways; if a
string is merely plucked or bowed at those places, the result will be
the same. It is important to remember that no matter where a string of
definite length is bowed, the note most distinctly heard will be the
fundamental; but the quality of the emitted tone will vary with the
bowing. For example, if a string is bowed in the middle, the effect
will be far less pleasing than though it were bowed near the end. In
the piano, the hammers are arranged so as to strike near one end of
the string, at a distance of about 1:7 to 1:9; and hence a large
number of overtones combine to reënforce and enrich the fundamental
tone.

270. The Individuality of Instruments. It has been shown that a
piano string when struck by a hammer, or a violin string when bowed,
or a mandolin string when plucked, vibrates not only as a whole, but
also in segments, and as a result gives forth not a simple tone, as we
are accustomed to think, but a very complex tone consisting of the
fundamental and one or more overtones. If the string whose fundamental
note is lower C (128 vibrations per second) is thrown into vibration,
the tone produced may contain, in addition to the prominent
fundamental, any one or more of the following overtones: C', G'', C'',
E'', C''', etc.

The number of overtones actually present depends upon a variety of
circumstances: in the piano, it depends largely upon the location of
the hammer; in the violin, upon the place and manner of bowing.
Mechanical differences in construction account for prominent and
numerous overtones in some instruments and for feeble and few
overtones in others. The oboe, for example, is so constructed that
only the high overtones are present, and hence the sound gives a
"pungent" effect; the clarinet is so constructed that the
even-numbered overtones are killed, and the presence of only
odd-numbered overtones gives individuality to the instrument. In these
two instruments we have vibrating air columns instead of vibrating
strings, but the laws which govern vibrating strings are applicable to
vibrating columns of air, as we shall see later. It is really the
presence or absence of overtones which enables us to distinguish the
note of the piano from that of the violin, flute, or clarinet. If
overtones could be eliminated, then middle C, or any other note on the
piano, would be indistinguishable from that same note sounded on any
other instrument. The fundamental note in every instrument is the
same, but the overtones vary with the instrument and lend
individuality to each. The presence of high overtones in the oboe and
the presence of odd-numbered overtones in the clarinet enable us to
distinguish without fail the sounds given out by these instruments.

The richness and individuality of an instrument are due, not only to
the overtones which accompany the fundamental, but also to the
"forced" vibrations of the inclosing case, or of the sounding board.
If a vibrating tuning fork is held in the hand, the sound will be
inaudible except to those quite near; if, however, the base of the
fork is held against the table, the sound is greatly intensified and
becomes plainly audible throughout the room.

The vibrations of the fork are transmitted to the table top and throw
it into vibrations similar to its own, and these additional vibrations
intensify the original sound. Any fork, no matter what its frequency,
can force the surface of the table into vibration, and hence the sound
of any fork will be intensified by contact with a table or box.

This is equally true of strings; if stretched between two posts and
bowed, the sound given out by a string is feeble, but if stretched
over a sounding board, as in the piano, or over a wooden shell, as in
the violin, the sound is intensified. Any note of the instrument will
force the sounding body to vibrate, thus reënforcing the volume of
sound, but some tones, or modes of vibration, do this more easily than
others, and while the sounding board or shell always responds, it
responds in varying degree. Here again we have not only enrichment of
sound but also individuality of instruments.

271. The Kinds of Stringed Instruments. Stringed instruments may be
grouped in the following three classes:--

_a_. Instruments in which the strings are set into motion by
hammers--piano.

_b_. Instruments in which the strings are set into motion by
bowing--violin, viola, violoncello, double bass.

_c_. Instruments in which the strings are set into motion by
plucking--harp, guitar, mandolin.

[Illustration: FIG. 186.--1, violin; 2, viola; 3, violoncello; 4,
double bass.]

     _a_. The piano is too well known to need comment. In
     passing, it may be mentioned that in the construction of the
     modern concert piano approximately 40,000 separate pieces of
     material are used. The large number of pieces is due,
     partly, to the fact that the single string corresponding to
     any one key is usually replaced by no less than three or
     four similar strings in order that greater volume of sound
     may be obtained. The hammer connected to a key strikes
     four or more strings instead of one, and hence produces a
     greater volume of tone.

     _b_. The viola is larger than the violin, has heavier and
     thicker strings, and is pitched to a lower key; in all other
     respects the two are similar. The violoncello, because of
     the length and thickness of its strings, is pitched a whole
     octave lower than the violin; otherwise it is similar. The
     unusual length and thickness of the strings of the double
     bass make it produce very low notes, so that it is
     ordinarily looked upon as the "bass voice" of the orchestra.

     _c_. The harp has always been considered one of the most
     pleasing and perfect of musical instruments. Here the
     skilled performer has absolutely free scope for his genius,
     because his fingers can pluck the strings at will and hence
     regulate the overtones, and his feet can regulate at will
     the tension, and hence the pitch of the strings.

     Guitar and mandolin are agreeable instruments for amateurs,
     but are never used in orchestral music.

[Illustration: FIG. 187.--A harp.]

272. Wind Instruments. In the so-called wind instruments, sound is
produced by vibrating columns of air inclosed in tubes or pipes of
different lengths. The air column is thrown into vibration either
directly, by blowing across a narrow opening at one end of a pipe as
in the case of the whistle, or indirectly, by exciting vibrations in a
thin strip of wood or metal, called a reed, which in turn communicates
its vibrations to the air column within.

The shorter the air column, the higher the pitch. This agrees with the
law of vibrating strings which gives high pitches for short lengths.

[Illustration: FIG. 188.--Open organ pipes of different pitch.]

The pitch of the sound emitted by a column of air vibrating within a
pipe varies according to the following laws:

1. The shorter the pipe, the higher the pitch.

2. The pitch of a note emitted by an open pipe is one octave higher
than that of a closed pipe of equal length.

3. Air columns vibrate in segments just as do strings, and the tone
emitted by a pipe of given length is complex, consisting of the
fundamental and one or more overtones. The greater the number of
overtones present, the richer the tone produced.

273. How the Various Pitches are Produced. With a pipe of fixed
length, for example, the clarinet (Fig. 189, 1), different pitches are
obtained by pressing keys which open holes in the tube and thus
shorten or lengthen the vibrating air column and produce a rise or
fall in pitch. Changes in pitch are also produced by variation in the
player's breathing. By blowing hard or gently, the number of
vibrations of the reed is increased or decreased and hence the pitch
is altered.

[Illustration: FIG. 189--1, clarinet; 2, oboe; 3, flute.]

In the oboe (Fig. 189, 2) the vibrating air column is set into motion
by means of two thin pieces of wood or metal placed in the mouthpiece
of the tube. Variations in pitch are produced as in the clarinet by
means of stops and varied breathing. In the flute, the air is set into
motion by direct blowing from the mouth, as is done, for instance,
when we blow into a bottle or key.

The sound given out by organ pipes is due to air blown across a sharp
edge at the opening of a narrow tube. The air forced across the sharp
edge is thrown into vibration and communicates its vibration to the
air within the organ pipe. For different pitches, pipes of different
lengths are used: for very low pitches long, closed pipes are used;
for very high pitches short, open pipes are used. The mechanism of the
organ is such that pressing a key allows the air to rush into the
communicating pipe and a sound is produced characteristic of the
length of the pipe.

[Illustration: FIG. 190.--1, horn; 2, trumpet; 3, trombone.]

[Illustration: FIG. 191.--1, kettledrum; 2, bass drum; 3, cymbals.]

[Illustration: FIG. 192.--The seating arrangement of the Philadelphia
orchestra.]

In the brass wind instruments such as horn, trombone, and trumpet, the
lips of the player vibrate and excite the air within. Varying pitches
are obtained partly by the varying wind pressure of the musician; if
he breathes fast, the pitch rises; if he breathes slowly, the pitch
falls. All of these instruments, however, except the trombone possess
some valves which, on being pressed, vary the length of the tube and
alter the pitch accordingly. In the trombone, valves are replaced by a
section which slides in and out and shortens or lengthens the tube.

274. The Percussion Instruments. The percussion instruments,
including kettledrums, bass drums, and cymbals, are the least
important of all the musical instruments; and are usually of service
merely in adding to the excitement and general effect of an orchestra.

In orchestral music the various instruments are grouped somewhat as
shown in Figure 192.



CHAPTER XXIX

SPEAKING AND HEARING


[Illustration: FIG. 193.--The vibration of the vocal cords produces
the sound of the human voice.]

275. Speech. The human voice is the most perfect of musical
instruments. Within the throat, two elastic bands are attached to the
windpipe at the place commonly called Adam's apple; these flexible
bands have received the name of vocal cords, since by their vibration
all speech is produced. In ordinary breathing, the cords are loose and
are separated by a wide opening through which air enters and leaves
the lungs. When we wish to speak, muscular effort stretches the cords,
draws them closer together, and reduces the opening between them to a
narrow slit, as in the case of the organ pipe. If air from the lungs
is sent through the narrow slit, the vocal cords or bands are thrown
into rapid vibration and produce sound. The pitch of the sound depends
upon the tension of the stretched membranes, and since this can be
altered by muscular action, the voice can be modulated at will. In
times of excitement, when the muscles of the body in general are in a
state of great tension, the pitch is likely to be uncommonly high.

Women's voices are higher than men's because the vocal cords are
shorter and finer; even though muscular tension is relaxed and the
cords are made looser, the pitch of a woman's voice does not fall so
low as that of a man's voice since his cords are naturally much
longer and coarser. The difference between a soprano and an alto voice
is merely one of length and tension of the vocal cords.

Successful singing is possible only when the vocal cords are readily
flexible and when the singer can supply a steady, continuous blast of
air through the slit between the cords. The hoarseness which
frequently accompanies cold in the head is due to the thickening of
the mucous membrane and to the filling up of the slit with mucus,
because when this happens, the vocal cords cannot vibrate properly.

The sounds produced by the vocal cords are transformed into speech by
the help of the tongue and lips, which modify the shape of the mouth
cavity. Some of the lower animals have a speaking apparatus similar to
our own, but they cannot perfectly transform sound into speech. The
birds use their vocal cords to beautiful advantage in singing, far
surpassing us in many ways, but the power of speech is lacking.

276. The Ear. The pulses created in the air by a sounding body are
received by the ear and the impulses which they impart to the auditory
nerve pass to the brain and we become conscious of a sound. The ear is
capable of marvelous discrimination and accuracy. "In order to form an
idea of the extent of this power imagine an auditor in a large music
hall where a full band and chorus are performing. Here, there are
sounds mingled together of all varieties of pitch, loudness, and
quality; stringed instruments, wood instruments, brass instruments,
and voices, of many different kinds. And in addition to these there
may be all sorts of accidental and irregular sounds and noises, such
as the trampling and shuffling of feet, the hum of voices, the rustle
of dress, the creaking of doors, and many others. Now it must be
remembered that the only means the ear has of becoming aware of these
simultaneous sounds is by the condensations and rarefactions which
reach it; and yet when the sound wave meets the nerves, the nerves
single out each individual element, and convey to the mind of the
hearer, not only the tones and notes of every instrument in the
orchestra, but the character of every accidental noise; and almost as
distinctly as if each single tone or noise were heard alone."--POLE.

[Illustration: FIG. 194.--The ear.]

277. The Structure of the Ear. The external portion of the ear acts
as a funnel for catching sound waves and leading them into the canal,
where they strike upon the ear drum, or tympanic membrane, and throw
it into vibration. Unless the ear drum is very flexible there cannot
be perfect response to the sound waves which fall upon it; for this
reason, the glands of the canal secrete a wax which moistens the
membrane and keeps it flexible. Lying directly back of the tympanic
membrane is a cavity filled with air which enters by the Eustachian
tube; from the throat air enters the Eustachian tube, moves along it,
and passes into the ear cavity. The dull crackling noise noticed in
the ear when one swallows is due to the entrance and exit of air in
the tube. Several small bones stretch across the upper portion of the
cavity and make a bridge, so to speak, from the ear drum to the far
wall of the cavity. It is by means of these three bones that the
vibrations of the ear drum are transmitted to the inner wall of the
cavity. Behind the first cavity is a second cavity so complex and
irregular that it is called the labyrinth of the ear. This labyrinth
is filled with a fluid in which are spread out the delicate sensitive
fibers of the auditory nerves; and it is to these that the vibrations
must be transmitted.

Suppose a note of 800 vibrations per second is sung. Then 800 pulses
of air will reach the ear each second, and the ear drum, being
flexible, will respond and will vibrate at the same rate. The
vibration of the ear drum will be transmitted by the three bones and
the fluid to the fibers of the auditory nerves. The impulses imparted
to the auditory nerve reach the brain and in some unknown way are
translated into sound.

