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Title: Science Primers, Introductory
Author: Huxley, Thomas Henry
Language: English
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Science Primers.
INTRODUCTORY.
BY
PROFESSOR HUXLEY, F.R.S.
=Toronto:=
CANADA PUBLISHING CO.,
(LIMITED.)
_Entered according to the Act of the Parliament of Canada, in the year
One Thousand Eight Hundred and Eighty-one by_ MACMILLAN & CO.,
LONDON, _in the Office of the Minister of Agriculture_.
TABLE OF CONTENTS.
PART. SECT.
I. NATURE AND SCIENCE.
PAGE
1. 〃 Sensations and Things 5
2. 〃 Causes and Effects 5
3. 〃 The reason Why. Explanation 6
4. 〃 Properties and Powers 7
5. 〃 Artificial and Natural Objects. Nature 8
6. 〃 Artificial Things are only Natural Things shaped and
brought together or separated by Men 8
7. 〃 Many Objects and Chains of Causes and Effects in
Nature are out of our reach 10
8. 〃 The Order of Nature: nothing happens by Accident, and
there is no such thing as Chance 10
9. 〃 Laws of Nature; Laws are not Causes 12
10. 〃 Knowledge of Nature is the Guide of Practical Conduct 14
11. 〃 Science: The Knowledge of the Laws of Nature obtained
by Observation, Experiment, and Reasoning 16
II. MATERIAL OBJECTS.—(A.) MINERAL BODIES.
12. 〃 The Natural Object Water 19
13. 〃 A Tumbler of Water 20
14. 〃 Water occupies Space; it offers Resistance; it has
Weight; and is able to transfer Motion which it has
acquired; it is therefore a form of Matter 20
15. 〃 Water is a liquid 21
16. 〃 Water is almost incompressible 22
17. 〃 The meaning of Weight 24
18. 〃 Gravity and Gravitation 25
19. 〃 The cause of Weight: Attraction: Force 27
20. 〃 The Weight of Water is Proportioned to its Bulk 28
21. 〃 The Measuring of Weights. The Balance 29
22. 〃 The Weight of the same Bulk or Volume of Water is
Constant under the same conditions. Mass. Density 30
23. 〃 Equal Volumes of Different Things under the same
circumstances, have Different Weights: the Density
of Different Bodies is Different 32
24. 〃 The Meaning of Heavy and Light—Specific Gravity 33
25. 〃 Things of greater Specific Gravity than Water sink in
Water; Things of less Specific Gravity float 34
26. 〃 A Body which Floats in Water always occupies as much
Space beneath the level of the Surface of the Water
as is equal to the Volume of Water which weighs as
much as that Body; in other words, it displaces its
own Weight of Water 36
27. 〃 Water Presses in all Directions 37
28. 〃 The Transference of Motion by Moving Water: the
Momentum of Moving Water 40
29. 〃 The Energy of Moving Water 43
30. 〃 The Properties of Water are Constant 47
31. 〃 Increase of Heat at first causes Water to Increase in
Volume 48
32. 〃 Increase of Heat at length causes Water to become
Steam 50
33. 〃 The taking away of Heat from Steam causes the Steam to
change into Hot Water 51
34. 〃 When Water is changed into Steam, its Volume becomes
about 1,700 times greater than it was at first 51
35. 〃 Gases or Elastic Fluids. Air 52
36. 〃 Steam is an Elastic Fluid or Gas 54
37. 〃 Gases and Vapours 55
38. 〃 The Evaporation of Water at ordinary Temperatures 56
39. 〃 When Hot Water is cooled, it Contracts to begin with,
but after a time Expands 57
40. 〃 Water cooled still further becomes the transparent
brittle solid Ice 58
41. 〃 Ice has less Specific Gravity than the Water from
which it was formed 59
42. 〃 Hoar Frost is the Gaseous Water which exists in the
Atmosphere, condensed and converted into Ice
Crystals 60
43. 〃 When Ice is warmed it begins to change back into Water
as soon as the Temperature reaches 32° 61
44. 〃 Ice the solid, Water the liquid, and Steam the gas,
are three states of one natural object; the
Condition of each State being a certain Amount of
Heat 62
45. 〃 The Phenomena of Heat are the Effects of a rapid
Motion of the Particles of Matter 63
46. 〃 The Structure of Water 65
47. 〃 Suppositions or Hypotheses; their Uses and their Value 67
48. 〃 The Hypothesis that Water is composed of Separate
Particles (Molecules) 68
49. 〃 All Matter is probably made up either of Molecules or
of Atoms 70
50. 〃 Elementary Bodies are neither destroyed nor is their
Quantity increased in Nature 72
51. 〃 Simple Mixture 73
52. 〃 Mixture followed by Increase of Density; Alcohol and
Water 74
53. 〃 Solution; Water Dissolves Salt 76
54. 〃 Quicklime and Water: Plaster of Paris and Water:
Combination 79
55. 〃 Mineral bodies may take on definite shapes and grow,
or increase in size, by the addition of like parts 82
(B.) LIVING BODIES.