278. Care of the Ear. Most catarrhal troubles are accompanied by an
oversupply of mucus which frequently clogs up the Eustachian tube and
produces deafness. For the same reason, colds and sore throat
sometimes induce temporary deafness.

The wax of the ear is essential for flexibility of the ear drum; if an
extra amount accumulates, it can be got rid of by bathing the ear in
hot water, since the heat will melt the wax. The wax should never be
picked out with pin or sharp object except by a physician, lest injury
be done to the tympanic membrane.

279. The Phonograph. The invention of the phonograph by Edison in
1878 marked a new era in the popularity and dissemination of music. Up
to that time, household music was limited to those who were rich
enough to possess a real musical instrument, and who in addition had
the understanding and the skill to use the instrument. The invention
of the phonograph has brought music to thousands of homes possessed
of neither wealth nor skill. That the music reproduced by a phonograph
is not always of the highest order does not, in the least, detract
from the interest and wonder of the instrument. It can reproduce what
it is called upon to reproduce, and if human nature demands the
commonplace, the instrument will be made to satisfy the demand. On the
other hand, speeches of famous men, national songs, magnificent opera
selections, and other pleasing and instructive productions can be
reproduced fairly accurately. In this way the phonograph, perhaps more
than any other recent invention, can carry to the "shut-ins" a lively
glimpse of the outside world and its doings.

[Illustration: FIG. 195.--A vibrating tuning fork traces a curved line
on smoked glass.]

The phonograph consists of a cylinder or disk of wax upon which the
vibrations of a sensitive diaphragm are recorded by means of a fine
metal point. The action of the pointer in reporting the vibrations of
a diaphragm is easily understood by reference to a tuning fork. Fasten
a stiff bristle to a tuning fork by means of wax, allowing the end of
the point to rest lightly upon a piece of smoked glass. If the glass
is drawn under the bristle a straight line will be scratched on the
glass, but if the tuning fork is struck so that the prongs vibrate
back and forth, then the straight line changes to a wavy line and the
type of wavy line depends upon the fork used.

In the phonograph, a diaphragm replaces the tuning fork and a cylinder
(or a disk) coated with wax replaces the glass plate. When the speaker
talks or the singer sings, his voice strikes against a delicate
diaphragm and throws it into vibration, and the metal point attached
to it traces on the wax of a moving cylinder a groove of varying shape
and appearance called the "record." Every variation in the speaker's
voice is repeated in the vibrations of the metal disk and hence in the
minute motion of the pointer and in the consequent record on the
cylinder. The record thus made can be placed in any other phonograph
and if the metal pointer of this new phonograph is made to pass over
the tracing, the process is reversed and the speaker's voice is
reproduced. The sound given out in the this way is faint and weak, but
can be strengthened by means of a trumpet attached to the phonograph.

[Illustration: FIG. 196.--A phonograph. In this machine the cylinder
is replaced by a revolving disk.]



CHAPTER XXX

ELECTRICITY


280. Many animals possess the five senses, but only man possesses
constructive, creative power, and is able to build on the information
gained through the senses. It is the constructive, creative power
which raises man above the level of the beast and enables him to
devise and fashion wonderful inventions. Among the most important of
his inventions are those which relate to electricity; inventions such
as trolley car, elevator, automobile, electric light, the telephone,
the telegraph. Bell, by his superior constructive ability, made
possible the practical use of the telephone, and Marconi that of
wireless telegraphy. To these inventions might be added many others
which have increased the efficiency and production of the business
world and have decreased the labor and strain of domestic life.

[Illustration: FIG. 197.--A simple electric cell.]

281. Electricity as first Obtained by Man. Until modern times the
only electricity known to us was that of the lightning flash, which
man could neither hinder nor make. But in the year 1800, electricity
in the form of a weak current was obtained by Volta of Italy in a very
simple way; and even now our various electric batteries and cells are
but a modification of that used by Volta and called a voltaic cell. A
strip of copper and a strip of zinc are placed in a glass containing
dilute sulphuric acid, a solution composed of oxygen, hydrogen,
sulphur, and water. As soon as the plates are immersed in the acid
solution, minute bubbles of gas rise from the zinc strip and it begins
to waste away slowly. The solution gradually dissolves the zinc and at
the same time gives up some of the hydrogen which it contains; but it
has little or no effect on the copper, since there is no visible
change in the copper strip.

If, now, the strips are connected by means of metal wires, the zinc
wastes away rapidly, numerous bubbles of hydrogen pass over to the
copper strip and collect on it, and a current of electricity flows
through the connecting wires. Evidently, the source of the current is
the chemical action between the zinc and the liquid.

Mere inspection of the connecting wire will not enable us to detect
that a current is flowing, but there are various ways in which the
current makes itself evident. If the ends of the wires attached to the
strips are brought in contact with each other and then separated, a
faint spark passes, and if the ends are placed on the tongue, a twinge
is felt.

282. Experiments which grew out of the Voltaic Cell. Since chemical
action on the zinc is the source of the current, it would seem
reasonable to expect a current if the cell consisted of two zinc
plates instead of one zinc plate and one copper plate. But when the
copper strip is replaced by a zinc strip so that the cell consists of
two similar plates, no current flows between them. In this case,
chemical action is expended in heat rather than in the production of
electricity and the liquid becomes hot. But if carbon and zinc are
used, a current is again produced, the zinc dissolving away as before,
and bubbles collecting on the carbon plate. By experiment it has been
found that many different metals may be employed in the construction
of an electric cell; for example, current may be obtained from a cell
made with a zinc plate and a platinum plate, or from a cell made with
a lead plate and a copper plate. Then, too, some other chemical, such
as bichromate of potassium, or ammonium chloride, may be used instead
of dilute sulphuric acid.

Almost any two different substances will, under proper conditions,
give a current, but the strength of the current is in some cases so
weak as to be worthless for practical use, such as telephoning, or
ringing a door bell. What is wanted is a strong, steady current, and
our choice of material is limited to the substances which will give
this result. Zinc and lead can be used, but the current resulting is
weak and feeble, and for general use zinc and carbon are the most
satisfactory.

283. Electrical Terms. The plates or strips used in making an
electric cell are called electrodes; the zinc is called the negative
electrode (-), and the carbon the positive electrode (+); the current
is considered to flow through the wire from the + to the-electrode. As
a rule, each electrode has attached to it a binding post to which
wires can be quickly fastened.

The power that causes the current is called the electromotive force,
and the value of the electromotive force, generally written E.M.F., of
a cell depends upon the materials used.

When the cell consists of copper, zinc, and dilute sulphuric acid, the
electromotive force has a definite value which is always the same no
matter what the size or shape of the cell. But the E.M.F. has a
decidedly different value in a cell composed of iron, copper, and
chromic acid. Each combination of material has its own specific
electromotive force.

284. The Disadvantage of a Simple Cell. When the poles of a simple
voltaic cell are connected by a wire, the current thus produced
slowly diminishes in strength and, after a short time, becomes feeble.
Examination of the cell shows that the copper plate is covered with
hydrogen bubbles. If, however, these bubbles are completely brushed
away by means of a rod or stick, the current strength increases, but
as the bubbles again gather on the + electrode the current strength
diminishes, and when the bubbles form a thick film on the copper
plate, the current is too weak to be of any practical value. The film
of bubbles weakens the current because it practically substitutes a
hydrogen plate for a copper plate, and we saw in Section 282 that a
change in any one of the materials of which a cell is composed changes
the current.

This weakening of the current can be reduced mechanically by brushing
away the bubbles as soon as they are formed; or chemically, by
surrounding the copper plate with a substance which will combine with
the free hydrogen and prevent it from passing onward to the copper
plate.

[Illustration: FIG 198. The gravity cell.]

In practically all cells, the chemical method is used in preference to
the mechanical one. The numerous types of cells in daily use differ
chiefly in the devices employed for preventing the formation of
hydrogen bubbles, or for disposing of them when formed. One of the
best-known cells in which weakening of the current is prevented by
chemical means is the so-called gravity cell.

285. The Gravity Cell. A large, irregular copper electrode is placed
in the bottom of a jar (Fig. 198), and completely covered with a
saturated solution of copper sulphate. Then a large, irregular zinc
electrode is suspended from the top of the jar, and is completely
covered with dilute sulphuric acid which does not mix with the copper
sulphate, but floats on the top of it like oil on water. The hydrogen
formed by the chemical action of the dilute sulphuric acid on the zinc
moves toward the copper electrode, as in the simple voltaic cell. It
does not reach the electrode, however, because, when it comes in
contact with the copper sulphate, it changes places with the copper
there, setting it free, but itself entering into the solution. The
copper freed from the copper sulphate solution travels to the copper
electrode, and is deposited on it in a clean, bright layer. Instead of
a deposit of hydrogen there is a deposit of copper, and falling off in
current is prevented.

The gravity cell is cheap, easy to construct, and of constant
strength, and is in almost universal use in telegraphic work.
Practically all small railroad stations and local telegraph offices
use these cells.

[Illustration: FIG. 199.--A dry cell.]

286. Dry Cells. The gravity cell, while cheap and effective, is
inconvenient for general use, owing to the fact that it cannot be
easily transported, and the _dry cell_ has largely supplanted all
others, because of the ease with which it can be taken from place to
place. This cell consists of a zinc cup, within which is a carbon rod;
the space between the cup and rod is packed with a moist paste
containing certain chemicals. The moist paste takes the place of the
liquids used in other cells.

[Illustration: FIG. 200.--A battery of three cells.]

287. A Battery of Cells. The electromotive force of one cell may not
give a current strong enough to ring a door bell or to operate a
telephone. But by using a number of cells, called a battery, the
current may be increased to almost any desired strength. If three
cells are arranged as in Figure 200, so that the copper of one cell is
connected with the zinc of another cell, the electromotive force of
the battery will be three times as great as the E.M.F. of a single
cell. If four cells are arranged in the same way, the E.M.F. of the
battery is four times as great as the E.M.F. of a single cell; when
five cells are combined, the resulting E.M.F. is five times as great.



CHAPTER XXXI

SOME USES OF ELECTRICITY


288. Heat. Any one who handles electric wires knows that they are
more or less heated by the currents which flow through them. If three
cells are arranged as in Figure 200 and the connecting wire is coarse,
the heating of the wire is scarcely noticeable; but if a shorter wire
of the same kind is used, the heat produced is slightly greater; and
if the coarse wire is replaced by a short, fine wire, the heating of
the wire becomes very marked. We are accustomed to say that a wire
offers resistance to the flow of a current; that is, whenever a
current meets resistance, heat is produced in much the same way as
when mechanical motion meets an obstacle and spends its energy in
friction. The flow of electricity along a wire can be compared to the
flow of water through pipes: a small pipe offers a greater resistance
to the flow of water than a large pipe; less water can be forced
through a small pipe than through a large pipe, but the friction of
the water against the sides of the small pipe is much greater than in
the large one.

So it is with the electric current. In fine wires the resistance to
the current is large and the energy of the battery is expended in heat
rather than in current. If the heat thus produced is very great,
serious consequences may arise; for example, the contact of a hot wire
with wall paper or dry beams may cause fire. Insurance companies
demand that the wires used in wiring a building for electric lights be
of a size suitable to the current to be carried, otherwise they will
not take the risk of insurance. The greater the current to be carried,
the coarser is the wire required for safety.

289. Electric Stoves. It is often desirable to utilize the electric
current for the production of heat. For example, trolley cars are
heated by coils of wire under the seats. The coils offer so much
resistance to the passage of a strong current through them that they
become heated and warm the cars.

[Illustration: FIG. 201.--An electric iron on a metal stand.]

Some modern houses are so built that electricity is received into them
from the great plants where it is generated, and by merely turning a
switch or inserting a plug, electricity is constantly available. In
consequence, many practical applications of electricity are possible,
among which are flatiron and toaster.

[Illustration: FIG. 202.--The fine wires are strongly heated by the
current which flows through them.]

Within the flatiron (Fig. 201), is a mass of fine wire coiled as shown
in Figure 202; as soon as the iron is connected with the house supply
of electricity, current flows through the fine wire which thus becomes
strongly heated and gives off heat to the iron. The iron, when once
heated, retains an even temperature as long as the current flows, and
the laundress is, in consequence, free from the disadvantages of a
slowly cooling iron, and of frequent substitution of a warm iron for a
cold one. Electric irons are particularly valuable in summer, because
they eliminate the necessity for a strong fire, and spare the
housewife intense heat. In addition, the user is not confined to the
laundry, but is free to seek the coolest part of the house, the only
requisite being an electrical connection.