56. 〃 The Wheat Plant and the substances of which it is
composed 83
57. 〃 The common Fowl and the Substances of which it is
Composed 85
58. 〃 Certain Constituents of the Body are very similar in
the Wheat Plant and in the Fowl 86
59. 〃 Proteid Substances are met with in Nature only in
Animals and Plants; and Animals and Plants always
contain Proteids 87
60. 〃 What is meant by the word Living? 88
61. 〃 The Living Plant increases in Size, by adding to the
Substances which compose its Body, like Substances;
these, however, are not derived from without, but
are manufactured within the Body of the Plant from
simpler Materials 88
62. 〃 The Living Plant, after it has grown up, detaches part
of its Substance, which has the Power of developing
into a similar Plant, as a Seed 90
63. 〃 The Living Animal increases in Size by adding to the
Substances which compose its Body, like Substances;
these, however, are chiefly derived directly from
other Animals or from Plants 90
64. 〃 The living Animal, after it has grown up, detaches
part of its Substance, which has the Power of
growing into a similar Animal, as an Egg 91
65. 〃 Living Bodies differ from Mineral Bodies in their
Essential Composition, in the manner of their
Growth, and in the fact that they are reproduced by
Germs 91
III. IMMATERIAL OBJECTS.
66. 〃 Mental Phenomena 92
67. 〃 The order of Mental Phenomena: Psychology 93
SCIENCE PRIMERS.
_INTRODUCTORY._
I. NATURE AND SCIENCE.
1. =Sensations and Things.=
All the time that we are awake we are learning by means of our =senses=
something about the world in which we live and of which we form a part;
we are constantly aware of feeling, or hearing, or smelling, and, unless
we happen to be in the dark, of seeing; at intervals we taste. We call
the information thus obtained =sensation=.
When we have any of these sensations we commonly say that we feel, or
hear, or smell, or see, or taste, something. A certain scent makes us
say we smell onions; a certain flavour, that we taste apples; a certain
sound, that we hear a carriage; a certain appearance before our eyes,
that we see a tree; and we call that which we thus perceive by the aid
of our senses a =thing= or an =object=.
2. =Causes and Effects.=
Moreover, we say of all these things, or objects, that they are the
=causes= of the sensations in question, and that the sensations are the
=effects= of these causes. For example, if we hear a certain sound, we
say it is caused by a carriage going along the road, or that it is the
effect, or the consequence, of a carriage passing along. If there is a
strong smell of burning, we believe it to be the effect of something on
fire, and look about anxiously for the cause of the smell. If we see a
tree, we believe that there is a thing or object, which is the cause of
that appearance in our field of view.
3. =The reason Why. Explanation.=
In the case of the smell of burning, when we find on looking about, that
something actually is on fire, we say indifferently either that we have
found out the cause of the smell, or that we know the =reason why= we
perceive that smell; or that we have =explained= it. So that to know the
reason why of anything, or to explain it, is to know the cause of it.
But that which is the cause of one thing is the effect of another. Thus,
suppose we find some smouldering straw to be the cause of the smell of
burning, we immediately ask what set it on fire, or what is the cause of
its burning? Perhaps we find that a lighted lucifer match has been
thrown into the straw, and then we say that the lighted match was the
cause of the fire. But a lucifer match would not be in that place unless
some person had put it there. That is to say, the presence of the
lucifer match is an effect produced by somebody as cause. So we ask why
did any one put the match there? Was it done carelessly, or did the
person who put it there intend to do so? And if so, what was his motive,
or the cause which led him to do such a thing? And what was the reason
for his having such a motive? It is plain that there is no end to the
questions, one arising out of the other, that might be asked in this
fashion.