[Illustration: FIG. 203.--Bread can be toasted by electricity.]

The toaster (Fig. 203) is another useful electrical device, since by
means of it toast may be made on a dining table or at a bedside. The
small electrical stove, shown in Figure 204, is similar in principle
to the flatiron, but in it the heating coil is arranged as shown in
Figure 205. To the physician electric stoves are valuable, since his
instruments can be sterilized in water heated by the stove; and that
without fuel or odor of gas.

A convenient device is seen in the heating pad (Fig. 206), a
substitute for a hot water bag. Embedded in some soft thick substance
are the insulated wires in which heat is to be developed, and over
this is placed a covering of felt.

[Illustration: FIG. 204.--An electric stove.]

290. Electric Lights. The incandescent bulbs which illuminate our
buildings consist of a fine, hairlike thread inclosed in a glass bulb
from which the air has been removed. When an electric current is sent
through the delicate filament, it meets a strong resistance. The heat
developed in overcoming the resistance is so great that it makes the
filament a glowing mass. The absence of air prevents the filament from
burning, and it merely glows and radiates the light.

[Illustration: FIG. 205.--The heating element in the electric stove.]

291. Blasting. Until recently, dynamiting was attended with serious
danger, owing to the fact that the person who applied the torch to the
fuse could not make a safe retreat before the explosion. Now a fine
wire is inserted in the fuse, and when everything is in readiness,
the ends of the wire are attached to the poles of a distant battery
and the heat developed in the wire ignites the fuse.

[Illustration: FIG. 206.--An electric pad serves the same purpose as a
hot water bag.]

292. Welding of Metals. Metals are fused and welded by the use of
the electric current. The metal pieces which are to be welded are
pressed together and a powerful current is passed through their
junction. So great is the heat developed that the metals melt and
fuse, and on cooling show perfect union.

293. Chemical Effects. _The Plating of Gold, Silver, and Other
Metals._ If strips of lead or rods of carbon are connected to the
terminals of an electric cell, as in Figure 208, and are then dipped
into a solution of copper sulphate, the strip in connection with the
negative terminal of the cell soon becomes thinly plated with a
coating of copper. If a solution of silver nitrate is used in place of
the copper sulphate, the coating formed will be of silver instead of
copper. So long as the current flows and there is any metal present in
the solution, the coating continues to form on the negative electrode,
and becomes thicker and thicker.

[Illustration: FIG. 207.--An incandescent electric bulb.]

The process by which metal is taken out of solution, as silver out of
silver nitrate and copper out of copper sulphate, and is in turn
deposited as a coating on another substance, is called electroplating.
An electric current can separate a liquid into some of its various
constituents and to deposit one of the metal constituents on the
negative electrode.

[Illustration: FIG. 208.--Carbon rods in a solution of copper
sulphate.]

Since copper is constantly taken out of the solution of copper
sulphate for deposit upon the negative electrode, the amount of copper
remaining in the solution steadily decreases, and finally there is
none of it left for deposit. In order to overcome this, the positive
electrode should be made of the same metal as that which is to be
deposited. The positive metal electrode gradually dissolves and
replaces the metal lost from the solution by deposit and
electroplating can continue as long as any positive electrode remains.

[Illustration: FIG. 209.--Plating spoons by electricity.]

Practically all silver, gold, and nickel plating is done in this way;
machine, bicycle, and motor attachments are not solid, but are of
cheaper material electrically plated with nickel. When spoons are to
be plated, they are hung in a bath of silver nitrate side by side with
a thick slab of pure silver, as in Figure 209. The spoons are
connected with the negative terminal of the battery, while the slab of
pure silver is connected with the positive terminal of the same
battery. The length of time that the current flows determines the
thickness of the plating.

294. How Pure Metal is obtained from Ore. When ore is mined, it
contains in addition to the desired metal many other substances. In
order to separate out the desired metal, the ore is placed in some
suitable acid bath, and is connected with the positive terminal of a
battery, thus taking the place of the silver slab in the last Section.
When current flows, any pure metal which is present is dissolved out
of the ore and is deposited on a convenient negative electrode, while
the impurities remain in the ore or drop as sediment to the bottom of
the vessel. Metals separated from the ore by electricity are called
electrolytic metals and are the purest obtainable.

295. Printing. The ability of the electric current to decompose a
liquid and to deposit a metal constituent has practically
revolutionized the process of printing. Formerly, type was arranged
and retained in position until the required number of impressions had
been made, the type meanwhile being unavailable for other uses.
Moreover, the printing of a second edition necessitated practically as
great labor as did the first edition, the type being necessarily set
afresh. Now, however, the type is set up and a mold of it is taken in
wax. This mold is coated with graphite to make it a conductor and is
then suspended in a bath of copper sulphate, side by side with a slab
of pure copper. Current is sent through the solution as described in
Section 293, until a thin coating of copper has been deposited on the
mold. The mold is then taken from the bath, and the wax is replaced by
some metal which gives strength and support to the thin copper plate.
From this copper plate, which is an exact reproduction of the original
type, many thousand copies can be printed. The plate can be preserved
and used from time to time for later editions, and the original type
can be put back into the cases and used again.



CHAPTER XXXII

MODERN ELECTRICAL INVENTIONS


296. An Electric Current acts like a Magnet. In order to understand
the action of the electric bell, we must consider a third effect which
an electric current can cause. Connect some cells as shown in Figure
200 and close the circuit through a stout heavy copper wire, dipping a
portion of the wire into fine iron filings. A thick cluster of filings
will adhere to the wire (Fig. 210), and will continue to cling to it
so long as the current flows. If the current is broken, the filings
fall from the wire, and only so long as the current flows through the
wire does the wire have power to attract iron filings. An electric
current makes a wire equivalent to a magnet, giving it the power to
attract iron filings.

[Illustration: FIG. 210.--A wire carrying current attracts iron
filings.]

[Illustration: FIG. 211.--A loosely wound coil of wire.]

Although such a straight current bearing wire attracts iron filings,
its power of attraction is very small; but its magnetic strength can
be increased by coiling as in Figure 211. Such an arrangement of wire
is known as a helix or solenoid, and is capable of lifting or pulling
larger and more numerous filings and even good-sized pieces of iron,
such as tacks. Filings do not adhere to the sides of the helix, but
they cling in clusters to the ends of the coil. This shows that the
ends of the helix have magnetic power but not the sides.

If a soft iron nail (Fig. 212) or its equivalent is slipped within the
coil, the lifting and attractive power of the coil is increased, and
comparatively heavy weights can be lifted.

[Illustration: FIG. 212.--Coil and soft iron rod.]

A coil of wire traversed by an electric current and containing a core
of soft iron has the power of attracting and moving heavy iron
objects; that is, it acts like a magnet. Such an arrangement is called
an electromagnet. As soon as the current ceases to flow, the
electromagnet loses its magnetic power and becomes merely iron and
wire without magnetic attraction.

If many cells are used, the strength of the electromagnet is
increased, and if the coil is wound closely, as in Figure 213, instead
of loosely, as in Figure 211, the magnetic strength is still further
increased. The strength of any electromagnet depends upon the number
of coils wound on the iron core and upon the strength of the current
which is sent through the coils.

[Illustration: FIG. 213.--An electromagnet.]

[Illustration: FIG. 214.--A horseshoe electromagnet is powerful enough
to support heavy weights.]

To increase the strength of the electromagnet still further, the
so-called horseshoe shape is used (Fig. 214). In such an arrangement
there is practically the strength of two separate electromagnets.

297. The Electric Bell. The ringing of the electric bell is due to
the attractive power of an electromagnet. By the pushing of a button
(Fig. 215) connection is made with a battery, and current flows
through the wire wound on the iron spools, and further to the screw
_P_ which presses against the soft iron strip or armature _S_; and
from _S_ the current flows back to the battery. As soon as the
current flows, the coils become magnetic and attract the soft iron
armature, drawing it forward and causing the clapper to strike the
bell. In this position, _S_ no longer touches the screw _P_, and hence
there is no complete path for the electricity, and the current ceases.
But the attractive, magnetic power of the coils stops as soon as the
current ceases; hence there is nothing to hold the armature down, and
it flies back to its former position. In doing this, however, the
armature makes contact at _P_ through the spring, and the current
flows once more; as a result the coils again become magnets, the
armature is again drawn forward, and the clapper again strikes the
bell. But immediately afterwards the armature springs backward and
makes contact at _P_ and the entire operation is repeated. So long as
we press the button this process continues producing what sounds like
a continuous jingle; in reality the clapper strikes the bell every
time a current passes through the electromagnet.

[Illustration: FIG. 215.--The electric bell.]

298. The Push Button. The push button is an essential part of every
electric bell, because without it the bell either would not ring at
all, or would ring incessantly until the cell was exhausted. When the
push button is free, as in Figure 216, the cell terminals are not
connected in an unbroken path, and hence the current does not flow.
When, however, the button is pressed, the current has a complete path,
provided there is the proper connection at _S_. That is, the pressure
on the push button permits current to flow to the bell. The flow of
this current then depends solely upon the connection at _S_, which is
alternately made and broken, and in this way produces sound.

[Illustration: FIG. 216.--Push button.]

The sign "Bell out of order" is usually due to the fact that the
battery is either temporarily or permanently exhausted. In warm
weather the liquid in the cell may dry up and cause stoppage of the
current. If fresh liquid is poured into the vessel so that the
chemical action of the acid on the zinc is renewed, the current again
flows. Another explanation of an out-of-order bell is that the liquid
may have eaten up all the zinc; if this is the case, the insertion of
a fresh strip of zinc will remove the difficulty and the current will
flow. If dry cells are used, there is no remedy except in the purchase
of new cells.

299. How Electricity may be lost to Use. In the electric bell, we
saw that an air gap at the push button stopped the flow of
electricity. If we cut the wire connecting the poles of a battery, the
current ceases because an air gap intervenes and electricity does not
readily pass through air. Many substances besides air stop the flow of
electricity. If a strip of glass, rubber, mica, or paraffin is
introduced anywhere in a circuit, the current ceases. If a metal is
inserted in the gap, the current again flows. Substances which, like
an air gap, interfere with the flow of electricity are called
non-conductors, or, more commonly, insulators. Substances which, like
the earth, the human body, and all other moist objects, conduct
electricity are conductors. If the telephone and electric light wires
in our houses were not insulated by a covering of thread, or cloth, or
other non conducting material, the electricity would escape into
surrounding objects instead of flowing through the wire and producing
sound and light.

In our city streets, the overhead wires are supported on glass knobs
or are closely wrapped, in order to prevent the escape of electricity
through the poles to the ground. In order to have a steady, dependable
current, the wire carrying the current must be insulated.

Lack of insulation means not only the loss of current for practical
uses, but also serious consequences in the event of the crossing of
current-bearing wires. If two wires properly insulated touch each
other, the currents flow along their respective wires unaltered; if,
however, two uninsulated wires touch, some of the electricity flows
from one to the other. Heat is developed as a result of this
transference, and the heat thus developed is sometimes so great that
fire occurs. For this reason, wires are heavily insulated and extra
protection is provided at points where numerous wires touch or cross.

Conductors and insulators are necessary to the efficient and economic
flow of a current, the insulator preventing the escape of electricity
and lessening the danger of fire, and the conductor carrying the
current.

300. The Telegraph. Telegraphy is the process of transmitting
messages from place to place by means of an electric current. The
principle underlying the action of the telegraph is the principle upon
which the electric bell operates; namely, that a piece of soft iron
becomes a magnet while a current flows around it, but loses its
magnetism as soon as the current ceases.

In the electric bell, the electromagnet, clapper, push button, and
battery are relatively near,--usually all are located in the same
building; while in the telegraph the current may travel miles before
it reaches the electromagnet and produces motion of the armature.

[Illustration: FIG. 217.--Diagram of the electric telegraph.]

The fundamental connections of the telegraph are shown in Figure 217.
If the key _K_ is pressed down by an operator in Philadelphia, the
current from the battery (only one cell is shown for simplicity) flows
through the line to New York, passes through the electromagnet _M_,
and thence back to Philadelphia. As long as the key _K_ is pressed
down, the coil _M_ acts as a magnet and attracts and holds fast the
armature _A_; but as soon as _K_ is released, the current is broken,
_M_ loses its magnetism, and the armature is pulled back by the spring
_D_. By a mechanical device, tape is drawn uniformly under the light
marker _P_ attached to the armature. If _K_ is closed for but a short
time, the armature is drawn down for but a short interval, and the
marker registers a dot on the tape. If _K_ is closed for a longer
time, a short dash is made by the marker, and, in general, the length
of time that _K_ is closed determines the length of the marks recorded
on the tape. The telegraphic alphabet consists of dots and dashes and
their various combinations, and hence an interpretation of the dot and
dash symbols recorded on the tape is all that is necessary for the
receiving of a telegraphic message.