Thus we believe that everything is the effect of something which
preceded it as its cause, and that this cause is the effect of something
else, and so on, through a chain of causes and effects which goes back
as far as we choose to follow it. Anything is said to be explained as
soon as we have discovered its cause, or the reason why it exists; the
explanation is fuller, if we can find out the cause of that cause; and
the further we can trace the chain of causes and effects, the more
satisfactory is the explanation. But no explanation of anything can be
complete, because human knowledge, at its best, goes but a very little
way back towards the beginning of things.
4. =Properties and Powers.=
When a thing is found always to cause a particular effect, we call that
effect sometimes a =property=, sometimes a =power= of the thing. Thus
the odor of onions is said to be a property of onions, because onions
always cause that particular sensation of smell to arise, when they are
brought near the nose; lead is said to have the property of heaviness,
because it always causes us to have the feeling of weight when we handle
it; a stream is said to have the power to turn a water-wheel, because it
causes the water-wheel to turn; and a venomous snake is said to have the
power to kill a man, because its bite may cause a man to die. Properties
and powers, then, are certain effects caused by the things which are
said to possess them.
5. =Artificial and Natural Objects. Nature.=
A great many of the things brought to our knowledge by our senses, such
as houses and furniture, carriages and machines, are termed =artificial
things= or =objects=, because they have been shaped by the =art= of man;
indeed, they are generally said to be made by man. But a far greater
number of things owe nothing to the hand of man, and would be just what
they are if mankind did not exist,—such as the sky and the clouds; the
sun, moon and stars; the sea with its rocks and shingly or sandy shores;
the hills and dales of the land; and all wild plants and animals. Things
of this kind are termed =natural objects=, and to the whole of them we
give the name of =Nature=.
6. =Artificial Things are only Natural Things shaped and brought
together or separated by Men.=
Although this distinction between =nature= and =art=, between =natural=
and =artificial= things, is very easily made and very convenient, it is
needful to remember that, in the long run, we owe everything to nature;
that even those artificial objects which we commonly say are made by
men, are only natural objects shaped and moved by men; and that, in the
sense of =creating=, that is to say, of causing something to exist which
did not exist in some other shape before, man can make nothing whatever.
Moreover, we must recollect that what men do in the way of shaping and
bringing together or separating natural objects, is done in virtue of
the powers which they themselves possess as natural objects.
Artificial things are, in fact, all produced by the action of that part
of nature which we call mankind, upon the rest.
We talk of “making” a box, and rightly enough, if we mean only that we
have shaped the pieces of wood and nailed them together; but the wood is
a natural object and so is the iron of the nails. A watch is “made” of
the natural objects gold and other metals, sand, soda, rubies, brought
together, and shaped in various ways; a coat is “made” of the natural
object, wool; and a frock of the natural objects, cotton or silk.
Moreover, the men who make all these things are natural objects.
Carpenters, builders, shoemakers, and all other artisans and artists,
are persons who have learned so much of the powers and properties of
certain natural objects, and of the chain of causes and effects in
nature, as enables them to shape and put together those natural objects,
so as to make them useful to man.
A carpenter could not, as we say, “make” a chair unless he knew
something of the properties and powers of wood; a blacksmith could not
“make” a horseshoe unless he knew that it is a property of iron to
become soft and easily hammered into shape when it is made red-hot; a
brickmaker must know many of the properties of clay; and a plumber could
not do his work unless he knew that lead has the properties of softness
and flexibility, and that a moderate heat causes it to melt.
So that the practice of every art implies a certain knowledge of natural
causes and effects; and the improvement of the arts depends upon our
learning more and more of the properties and powers of natural objects,
and discovering how to turn the properties and the powers of things and
the connections of cause and effect among them to our own advantage.
7. =Many Objects and Chains of Causes and Effects in Nature are out of
our reach.=
Among natural objects, as we have seen, there are some that we can get
hold of and turn to account. But all the greatest things in nature and
the links of cause and effect which connect them, are utterly beyond our
reach. The sun rises and sets; the moon and the stars move through the
sky; fine weather and storms, cold and heat, alternate. The sea changes
from violent disturbance to glassy calm, as the winds sweep over it with
varying strength or die away; innumerable plants and animals come in
being and vanish again, without our being able to exert the slightest
influence on the majestic procession of the series of great natural
events. Hurricanes ravage one spot; earthquakes destroy another;
volcanic eruptions lay waste a third. A fine season scatters wealth and
abundance here, and a long drought brings pestilence and famine there.
In all such cases, the direct influence of man avails him nothing; and,
so long as he is ignorant, he is the mere sport of the greater powers of
nature.