The Morse telegraphic code, consisting of dots, dashes, and spaces, is
given in Figure 218.

[Illustration:

  |A .-   |H .... |O . .   |U ..-   |
  |B -... |I ..   |P ..... |V ...-  |
  |C .. . |J -.-. |Q ..-.  |W .--   |
  |D -..  |K -.-  |R . ..  |X .-..  |
  |E .    |L ---  |S ...   |Y .. .. |
  |F .-.  |M - -  |T -     |Z ... . |
  |G --.  |N -.   |        |        |

FIG. 218.--The Morse telegraphic code.]

The telegraph is now such a universal means of communication between
distant points that one wonders how business was conducted before its
invention in 1832 by S.F.B. Morse.

[Illustration: FIG. 219.--The sounder.]

301. Improvements. _The Sounder._ Shortly after the invention of
telegraphy, operators learned that they could read the message by the
click of the marker against a metal rod which took the place of the
tape. In practically all telegraph offices of the present day the
old-fashioned tape is replaced by the sounder, shown in Figure 219.
When current flows, a lever, _L_, is drawn down by the electromagnet
and strikes against a solid metal piece with a click; when the current
is broken, the lever springs upward, strikes another metal piece and
makes a different click. It is clear that the working of the key which
starts and stops the current in this line will be imitated by the
motion and the resulting clicks of the sounder. By means of these
varying clicks of the sounder, the operator interprets the message.

[Illustration: FIG. 220.--Diagram of a modern telegraph system.]

_The Relay._ When a telegraph line is very long, the resistance of the
wire is great, and the current which passes through the electromagnet
is correspondingly weak, so feeble indeed that the armature must be
made very thin and light in order to be affected by the makes and
breaks in the current. The clicks of an armature light enough to
respond to the weak current of a long wire are too faint to be
recognized by the ear, and hence in such long circuits some device
must be introduced whereby the effect is increased. This is usually
done by installing at each station a local battery and a very delicate
and sensitive electromagnet called the _relay_. Under these conditions
the current of the main line is not sent through the sounder, but
through the relay which opens and closes a local battery in connection
with the strong sounder. For example, the relay is so arranged that
current from the main line runs through it exactly as it runs through
_M_ in Figure 217. When current is made, the relay attracts an
armature, which thereby closes a circuit in a local battery and thus
causes a click of the sounder. When the current in the main line is
broken, the relay loses its magnetic attraction, its armature springs
back, connection is broken in the local circuit, and the sounder
responds by allowing its armature to spring back with a sharp sound.

302. The Earth an Important Part of a Telegraphic System. We learned
in Section 299 that electricity could flow through many different
substances, one of which was the earth. In all ordinary telegraph
lines, advantage is taken of this fact to utilize the earth as a
conductor and to dispense with one wire. Originally two wires were
used, as in Figure 217; then it was found that a railroad track could
be substituted for one wire, and later that the earth itself served
equally well for a return wire. The present arrangement is shown in
Figure 220, where there is but one wire, the circuit being completed
by the earth. No fact in electricity seems more marvelous than that
the thousands of messages flashing along the wires overhead are
likewise traveling through the ground beneath. If it were not for this
use of the earth as an unfailing conductor, the network of overhead
wires in our city streets would be even more complex than it now is.

303. Advances in Telegraphy. The mechanical improvements in
telegraphy have been so rapid that at present a single operator can
easily send or receive forty words a minute. He can telegraph more
quickly than the average person can write; and with a combination of
the latest improvements the speed can be enormously increased.
Recently, 1500 words were flashed from New York to Boston over a
single wire in one second.

In actual practice messages are not ordinarily sent long distances
over a direct line, but are automatically transferred to new lines at
definite points. For example, a message from New York to Chicago does
not travel along an uninterrupted path, but is automatically
transferred at some point, such as Lancaster, to a second line which
carries it on to Pittsburgh, where it is again transferred to a third
line which takes it farther on to its destination.



CHAPTER XXXIII

MAGNETS AND CURRENTS


304. In the twelfth century, there was introduced into Europe from
China a simple instrument which changed journeying on the sea from
uncertain wandering to a definite, safe voyage. This instrument was
the compass (Fig. 221), and because of the property of the compass
needle (a magnet) to point unerringly north and south, sailors were
able to determine directions on the sea and to steer for the desired
point.

[Illustration: FIG. 221.--The compass.]

Since an electric current is practically equivalent to a magnet
(Section 296), it becomes necessary to know the most important facts
relative to magnets, facts simple in themselves but of far-reaching
value and consequences in electricity. Without a knowledge of the
magnetic characteristics of currents, the construction of the motor
would have been impossible, and trolley cars, electric fans, motor
boats, and other equally well-known electrical contrivances would be
unknown.

305. The Attractive Power of a Magnet. The magnet best known to us
all is the compass needle, but for convenience we will use a magnetic
needle in the shape of a bar larger and stronger than that employed in
the compass. If we lay such a magnet on a pile of iron filings, it
will be found on lifting the magnet that the filings cling to the ends
in tufts, but leave it almost bare in the center (Fig. 222). The
points of attraction at the two ends are called the poles of the
magnet.

[Illustration: FIG. 222.--A magnet.]

If a delicately made magnet is suspended as in Figure 223, and is
allowed to swing freely, it will always assume a definite north and
south position. The pole which points north when the needle is
suspended is called the north pole and is marked _N_, while the pole
which points south when the needle is suspended is called the south
pole and is marked _S_.

A freely suspended magnet points nearly north and south.

A magnet has two main points of attraction called respectively the
north and south poles.

[Illustration: FIG. 223.--The magnetic needle.]

306. The Extent of Magnetic Attraction. If a thin sheet of paper or
cardboard is laid over a strong, bar-shaped magnet and iron filings
are then gently strewn on the paper, the filings clearly indicate the
position of the magnet beneath, and if the cardboard is gently tapped,
the filings arrange themselves as shown in Figure 224. If the paper is
held some distance above the magnet, the influence on the filings is
less definite, and finally, if the paper is held very far away, the
filings do not respond at all, but lie on the cardboard as dropped.

The magnetic power of a magnet, while not confined to the magnet
itself, does not extend indefinitely into the surrounding region; the
influence is strong near the magnet, but at a distance becomes so weak
as to be inappreciable. The region around a magnet through which its
magnetic force is felt is called the field of force, or simply the
magnetic field, and the definite lines in which the filings arrange
themselves are called lines of force.

[Illustration: FIG. 224.--Iron filings scattered over a magnet arrange
themselves in definite lines.]

The magnetic power of a magnet is not limited to the magnet, but
extends to a considerable distance in all directions.

307. The Influence of Magnets upon Each Other. If while our
suspended magnetic needle is at rest in its characteristic
north-and-south direction another magnet is brought near, the
suspended magnet is turned; that is, motion is produced (Fig. 225). If
the north pole of the free magnet is brought toward the south pole of
the suspended magnet, the latter moves in such a way that the two
poles _N_ and _S_ are as close together as possible. If the north pole
of the free magnet is brought toward the north pole of the suspended
magnet, the latter moves in such a way that the two poles _N_ and _N_
are as far apart as possible. In every case that can be tested, it is
found that a north pole repels a north pole, and a south pole repels a
south pole; but that a north and a south pole always attract each
other.

[Illustration: FIG. 225.--A south pole attracts a north pole.]

The main facts relative to magnets may be summed up as follows:--

_a_. A magnet points nearly north and south if it is allowed to swing
freely.

_b_. A magnet contains two unlike poles, one of which persistently
points north, and the other of which as persistently points south, if
allowed to swing freely.

_c_. Poles of the same name repel each other; poles of unlike name
attract each other.

_d_. A magnet possesses the power of attracting certain substances,
like iron, and this power of attraction is not limited to the magnet
itself but extends into the region around the magnet.

308. Magnetic Properties of an Electric Current. If a
current-bearing wire is really equivalent in its magnetic powers to a
magnet, it must possess all of the characteristics mentioned in the
preceding Section. We saw in Section 296 that a coiled wire through
which current was flowing would attract iron filings at the two ends
of the helix. That a coil through which current flows possesses the
characteristics _a_, _b_, _c_, and _d_ of a magnet is shown as follows:--

_a_, _b_. If a helix marked at one end with a red string is arranged so
that it is free to rotate and a strong current is sent through it,
the helix will immediately turn and face about until it points north
and south. If it is disturbed from this position, it will slowly swing
back until it occupies its characteristic north and south position.
The end to which the string is attached will persistently point either
north or south. If the current is sent through the coil in the
opposite direction, the two poles exchange positions and the helix
turns until the new north pole points north.

[Illustration: FIG. 226.--A helix through which current flows always
points north and south, if it is free to rotate.]

_c_. If a coil conducting a current is held near a suspended magnet,
one end of the helix will be found to attract the north pole of the
magnet, while the opposite end will be found to repel the north pole
of the magnet. In fact, the helix will be found to behave in every
way as a magnet, with a north pole at one end and a south pole at the
other. If the current is sent through the helix in the opposite
direction, the north and south poles exchange places.

[Illustration: FIG. 227.--A wire through which current flows is
surrounded by a field of magnetic force.]

If the number of turns in the helix is reduced until but a single loop
remains, the result is the same; the single loop acts like a flat
magnet, one side of the loop always facing northward and one
southward, and one face attracting the north pole of the suspended
magnet and one repelling it.

_d_. If a wire is passed through a card and a strong current is sent
through the wire, iron filings will, when sprinkled upon the card,
arrange themselves in definite directions (Fig. 227). A wire carrying
a current is surrounded by a magnetic field of force.

A magnetic needle held under a current-bearing wire turns on its pivot
and finally comes to rest at an angle with the current. The fact that
the needle is deflected by the wire shows that the magnetic power of
the wire extends into the surrounding medium.

The magnetic properties of current electricity were discovered by
Oersted of Denmark less than a hundred years ago; but since that time
practically all important electrical machinery has been based upon one
or more of the magnetic properties of electricity. The motors which
drive our electric fans, our mills, and our trolley cars owe their
existence entirely to the magnetic action of current electricity.

[Illustration: FIG. 228.--The coil turns in such a way that its north
pole is opposite the south pole of the magnet.]

309. The Principle of the Motor. If a close coil of wire is
suspended between the poles of a strong horseshoe magnet, it will not
assume any characteristic position but will remain wherever placed.
If, however, a current is sent through the wire, the coil faces about
and assumes a definite position. This is because a coil, carrying a
current, is equivalent to a magnet with a north and south face; and,
in accordance with the magnetic laws, tends to move until its north
face is opposite the south pole of the horseshoe magnet, and its south
face opposite the north pole of the magnet. If, when the coil is at
rest in this position, the current is reversed, so that the north pole
of the coil becomes a south pole and the former south pole becomes a
north pole, the result is that like poles of coil and magnet face each
other. But since like poles repel each other, the coil will move, and
will rotate until its new north pole is opposite to the south pole of
the magnet and its new south pole is opposite the north pole. By
sending a strong current through the coil, the helix is made to rotate
through a half turn; by reversing the current when the coil is at the
half turn, the helix is made to continue its rotation and to swing
through a whole turn. If the current could be repeatedly reversed just
as the helix completed its half turn, the motion could be prolonged;
periodic current reversal would produce continuous rotation. This is
the principle of the motor.

[Illustration: FIG. 229.--Principle of the motor.]

It is easy to see that long-continued rotation would be impossible in
the arrangement of Figure 228, since the twisting of the suspending
wire would interfere with free motion. If the motor is to be used for
continuous motion, some device must be employed by means of which the
helix is capable of continued rotation around its support.

In practice, the rotating coil of a motor is arranged as shown in
Figure 229. Wires from the coil terminate on metal disks and are
securely soldered there. The coil and disks are supported by the
strong and well-insulated rod _R_, which rests upon braces, but which
nevertheless rotates freely with disks and coil. The current flows to
the coil through the thin metal strips called brushes, which rest
lightly upon the disks.

When the current which enters at _B_ flows through the wire, the coil
rotates, tending to set itself so that its north face is opposite the
south face of the magnet. If, when the helix has just reached this
position, the current is reversed--entering at _B'_ instead of
_B_--the poles of the coil are exchanged; the rotation, therefore,
does not cease, but continues for another half turn. Proper reversals
of the current are accompanied by continuous motion, and since the
disk and shaft rotate with the coil, there is continuous rotation.

If a wheel is attached to the rotating shaft, weights can be lifted,
and if a belt is attached to the wheel, the motion of the rotating
helix can be transferred to machinery for practical use.

The rotating coil is usually spoken of as the armature, and the large
magnet as the field magnet.

310. Mechanical Reversal of the Current. _The Commutator_. It is not
possible by hand to reverse the current with sufficient rapidity and
precision to insure uninterrupted rotation; moreover, the physical
exertion of such frequent reversals is considerable. Hence, some
mechanical device for periodically reversing the current is necessary,
if the motor is to be of commercial value.

[Illustration: FIG. 230.--The commutator.]

The mechanical reversal of the current is accomplished by the use of
the commutator, which is a metal ring split into halves, well
insulated from each other and from the shaft. To each half of this
ring is attached one of the ends of the armature wire. The brushes
which carry the current are set on opposite sides of the ring and do
not rotate. As armature, commutator, and shaft rotate, the brushes
connect first with one segment of the commutator and then with the
other. Since the circuit is arranged so that the current always enters
the commutator through the brush _B_, the flow of the current into the
coil is always through the segment in contact with _B_; but the
segment in contact with _B_ changes at every half turn of the coil,
and hence the direction of the current through the coil changes
periodically. As a result the coil rotates continuously, and produces
motion so long as current is supplied from without.

311. The Practical Motor. A motor constructed in accordance with
Section 309 would be of little value in practical everyday affairs;
its armature rotates too slowly and with too little force. If a motor
is to be of real service, its armature must rotate with sufficient
strength to impart motion to the wheels of trolley cars and mills, to
drive electric fans, and to set into activity many other forms of
machinery.

The strength of a motor may be increased by replacing the singly
coiled armature by one closely wound on an iron core; in some
armatures there are thousands of turns of wire. The presence of soft
iron within the armature (Section 296) causes greater attraction
between the armature and the outside magnet, and hence greater force
of motion. The magnetic strength of the field magnet influences
greatly the speed of the armature; the stronger the field magnet the
greater the motion, so electricians make every effort to strengthen
their field magnets. The strongest known magnets are electromagnets,
which, as we have seen, are merely coils of wire wound on an iron
core. For this reason, the field magnet is usually an electromagnet.

When very powerful motors are necessary, the field magnet is so
arranged that it has four or more poles instead of two; the armature
likewise consists of several portions, and even the commutator may be
very complex. But no matter how complex these various parts may seem
to be, the principle is always that stated in Section 309, and the
parts are limited to field magnet, commutator, and armature.

[Illustration: FIG. 231.--A modern power plant.]

[Illustration: FIG. 232.--The electric street car.]

The motor is of value because by means of it motion, or mechanical
energy, is obtained from an electric current. Nearly all electric
street cars (Fig. 232), are set in motion by powerful motors placed
under the cars. As the armature rotates, its motion is communicated by
gears to the wheels, the necessary current reaching the motor through
the overhead wires. Small motors may be used to great advantage in the
home, where they serve to turn the wheels of sewing machines, and to
operate washing machines. Vacuum cleaners are frequently run by
motors.



CHAPTER XXXIV

HOW ELECTRICITY MAY BE MEASURED


312. Danger of an Oversupply of Current. If a small toy motor is
connected with one cell, it rotates slowly; if connected with two
cells, it rotates more rapidly, and in general, the greater the number
of cells used, the stronger will be the action of the motor. But it is
possible to send too strong a current through our wire, thereby
interfering with all motion and destroying the motor. We have seen in
Section 288 that the amount of current which can safely flow through a
wire depends upon the thickness of the wire. A strong current sent
through a fine wire has its electrical energy transformed largely into
heat; and if the current is very strong, the heat developed may be
sufficient to burn off the insulation and melt the wire itself. This
is true not only of motors, but of all electric machinery in which
there are current-bearing wires. The current should not be greater
than the wires can carry, otherwise too much heat will be developed
and damage will be done to instruments and surroundings.

The current sent through our electric stoves and irons should be
strong enough to heat the coils, but not strong enough to melt them.
If the current sent through our electric light wires is too great for
the capacity of the wires, the heat developed will injure the wires
and may cause disastrous results. The overloading of wires is
responsible for many disastrous fires.

The danger of overloading may be eliminated by inserting in the
circuit a fuse or other safety device. A fuse is made by combining a
number of metals in such a way that the resulting substance has a low
melting point and a high electrical resistance. A fuse is inserted in
the circuit, and the instant the current increases beyond its normal
amount the fuse melts, breaks the circuit, and thus protects the
remaining part of the circuit from the danger of an overload. In this
way, a circuit designed to carry a certain current is protected from
the danger of an accidental overload. The noise made by the burning
out of a fuse in a trolley car frequently alarms passengers, but it is
really a sign that the system is in good working order and that there
is no danger of accident from too strong a current.

313. How Current is Measured. The preceding Section has shown
clearly the danger of too strong a current, and the necessity for
limiting the current to that which the wire can safely carry. There
are times when it is desirable to know accurately the strength of a
current, not only in order to guard against an overload, but also in
order to determine in advance the mechanical and chemical effects
which will be produced by the current. For example, the strength of
the current determines the thickness of the coating of silver which
forms in a given time on a spoon placed in an electrolytic bath; if
the current is weak, a thin plating is made on the spoon; if the
current is strong, a thick plating is made. If, therefore, the exact
value of the current is known, the exact amount of silver which will
be deposited on the spoon in a given time can be definitely
calculated.

[Illustration: FIG. 233.--The principle of the galvanometer.]

Current-measuring instruments, or galvanometers, depend for their
action on the magnetic properties of current electricity. The
principle of practically all galvanometers is as follows:--

A closely wound coil of fine wire free to rotate is suspended as in
Figure 233 between the poles of a strong magnet. When a current is
sent through the coil, the coil becomes a magnet and turns so that its
faces will be towards the poles of the permanent magnet. But as the
coil turns, the suspending wire becomes twisted and hinders the
turning. For this reason, the coil can turn only until the motion
caused by the current is balanced by the twist of the suspending wire.
But the stronger the current through the coil, the stronger will be
the force tending to rotate the coil, and hence the less effective
will be the hindrance of the twisting string. As a consequence, the
coil swings farther than before; that is, the greater the current, the
farther the swing. Usually a delicate pointer is attached to the
movable coil and rotates freely with it, so that the swing of the
pointer indicates the relative values of the current. If the source of
the current is a gravity cell, the swing is only two thirds as great
as when a dry cell is used, indicating that the dry cell furnishes
about 1-1/2 times as much current as a gravity cell.

314. Ammeters. A galvanometer does not measure the current, but
merely indicates the relative strength of different currents. But it
is desirable at times to measure a current in units. Instruments for
measuring the strength of currents in units are called ammeters, and
the common form makes use of a galvanometer.

A current is sent through a movable coil (the field magnet and coil
are inclosed in the case) (Fig. 234), and the magnetic field thus
developed causes the coil to turn, and the pointer attached to it to
move over a scale graduated so that it reads current strengths. This
scale is carefully graduated by the following method.

If two silver rods (Fig. 208) are weighed and placed in a solution of
silver nitrate, and current from a single cell is passed through the
liquid for a definite time, we find, on weighing the two rods, that
one has gained in weight and the other has lost. If the current is
allowed to flow twice as long, the amount of silver lost and gained by
the electrodes is doubled; and if twice the current is used, the
result is again doubled.

As a result of numerous experiments, it was found that a definite
current of electricity will deposit a definite amount of silver in a
definite time, and that the amount of silver deposited on an electrode
in one second might be used to measure the current of electricity
which has flowed through the circuit in one second.

A current is said to be one ampere strong if it will deposit silver on
an electrode at the rate of 0.001118 gram per second.

[Illustration: FIG. 234.--An ammeter.]

In marking the scale, an ammeter is placed in the circuit of an
electrolytic cell and the position of the pointer is marked on the
blank card which lies beneath and which is to serve as a scale (Fig.
235). After the current has flowed for about an hour, the amount of
silver which has been deposited is measured. Knowing the time during
which the current has run, and the amount of deposit, the strength of
the current in amperes can be calculated. This number is written
opposite the place at which the pointer stood during the experiment.

The scale may be completed by marking the positions of the pointer
when other currents of known strength flow through the ammeter.

[Illustration: FIG. 235.--Marking the scale of an ammeter.]

All electric plants, whether for heating, lighting, or for machinery,
are provided with ammeters, such instruments being as important to an
electric plant as the steam gauge is to the boiler.

315. Voltage and Voltmeters. Since electromotive force, or voltage,
is the cause of current, it should be possible to compare different
electromotive forces by comparing the currents which they produce in a
given circuit. But two voltages of equal value do not give equal
currents unless the resistances met by the currents are equal. For
example, the simple voltaic cell and the gravity cell have
approximately equal voltages, but the current produced by the voltaic
cell is stronger than that produced by the gravity cell. This is
because the current meets more resistance within the gravity cell
than within the voltaic cell. Every cell, no matter what its nature,
offers resistance to the flow of electricity through it and is said to
have internal resistance. If we are determining the voltages of
various cells by a comparison of the respective currents produced, the
result will be true only on condition that the resistances in the
various circuits are equal. If a very large external resistance of
fine wire is placed in circuit with a gravity cell, the _total_
resistance of the circuit (made up of the relatively small resistance
in the cell and the larger resistance in the rest of the circuit) will
differ but little from that of another circuit in which the gravity
cell is replaced by a voltaic cell, or any other type of cell.

With a high resistance in the outside circuit, the deflections of the
ammeter will be small, but such as they are, they will fairly
accurately represent the electromotive forces which produce them.

Voltmeters (Fig. 236), or instruments for measuring voltage, are like
ammeters except that a wire of very high resistance is in circuit with
the movable coil. In external appearance they are not distinguishable
from ammeters.

[Illustration: FIG. 236.--A voltmeter.]

The unit of electromotive force is called the _volt_. The voltage of a
dry cell is approximately 1.5 volts, and the voltage of a voltaic cell
and of a gravity cell is approximately 1 volt.

316. Current, Voltage, Resistance. We learned in Section 287 that
the strength of a current increases when the electromotive force
increases, and diminishes when the electromotive force diminishes.
Later, in Section 288, we learned that the strength of the current
decreases as the resistance in circuit increases.

The strength of a steady current depends upon these two factors only,
the electromotive force which causes it and the resistance which it
has to overcome.

317. Resistance. Since resistance plays so important a rôle in
electricity, it becomes necessary to have a unit of resistance. The
practical unit of resistance is called an ohm, and some idea of the
value of an ohm can be obtained if we remember that a 300-foot length
of common iron telegraph wire has a resistance of 1 ohm. An
approximate ohm for rough work in the laboratory may be made by
winding 9 feet 5 inches of number 30 copper wire on a spool or
arranging it in any other convenient form.

In Section 299 we learned that substances differ very greatly in the
resistance which they offer to electricity, and so it will not
surprise us to learn that while it takes 300 feet of iron telegraph
wire to give 1 ohm of resistance, it takes but 39 feet of number 24
copper wire, and but 2.2 feet of number 24 German silver wire, to give
the same resistance.

   NOTE. The number of a wire indicates its diameter; number
   30, for example, being always of a definite fixed diameter,
   no matter what the material of the wire.

If we wish to avoid loss of current by heating, we use a wire of low
resistance; while if we wish to transform electricity into heat, as in
the electric stove, we choose wire of high resistance, as German
silver wire.



CHAPTER XXXV

HOW ELECTRICITY IS OBTAINED ON A LARGE SCALE


318. The Dynamo. We have learned that cells furnish current as a
result of chemical action, and that the substance usually consumed
within the cell is zinc. Just as coal within the furnace furnishes
heat, so zinc within the cell furnishes electricity. But zinc is a
much more expensive fuel than coal or oil or gas, and to run a large
motor by electricity produced in this way would be very much more
expensive than to run the motor by water or steam. For weak and
infrequent currents such as are used in the electric bell, only small
quantities of zinc are needed, and the expense is small. But for the
production of such powerful currents as are needed to drive trolley
cars, elevators, and huge machinery, enormous quantities of zinc would
be necessary and the cost would be prohibitive. It is safe to say that
electricity would never have been used on a large scale if some less
expensive and more convenient source than zinc had not been found.

319. A New Source of Electricity. It came to most of us as a
surprise that an electric current has magnetic properties and
transforms a coil into a veritable magnet. Perhaps it will not
surprise us now to learn that a magnet in motion has electric
properties and is, in fact, able to produce a current within a wire.
This can be proved as follows:--

[Illustration: FIG. 237.--The motion of a magnet within a coil of wire
produces a current of electricity.]

Attach a closely wound coil to a sensitive galvanometer (Fig. 237);
naturally there is no deflection of the galvanometer needle, because
there is no current in the wire. Now thrust a magnet into the coil.
Immediately there is a deflection of the needle, which indicates that
a current is flowing through the circuit. If the magnet is allowed to
remain at rest within the coil, the needle returns to its zero
position, showing that the current has ceased. Now let the magnet be
withdrawn from the coil; the needle is deflected as before, but the
deflection is in the opposite direction, showing that a current
exists, but that it flows in the opposite direction. We learn,
therefore, that a current may be induced in a coil by moving a magnet
back and forth within the coil, but that a magnet at rest within the
coil has no such influence.

An electric current transforms a coil into a magnet. A magnet in
motion induces electricity within a coil; that is, causes a current to
flow through the coil.

A magnet possesses lines of force, and as the magnet moves toward the
coil it carries lines of force with it, and the coil is cut, so to
speak, by these lines of force. As the magnet recedes from the coil,
it carries lines of force away with it, this time reducing the number
of the lines which cut the coil.

[Illustration: FIG. 238.--As long as the coil rotates between the
poles of the magnet, current flows.]

320. A Test of the Preceding Statement. We will test the statement
that a magnet has electric properties by another experiment. Between
the poles of a strong magnet suspend a movable coil which is connected
with a sensitive galvanometer (Fig. 237). Starting with the coil in
the position of Figure 228, when many lines of force pass through it,
let the coil be rotated quickly until it reaches the position
indicated in Figure 238, when no lines of force pass through it.
During the motion of the coil, a strong deflection of the galvanometer
is observed; but the deflection ceases as soon as the coil ceases to
rotate. If, now, starting with the position of Figure 238, the coil is
rotated forward to its starting point, a deflection occurs in the
opposite direction, showing that a current is present, but that it
flows in the opposite direction. So long as the coil is in motion, it
is cut by a varying number of lines of force, and current is induced
in the coil.

_The above arrangement is a dynamo in miniature_. By rotation of a
coil (armature) within a magnetic field, that is, between the poles of
a magnet, current is obtained.

In the _motor_, current produces motion. In the _dynamo_, motion
produces current.

321. The Dynamo. As has been said, the arrangement of the preceding
Section is a dynamo in miniature. Every dynamo, no matter how complex
its structure and appearance, consists of a coil of wire which can
rotate continuously between the poles of a strong magnet. The
mechanical devices to insure easy rotation are similar in all respects
to those previously described for the motor.

[Illustration: FIG. 239.--A modern electrical machine.]

The current obtained from such a dynamo alternates in direction,
flowing first in one direction and then in the opposite direction.
Such alternating currents are unsatisfactory for many purposes, and to
be of service are in many cases transformed into direct currents; that
is, current which flows steadily in one direction. This is
accomplished by the use of a commutator. In the construction of the
motor, continuous _motion_ in one direction is obtained by the use of
a commutator (Section 310); in the construction of a dynamo,
continuous _current_ in one direction is obtained by the use of a
similar device.

322. Powerful Dynamos. The power and efficiency of a dynamo are
increased by employing the devices previously mentioned in connection
with the motor. Electromagnets are used in place of simple magnets,
and the armature, instead of being a simple coil, may be made up of
many coils wound on soft iron. The speed with which the armature is
rotated influences the strength of the induced current, and hence the
armature is run at high speed.

[Illustration: FIG. 240.--Thomas Edison, one of the foremost
electrical inventors of the present day.]

A small dynamo, such as is used for lighting fifty incandescent lamps,
has a horse power of about 33.5, and large dynamos are frequently as
powerful as 7500 horse power.

323. The Telephone. When a magnet is at rest within a closed coil of
wire, as in Section 319, current does not flow through the wire. But
if a piece of iron is brought near the magnet, current is induced and
flows through the wire; if the iron is withdrawn, current is again
induced in the wire but flows in the opposite direction. As iron
approaches and recedes from the magnet, current is induced in the wire
surrounding the magnet. This is in brief the principle of the
telephone. When one talks into a receiver, _L_, the voice throws into
vibration a sensitive iron plate standing before an electromagnet. The
back and forth motion of the iron plate induces current in the
electromagnet _c_. The current thus induced makes itself evident at
the opposite end of the line _M_, where by its magnetic attraction, it
throws a second iron plate into vibrations. The vibrations of the
second plate are similar to those produced in the first plate by the
voice. The vibrations of the far plate thus reproduce the sounds
uttered at the opposite end.

[Illustration: FIG. 241.--Diagram of a simple telephone circuit.]

324. Cost of Electric Power. The water power of a stream depends
upon the quantity of water and the force with which it flows. The
electric power of a current depends upon the quantity of electricity
and the force under which it flows. The unit of electric power is
called the watt; it is the power furnished by a current of one ampere
with a voltage of one volt.

One watt represents a very small amount of electric power, and for
practical purposes a unit 1000 times as large is used, namely, the
kilowatt. By experiment it has been found that one kilowatt is
equivalent to about 1-1/3 horse power. Electric current is charged for
by the watt hour. A current of one ampere, having a voltage of one
volt, will furnish in the course of one hour one watt hour of energy.
Energy for electric lighting is sold at the rate of about ten cents
per kilowatt hour. For other purposes it is less expensive. The meters
commonly used measure the amperes, volts, and time automatically, and
register the electric power supplied in watt hours.



    INDEX


    Absorption, of heat by lampblack, 143-144.
      of gases by charcoal, 57.
      of light waves, 135-138.

    Accommodation of the eye, 123.

    Acetanilid, 259.

    Acetylene, as illuminant, 152-153.
      manufacture of, 152-153.
      properties of, 220.

    Acid, boric, 253.
      carbolic, 152, 251, 252.
      hydrochloric, 55, 80, 227, 238, 241.
      lactic, 230.
      oxalic, 247, 248.
      salicylic, 253.
      sulphuric, 55, 80, 240, 241, 307.
      sulphurous, 242.

    Acids, action on litmus, 220.

    Adenoids, 51.

    Adulterants, detection of, 16.

    Air, characteristics of, 81-83, 86, 189.
      compressibility of, 91.
      expansion of, 10-11.
      humidity, 38, 39.
      pumps, 201-205.
      transmits sound, 269.
      weight of, 86.
     _See_ Atmosphere.

    Alcohol, 234.
      in patent medicines, 260.

    Alizarin, 248.

    Alkali, 222.

    Alternating current, 351.

    Alum, 247.
      in baking powder, 230.

    Ammeter, 341, 343.

    Ammonia, 152.
      a base, 221-222.
      in bath, 226.
      in manufacture of ice, 98.
      neutralizing chlorine, 240.

    Ampere, 342.

    Anemia, 259.

    Angle, of incidence, 110.
      of reflection, 110.
      of refraction, 114.

    Aniline, 152, 245.

    Animal charcoal, 58.

    Animal transportation, 132.

    Antichlor, 240.

    Antipyrin, 259.

    Armature, 319, 320.
      dynamo, 350.
      motor, 335.

    Artificial lighting, 148-153.

    Atmosphere, 81.
      carbon dioxide in, 54-55.
      height of, 81.
      nitrogen and oxygen in, 262.
      pressure of, 82-86.
      water vapor in, 36-38.
      weight, 86.
      _See_ Air.

    Atmospheric pressure, 82-86.

    Atomizer, 92.

    Atoms, 102.

    Automobiles, gas engines, 185.

    Axis of a lens, 119.


    Bacteria, 133.
      as nitrogen makers, 263.
      destroyed by sunlight, etc., 133, 250, 251.
      diseases caused by, 133.
      in butter and cheese, 133.

    Baking powder, 229-230.

    Baking soda, 227-229.

    Barograph, 87.

    Barometer, aneroid, 84-85.
      mercury, 84.
      use in weather predictions, 86-87.

    Bases, action on litmus, 221-222.
      properties, 220-222.

    Battery, electric, 311.

    Beans, as food, 66.
      roots take in nitrogen, 263.

    Bell, electric, 319-321.

    Benzine, 150.
      as a cleaning agent, 227.

    Benzoate of soda, 253.

    Bicarbonate of soda, in fire extinguisher, 55, 56.
      in Rochelle salt, 227.
      in soda mints, 231.
      in seidlitz powder, 231.

    Bicycle pumps, 202.

    Blasting, by electricity, 314.

    Bleaching, 237-243.
      by chlorine, 238-240.

    Bleaching powder, 239-240.

    Body, human, 63-64.
      a conductor of electricity, 292.

    Boiling, 31.
      amount of heat absorbed, 31-32.
      of milk, 32.
      of water, 77.
      point, 15.

    Bomb calorimeter, 61.

    Borax, as meat preservative, 253.
      as washing powder, 226.

    Boric acid, as meat preservative, 253.

    Boyle's law, 95-96.

    Bread, 232-233.
      unleavened, 233.

    Bread making, 232-235.

    Breathing, hygienic habits of, 50.
      by mouth, 50-51.

    Burns, treatment of, 52-53.

    Butter, adulteration test, 16.
      bacteria in, 133.

    Buttermilk, 230.


    Caisson, 203-204.

    Calcium carbide, 152-153.
      in making nitrogenous fertilizer, 264.

    Calico printing, 249.

    Calorie, 27-28, 61-62.

    Calorimeter, 61.

    Camera, 128-129.
      films, 129.
      lens, 129.
      plates, 129.

    Camping, water supply, 195-197.

    Candle, 148-149.
      as standard for light-measure, 104-105.

    Candle-power, 105-107.

    Carbide, calcium, 152-153, 264.

    Carbohydrates, 64-65, 149.

    Carbolic acid, 152.
      as disinfectant, 251.

    Carbon, 56, 66.
      in voltaic cells, 308.

    Carbon dioxide, 53.
      as fire extinguisher, 55-56.
      commercial use, 55-56.
      in baking soda, 228.
      in fermentation, 234.
      in health, 54.
      in plants, 55.
      preparation of, 55.
      source of, 53.
      test for, 228.

    Catarrh, 259.

    Caustic lime, 222..

    Caustic potash, 222.

    Caustic soda, 218, 222.
      to make a salt, 227.

    Caves and caverns, 71.

    Cell, dry, 310.
      gravity, 309-310.
      voltaic, 306-308, 310.

    Cells of human body, 63, 64, 66.

    Centigrade thermometer, 15.

    Central heating plant, 19.

    Chalk, in making carbon dioxide, 55.

    Charcoal as a filter, 57.
      commercially, 57.
      preparation, 57-58.

    Chemical action, and electricity, 307, 315-317.
      and light, 126, 127.

    Chemistry, in daily life, 218, 219.

    Chills, 38.

    Chloride of lime, in bleaching, 240.
      disinfectant, 251.

    Chlorine, and hydrogen, 239.
      effect upon human body, 239.
      in bleaching, 238-240.
      influence of light upon, 126.
      presence in salt, 227.

    Circuit, electric, 321.
      local, in telegraph, 325-326.

    City water supply, 206-212.

    Clarinet, 297.

    Cleaning of material, 226, 243.

    Climate, influenced by presence of water, 29, 40.

    Clover, nitrogen producers, 263.

    Coal, 30.

    Coal gas, 150, 151.
      by-products, 152.

    Coal oil, 149, 150.

    Coal tar dyes, 152, 218, 245.

    Cogwheels, 170.

    Coil, current-bearing, 320.
      magnetic field about, 331-333.

    Coke, 152.

    Cold storage, 97.

    Color, 134-141.
      and heat, 142, 143.
      influenced by light, 137.
      of opaque bodies, 136, 137.
      of transparent bodies, 135, 136.

    Color blindness, 140, 141.
      designs in cloth, 248, 249.

    Colors, compound, 138, 139.
      essential, 139-140.
      primary, 135.
      simple, 138.
      spectrum, 134-135.
      variety in dyeing, 247, 248.

    Combustion, heat of, 45.
      spontaneous, 52.

    Commutator, 335.

    Compass, 328.

    Compound colors, 138, 139.

    Compound machine, 171.

    Compound substances, 103.

    Compression of air, 91, 92.
      cause of heat, 96.

    Compression pumps, 201, 205.

    Concave lens, 118.

    Condensation, 33.
      heat set free, 40.

    Conduction of heat, 25.

    Conductivity metals, 321.

    Conductors, electric, 321, 322.

    Conservation, of energy, 58, 59.
      of matter, 58, 59.

    Convection, 24, 25.

    Convex lens, 118.

    Cooling, by evaporation, 35-36.
      by expansion, 97.

    Copper, in electric cell, 307.

    Core, iron, 319.

    Corn, bleached with sulphurous acid, 242.

    Cotton, mercerized, 218.
      bleaching, 241.
      dyeing, 245-247.

    Cough sirup, 258.

    Crane, compound machine, 172.

    Cream of tartar, 229.

    Creosote oil, 254.

    Crude petroleum, 149, 150.

    Current, electric, 306, 312.
      alternating, 349.
      induced, 346-347.
      measurement of, 340.
      resistance, 312, 343, 345.
      strength, 339, 340, 344.


    Dams, 214-216.

    Decay, 49.

    Decomposition of soil by water, 70-74.

    Degrees Fahrenheit and Centigrade, 15.

    Density, 11.

    Designs in cloth, printed, 248, 249.
      woven, 249.

    Developer in photography, 128.

    Dew, 36, 37.

    Dew point, 38.

    Diarrhea, 251.

    Diet, 62, 66.
      economy on table, 66-69.

    Discord, reason for, 271.

    Disease, and surface water, 76.
      relation of light to, 131-132.

    Disease disinfectants, 250, 251, 252.

    Distillation, 34-35.
      in commerce, 35.
      of petroleum, 149-150.
      of soft coal, 150.
      of water, 34, 35, 77.

    Diving suits, 204.

    Door bells, 319-321.

    Drainage, of land, 194, 195.
      sewage, 196, 198, 199, 201.

    Drilled well, 199.

    Drinking water, 75-77.
      in camping, 195-196.
      and rural supplies, 198, 201.

    Driven well, 196-197.

    Drought, 217.

    Drugs, 255, 260.

    Dry cell, 312.

    Dyeing, 244-249.
      color designs, 248.

    Dyeing, direct, 245.
      home, 247.
      indirect, 247.
      variety of color, 247.

    Dyes, 218, 244, 245.

    Dynamo, 346.
      alternating current, 349.
      source of energy, 346-347.


    Ear, in man, 301-303.
      care of, 303.

    Earth, conductor of electricity, 326.

    Echo, 277.

    Economy in buying food, 66-69.

    Effort, muscular, 155, 160.

    Electric, battery, 311.
      bell, 319-321.
      bread toasters, 314.
      conductors and non-conductors, 321-322.
      cost of, energy, 352.
      current, 306, 312.
      flatiron, 313.
      heating pad, 314.
      lights, 314.
      street cars, 337.

    Electricity, heat, 312-315, 339.
      as a magnet, 319, 331-333.
      practical uses of, 312-317.

    Electrodes, of cell, 308.

    Electrolytic metals, 317.

    Electromagnets, 319.

    Electromotive force, 308.
      unit of, 344.

    Electroplating, 315.

    Electrotyping, 317.

    Elements, 102-103.

    Emulsion, 224.

    Energy, conservation of, 58, 59.
      transformations of, 58, 59.

    Engine, steam, 183-185.
      gas, 185-186.
      horse power, 173.

    Erosion, 73-74.

    Essential colors, 139-140.

    Evaporation, 35-39.
      cooling effect, 35-36.
      effect of temperature on, 35, 36.
      effect of air on, 38.
      freezing by, 98.
      heat absorbed, 36.
      of perspiration, 38.

    Expansion, of air, 10, 11.
      cooling effect of, 97.
      disadvantage and advantage of, 11-13.
      of liquids, 9-11.
      of solids, 10, 11.
      of water, 9, 10, 11, 12.
    Eye, 122-125.
      headache, 124, 125.
      how focused, 122, 123.
      nearsighted and farsighted, 123.
      strain, 125.


    Fahrenheit thermometer, 15.

    Fats, 65.
      in soap making, 223.

    Fermentation, 232-236.
      by yeast, 234-236.

    Ferric compounds, 248.

    Fertilizers, 262-265.
      nitrogen, 262.
      phosphorus, 263, 264.
      potash, 263-265.

    Field magnet, 336.

    Filings, iron, 329.

    Film, photographic, 129.

    Filter, charcoal, 57.

    Filtering water, 77.

    Fire, 9.
      and oxygen, 45, 47.
      and tinder box, 47.
      making of, 51.
      primitive production of, 47.
      produced by friction, 47.
      spontaneous combustion, 52.
      sores and burns, 52-53.
      extinguisher, 55, 56.

    Fireless cooker, 25, 26.

    Fireplaces, 17, 18.

    Fixing, in photography, 128.

    Flame, hydrogen, 80.

    Flood, Johnstown, 214, 215.
      relation to forests, 217.

    Flour, self-raising, 231.

    Flume, 177.

    Flute, 297.

    Focal length, 118.

    Focus, of lens, 118.

    Fog, 37.

    Food, 60-69.
      carbohydrates, 64, 65.
      economy in buying, 66-69.
      fats, 65.
      fuel value of, 60-62.
      need of, 63, 64.
      preservatives, 252.
      proteids, 66.
      value, 67.
      waste, 60.
      water in, 75.

    Foot pound, 172.

    Force and motion, 156, 157.
      and work, 156, 157.
      magnetic lines of, 329-331, 334.
      muscular, 155, 160.

    Force pumps, 192, 193.

    Forests and water supply, 216-217.

    Forging of iron, 40, 41.

    Formaldehyde, 253.

    Freezing, effect of salt, 44.
      effect on ground and rocks, 42.
      expansion of water on, 41.
      ice cream freezer, 44.

    Frequency in music, 273, 275.

    Fresh air, 22-24, 49.
      amount consumed by gas burner, 22.
      and health, 49, 50.
      in underground work, 202.
      in work under water, 203-205.

    Friction, 173, 174.
      losses by, 174, 210.
      source of heat and fire, 47.

    Frost, 36, 37.

    Fruit, canned, bleached with sulphurous acid, 242.
      colored with coal tar dyes, 253.

    Fuel value of foods, 60-62.
      table of fuel values, 67.

    Fulcrum, 159, 160.

    Fumigation, 251.

    Fundamental tone, 290, 291, 292.

    Furnace, hot air, 19.

    Fuse, 340.

    Fusion, heat of, 40.


    Galvanometer, 341.

    Gas, acetylene, 152, 153.
      and unburned carbon, 151.
      coal, 151, 152.
      effect of heat on volume, 96, 97.
      effect of pressure on volume, 95-96.
      engine, 185-186.
      for cooking, 151, 152.
      illuminating, 92, 93, 150, 151.
      liquefaction, 97, 98.
      meter, 93, 94.
      natural, 152.

    Gasolene, 149, 150.
      as cleaning agent, 227, 243.
      in gas engine, 185, 186.

    Gauge, pressure, 92-94.

    Gelatin, plate and film, 129.

    Glass, kinds of, 119.
      molding of, 40.
      non-conductor, 321.

    Grape juice, fermented with millet, 233.

    Gravity cell, 309, 310.

    Grease, and lye, 221.
      and soap making, 223.

    Gulf Stream, 24.


    Hard water, and soap, 225.

    Harp, 295.

    Headache, 124, 125.
      powders, 259.

    Health, effect of diet, 62, 64.

    Heat, 9.
      absorbed in boiling, 31-32.
      and disease germs, 250.
      and food, 252.
      and friction, 47.
      and light, 142, 147.
      and oxidation, 45, 48, 49.
      and wave motion, 145-147.
      conduction, 25.
      convection, 24, 25.
      from burning hydrogen, 80.
      from electricity, 312-315, 339.
      needed to melt substances, 39.
      of fusion, 40.
      of vaporization, 32.
      produced by compression, 96.
      relation of water to weather, 29, 40.
      set free by freezing water, 40.
      sources of, 29-30.
      specific, 28-29.
      temperature, 27.
      unit of, 27, 28.

    Heating effect of electric current, 312-315.

    Heating of buildings: central heating plant, 19.
      fireplaces, 17-18.

    Heating, furnaces, 19.
      hot water, 19-22.

    Helix, 318.

    Horse power, 173, 351.

    Hot water heating, 19-22.

    Hues, primary, 135.

    Humidity, 38.
      proper percentage for health and comfort, 38, 39.

    Humus, 216, 217.

    Hydrocarbons, 149.

    Hydrochloric acid, composition, 227.
      in bleaching, 241.
      to make a salt, 227.
      to make carbon dioxide, 55.
      to make chlorine, 238.
      to make hydrogen, 80.

    Hydrogen, 65, 66.
      and chlorine, 239.
      and water, 79.
      chemical conduct, 126-127.
      flame, 80.
      in voltaic cell, 307.
      peroxide, 53, 252.
      preparation, 80.
      to liquefy, 97.


    Ice, lighter than water, 42.
      manufacture of, 98, 99.

    Ice cream freezers, 44.

    Illuminating gas, manufacture of, 150, 151.
      measurement of quantity consumed, 93, 94.
      test of pressure, 92, 93.

    Illumination, intensity of, 105, 106.

    Image, in mirror, 108, 111.

    Incandescent lighting, 107, 314.

    Incidence, angle of, 110.

    Inclined plane, 162-166.
      screw, 166.
      wedge, 166.

    Indigo, 218.

    Induced current, 346-347.

    Ink spots, removal of, 243.

    Insoluble substances, 71.

    Insulators, electric, 324.

    Intensity, of light, 105-107.
      of sound, 270-271.

    Interval, in musical scale, 283.

    Iron, forging, 41.
      filings, 329.
      galvanizing, 49.
      oxidation of, 48.

    Irrigation, 193-194.

    Isobaric lines, 88, 91.

    Isothermal lines, 89, 91.


    Johnstown flood, 214, 215.


    Kerosene, 149, 150.

    Kilowatt, 351.


    Lactic acid, 230.

    Leaves, 132, 262.

    Lens, 117-121.
      concave, 118.
      converging, 118.
      crystalline, of eye, 122.
      focal length, 118.
      material, 119.
      refractive power, 119.

    Lever, 158-162.
      examples, 160-162.
      fulcrum, 159, 160.

    Life, and carbon dioxide, 54.
      and nitrogen, 261.
      and oxygen, 49, 54.

    Lifting pumps, 189-192.

    Light, absorption, 135-138.
      and heat, 142-147.
      a wave motion, 145-147.
      bent rays, 113, 114.
      chemical action, 126-127.
      disease, 131-132.
      essential to life, 131, 132.
      fading illumination, 105, 106.
      influence on color, 134.
      reflection of, 109-112.
      refraction of, 113-125.
      travels in a straight line, 108.
      white, composed of colors, 134.

    Lighting, artificial, 148-153.

    Lime, chloride of, 240, 251.

    Limewater, 220.
      and carbon dioxide, 228.

    Linen, bleaching, 241.
      dyeing, 245-247.

    Lines, of force, 329-331, 334.
      isobaric, 88, 91.
      isothermal, 89, 91.

    Liquefaction of gases, 97, 98.

    Liquid air, 98.

    Liquid soap, 223, 224.

    Litmus, action of acids, 220.
      action of bases, 221, 222.
      action of neutral substance, 222.

    Logwood dyes, 245, 247, 248.

    Los Angeles aqueduct, 211.

    Lye, 221, 222.


    Machines, compound, 171.
      inclined plane, 162-166.
      lever, 158-162.
      pulley, 166-169.
      wheel and axle, 169-171.

    Madder, for dyes, 245.

    Magnet, 328.
      electro-, 319.
      field of, 329-331.
      lines of force about, 329-331.
      poles of, 330-332.
      properties of electricity, 318.

    Magnetic, needle, 328.
      poles, 329-331.

    Magnifying power, of a lens, 115.
      of a microscope, 115.
      of a telescope, 115.

    Mammoth Cave of Kentucky, 71.

    Manganese dioxide, 46.
      chlorine made from, 238.
      oxygen made from, 46.

    Marble, for carbon dioxide, 55.

    Matches, 47.
      safety, 47-48.

    Matching colors, 137.

    Matter, conservation of, 58, 59.

    Meat, 66.
      preservation of, 253.

    Mechanical devices, 154, 155.

    Melting, 39, 40.

    Melting point, 40.

    Melting substances without a definite melting point, 40.

    Mercerized cotton, 218.

    Mercury, barometer, 84.
      thermometer, 14-17.

    Metals, electroplating, 317.
      preservation by paint, 253-254.
      veins deposited by precipitation, 72, 73.
      welding, 315.

    Meter, gas, 93, 94.

    Microörganisms, 132, 133.

    Microscope, 115.

    Milk, boiling point, 32.
      Pasteurized, 250.

    Minerals, in foods, 62, 63.
      in water, 70, 71.

    Mirrors, 108-112.
      distance of image behind mirror, 111.
      distance of object in front of mirror, 111.
      image a duplicate of object. 111.

    Molding of glass, 40.

    Molecule, 100-103.

    Mordants, 247, 248, 249.

    Morphine, 257.

    Morse, telegraphic code, 324.

    Motion, in sound, 266, 278, 280.
      in work, 156.

    Motor, electric, 336.
      principle of, 333.
      street car, 337.

    Mouth breathing, 50.
      cause of, 51.

    Movable pulley, 167, 168.

    Music, 278.

    Musical instruments, percussion, 299.
      stringed, 284-295.
      wind, 295, 299.

    Musical scale, 282.


    Naphtha in gas engines, 185.

    Naphthalene, 152.

    Narcotics, 255.

    Natural gas, 152.

    Needle, magnetic, 328.

    Negative, electrode, 308.
      photographic, 130.

    Neutral substance, 222.
      and litmus, 222.

    Neutralization, 222.

    Niagara Falls, 176.

    Nitrogen, 66.
      and bacteria, 263.
      and plant life, 261.
      in atmosphere, 261.
      in fertilizer, 262-265.
      in food, 66.
      preparation of, 261.
      properties of, 261.

    Noise in music, 280.

    Non-conductors, of electricity, 321-322.
      of heat, 25.

    Nutcracker, as a lever, 162.


    Oboe, 297.

    Octave, 284.

    Odors, 101.

    Ohm, unit of resistance, 345.

    Oil, gasoline, 149, 150.
      kerosene, 149, 150.
      lubricating, 174.
      olive, 16.

    Orchestra grouping, 299.

    Ore, 72.

    Organ pipes, 297.

    Overtones, 290-293.

    Oxalic acid, 247, 248.

    Oxidation, 45-59.
      and decay, 49.
      heat the result of, 49-52.
      in human body, 49, 53.
      of iron, 48.

    Oxygen, 66.
      and bleaching, 239.
      and combustion, 45.
      and food, 66.
      and plants, 55.
      and the human body, 50.
      and water, 79, 80.
      in the atmosphere, 45.
      preparation of, 46.


    Paint, as wood and metal preservatives, 253, 254.
      removal of stains, 243.

    Paper making, 219.

    Paraffin, 150, 321.

    Pasteurized milk, 250.

    Patent medicines, 257-260.

    Peas, sources of nitrogen, 263.

    Pelton wheel, 177.

    Percussion instruments, 299.

    Period of a body, 273.

    Peroxide of hydrogen, 53, 252

    Petrolatum, 150.

    Petroleum, 149, 150.

    Phonograph, 303-305.

    Phosphorus, in fertilizer, 263, 264.
      in making nitrogen, 261.
      in matches, 47, 48.
      poisoning by, 47.

    Photography, 127-131.

    Photometer, 107.

    Pianos, 284-292.

    Pin wheel, 181.

    Pitch of sound, 280, 281.
      cause of, 282.
      in wind instruments, 296-299.

    Plane, inclined, 162-166.

    Plants, and atmosphere, 55.
      and light, 131-132.
      and nitrogen, 261.

    Plate developing, photographic, 128.

    Pneumatic dispatch tube, 205.

    Poles, magnetic, 330-332.
      of cell, 308.

    Positive electrode, 308.

    Potash, in fertilizer, 263-265.

    Potassium chlorate and oxygen, 46.
      permanganate, 100.
      tartrate and Rochelle salt, 227.

    Power, candle, 105-107.
      electric, 351.
      horse, 173, 351.
      sources of, 174, 175, 185.
      transmission by belts, 171.
      water, 176-180.

    Precipitation, 72, 73.

    Preservatives, food, 252.
      wood and metal, 253-254.

    Pressure, atmospheric, 82-86.
      calculation of atmospheric, 83, 84.
      calculation of gas, 92, 93.
      calculation of water, 94.
      gauge, 92-94.
      of illuminating gas, 93.
      relation of pressure of gas to volume, 95, 96.
      water pressure, 208-211, 214-216.
      within the body, 86.

    Primary colors, 135.

    Print, photographic, 131.

    Printing, color designs in cloth, 248, 249.
      electrotype, 317.

    Prisms, 135.
      refraction through, 117.

    Proteids, 66.

    Pulleys, 166-169.
      applications of, 169.

    Pump, 187-205.
      air, 201-205.
      force, 192, 193.
      lifting, 189-192.

    Pupil of the eye, 122.

    Pure food laws, bleaching, 242.
      preservatives, 252.

    Purification of water, 77, 196.

    Push button, 321.


    Radiator, 19-21.

    Railroads, grading of, 165-166.

    Rain, 36, 37.

    Rainbow, 134.

    Rain water, 225.

    Reflection, angle of, 110.
      of light, 109-112.
      of sound, 278, 279.

    Refraction, angle of, 114.
      by atmosphere, 114.
      of light, 113.
      uses of, 115-116.

    Relay, telegraph, 325.

    Reservoir, 214.
      artificial, 211.
      construction of, 214-216.
      natural, 211.

    Resistance, electrical, 312.
      internal, of cell, 343.
      unit of, 345.

    Resonance, 276.

    River, volume and value of, 180.

    Roads, application of inclined plane to, 165-166.

    Rochelle salt, 227, 231.

    Rocks, effect of freezing water on, 42-43.
      water as a solvent, 71.

    Rosin, obtained by distillation, 35.


    Safety matches, 47-48.

    Salicylic acid, 253.

    Salt, 227-228.

    Salts, 227.
      general properties, 227.
      in ocean, 227.
      smelling, 222.

    Saturation of air, 37.

    Scale, musical, 282.

    Screw, and inclined plane, 166.

    Seaweed, 265.

    Seidlitz powder, 231.

    Self-raising flour, 231.

    Sewage, disposition of, 198-199.
      of camps, 196.
      source of revenue, 201.

    Sewer gas, 57.

    Silk, bleaching, 241.
      dyeing, 245-247.

    Silver chloride, 127-131.

    Simple colors, 138.

    Simple substances, 103.

    Siren, 280.

    Smelling salts, 222.

    Snow, 36-37.

    Soap, 222-224.
      and hard water, 225.
      liquid, 223-224.
      preparation, 223.

    Soda, baking, 227, 228-229.
      benzoate, 253.
      caustic, 218, 222, 223, 227.
      washing, 225, 226, 229.

    Soda mints, 231.

    Sodium, bicarbonate, 56, 227, 228, 230-231.
      carbonate, 228.
      chloride, 228.

    Soil, deposited by streams, 73.

    Solenoid, 318.

    Solution, 70.

    Soothing sirup, 258.

    Sound, and motion, 266, 278.
      musical, 278.
      nature of, 266.
      reflection, 277.
      speed of, 271-272.
      transmission of, 267-271.
      velocity of, 271-272.
      waves, 272-274.

    Sounder, telegraph, 324.

    Sounding board, 277.

    Sour milk in cooking, 230.

    Specific heat, 28-29.

    Spectrum, 134-135.

    Speed, of sound, 271, 272.

    Spontaneous combustion, 52.

    Stains, removal of, 226, 243.

    Standpipes, 212.

    Starch, 65.

    Steam, and work, 183-184.
      engine, 183-185.
      heat of vaporization, 32.
      heating by, 33.
      turbine, 183-184.

    Steel, forging and annealing, 16.

    Stoves, 18-19.

    Streams, carriers of mud, 73.
      volume of, 179-180.

    Street cars, electric, 337.

    Stringed instruments, 284-295.

    Strings, vibrating, 286-290.

    Sugar, 16, 65.
      fermented by yeast, 234.

    Sulphur, 66.
      as disinfectant, 251.
      in making sulphurous acid, 242.

    Sulphuric acid, in bleaching, 240,241.
      in fire extinguisher, 55.
      in making of hydrogen, 80.
      in voltaic cell, 307.

    Sulphurous acid, in bleaching, 242.
      preparation, 242.

    Sun, energy derived from, 143-144.
      source of heat, 29-30.

    Sunlight, 135.
      and bacteria, 133.
      and chemical action, 126-127.

    Sympathetic vibrations, 274-277.


    Tallow, 105, 148.

    Tartar, cream of, 229.

    Telegraph, 322.
      long distance, 327.
      relay, 325.
      sounder, 324.

    Telephone, 350-351.

    Temperature, 13-14.
      as measurement of heat present, 27.
      in detecting adulterants, 17.
      in forging steel, 16.
      in making sirups, 16.
      measurement of, 14-15.

    Thermometer, 14-17.
      Centigrade, 15.
      Fahrenheit, 15.

    Tinder box, 47.

    Transmission, of light, 145-147.
      of sound, 267-271.

    Tuning fork, 266, 273, 278, 290.

    Turbine, steam, 183.
      water, 178.

    Turpentine, and grease, 226.
      by distillation, 35.


    Unleavened bread, 233.


    Vacuum, sound in, 268.

    Vapor, in atmosphere, 36-38.

    Vaporization, heat of, 32.

    Varnish, on candies, 253.

    Vegetable matter, and coal, 30.
      and gas, 30.
      and oil, 30.

    Veins, formation in rock, 72-73.

    Velocity, of sound, 271-272.

    Ventilation, 21-24, 54.
      need of, 38.

    Vibration, of strings, 286-290.
      sympathetic, 274-277.

    Viola, 295.

    Violin, 295.

    Violoncello, 295.

    Vocal cords, 300.

    Voice, 300.

    Volt, 344.

    Voltage, 345.

    Voltaic cell, 306-308, 310.

    Voltmeter, 344.

    Volume, of a stream, 179-180.
      relation of pressure of a gas, 95-96.


    Washing powders, 224-226.
      soda, 229.

    Water, action in nature, 70-74.
      amount used daily per person, 181.
      and hydrogen, 79.
      and oxygen, 79, 80.
      as solvent, 70-71.
      boiling, 77.
      boiling point, 15.
      composition, 79-80.
      condensation, 33.
      dams and reservoirs, 214-216.
      density, 11.
      distilled, 34, 77.
      drinking, 75-77, 195-201.
      electrolysis, 79-80.
      evaporation, 33-34.
      expansion, 9-10, 41-42.
      filtration, 77.
      freezing, 40-41.
      hard, 225.
      heat of fusion, 40.
      impurities, 76-77.
      in atmosphere, 36-38.
      in food, 75.
      in human body, 75.
      in vegetables, 75.
      influence on climate, 29, 40.
      irrigation, 193-194.
      minerals in, 70-71.
      ocean, 265.
      power, 176-180.
      precipitates, 72, 73.
      pressure, 208-211, 214-216.
      purification, 77.
      rain, 225.
      running, value of, 178-180.
      source of, 78.
      steam, 32.
      waves, 145-147.
      weight, 208-209, 215.
      wells, 195-201.
      wheels, 176-180.
      work under, 203-205.

    Water supply, and forests, 216-217.
      cost, 212-214.
      of city, 206-212, 217.

    Watt, 351.

    Waves, heat, 145-147.
      light, 145-147.
      sound, 268, 272-274.
      water, 145-147.

    Weather, bureau, 87-91.
      forecasts, 38-39, 86-88.
      relation of water to, 29, 40.

    Weather maps, 89-91.

    Wedge, and inclined plane, 166.

    Weight, of air, 86.
      of water, 208-209, 215.

    Welding, by electricity, 315.

    Wells, 195-201.
      drilled, 199.
      driven, 196-197.

    Wheel and axle, 169-171.
      cogwheels, 170.
      windlass, 169.

    Wheelbarrow as lever, 160-161.

    White light, nature of, 135.

    Wind instruments, 297-301.

    Windlass, 169.

    Windmill, 174-175, 180-182.

    Winds, 24.

    Wine, 232, 234.

    Wood, as source of charcoal, 58.
      ashes in soap making, 223.
      in paper making, 219.
      preservation, 253-254.

    Wool, bleaching, 241.
      dyeing, 245-247.

    Work, 156-186.
      and steam, 183-184.
      and water, 176-180.
      conservation, 174-175.
      formula, 157.
      machines, 157-175.
      unit of, 172-173.
      waste, 173.

    Woven designs in cloth, 249.


    Yeast, 234-236.
      wild, 235-236.


    Zinc, in galvanizing iron, 49.
      in making hydrogen, 80.
      in voltaic cell, 307-308.



PLANT LIFE AND PLANT USES

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A NEW ASTRONOMY, $1.30

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An introductory course in geology, complete enough for college
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ESSENTIALS OF PHYSICS

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