8. =The Order of Nature: nothing happens by Accident, and there is no
such thing as Chance.=
But the first thing that men learned, as soon as they began to study
nature carefully, was that some events take place in regular order and
that some causes always give rise to the same effects. The sun always
rises on one side and sets on the other side of the sky; the changes of
the moon follow one another in the same order and with similar
intervals; some stars never sink below the horizon of the place in which
we live; the seasons are more or less regular; water always flows
down-hill; fire always burns; plants grow up from seed and yield seed,
from which like plants grow up again; animals are born, grow, reach
maturity, and die, age after age, in the same way. Thus the notion of an
=order of nature= and of a fixity in the relation of cause and effect
between things gradually entered the minds of men. So far as such order
prevailed it was felt that things were explained; while the things that
could not be explained were said to have come about by =chance=, or to
happen by =accident=.
But the more carefully nature has been studied, the more widely has
order been found to prevail, while what seemed disorder has proved to be
nothing but complexity; until, at present, no one is so foolish as to
believe that anything happens by chance, or that there are any real
accidents, in the sense of events which have no cause. And if we say
that a thing happens by chance, everybody admits that all we really mean
is, that we do not know its cause or the reason why that particular
thing happens. Chance and accident are only _aliases_ of ignorance.
At this present moment, as I look out of my window, it is raining and
blowing hard, and the branches of the trees are waving wildly to and
fro. It may be that a man has taken shelter under one of these trees;
perhaps, if a stronger gust than usual comes, a branch will break, fall
upon the man, and seriously hurt him. If that happens it will be called
an “accident,” and the man will perhaps say that by “chance” he went
out, and then “chanced” to take refuge under the tree, and so the
“accident” happened. But there is neither chance nor accident in the
matter. The storm is the effect of causes operating upon the atmosphere,
perhaps hundreds of miles away; every vibration of a leaf is the
consequence of the mechanical force of the wind acting on the surface
exposed to it; if the bough breaks, it will do so in consequence of the
relation between its strength and the force of the wind; if it falls
upon the man it will do so in consequence of the action of other
definite natural causes; and the position of the man under it is only
the last term in a series of causes and effects, which have followed one
another in natural order, from that cause, the effect of which was his
setting out, to that the effect of which was his stepping under the
tree.
But, inasmuch as we are not wise enough to be able to unravel all these
long and complicated series of causes and effects which lead to the
falling of the branch upon the man, we call such an event an accident.
9. =Laws of Nature; Laws are not Causes.=
When we have made out by careful and repeated observation that something
is always the cause of a certain effect, or that certain events always
take place in the same order, we speak of the truth thus discovered as a
=law of Nature=. Thus it is a law of nature that anything heavy falls to
the ground if it is unsupported; it is a law of nature that, under
ordinary conditions, lead is soft and heavy, while flint is hard and
brittle; because experience shows us that heavy things always do fall if
they are unsupported, that, under ordinary conditions, lead is always
soft and that flint is always hard.
In fact, everything that we know about the powers and properties of
natural objects and about the order of nature may properly be termed a
law of nature. But it is desirable to remember that which is very often
forgotten, that the laws of nature are not the causes of the order of
nature, but only our way of stating as much as we have made out of that
order. Stones do not fall to the ground in consequence of the law just
stated, as people sometimes carelessly say; but the law is a way of
asserting that which invariably happens when heavy bodies at the surface
of the earth, stones among the rest, are free to move.
The laws of nature are, in fact, in this respect, similar to the laws
which men make for the guidance of their conduct towards one another.
There are laws about the payment of taxes, and there are laws against
stealing or murder. But the law is not the cause of a man’s paying his
taxes, nor is it the cause of his abstaining from theft and murder. The
law is simply a statement of what will happen to a man if he does not
pay his taxes, and if he commits theft or murder; and the cause of his
paying his taxes, or abstaining from crime (in the absence of any better
motive) is the fear of consequences which is the effect of his belief in
that statement. A law of man tells what we may expect society will do
under certain circumstances; and a law of nature tells us what we may
expect natural objects will do under certain circumstances. Each
contains information addressed to our intelligence, and except so far as
it influences our intelligence, it is merely so much sound or writing.
While there is this much analogy between human and natural laws,
however, certain essential differences between the two must not be
overlooked. Human law consists of commands addressed to voluntary
agents, which they may obey or disobey; and the law is not rendered null
and void by being broken. Natural laws, on the other hand, are not
commands but assertions respecting the invariable order of nature; and
they remain laws only so long as they can be shown to express that
order. To speak of the violation, or the suspension, of a law of nature
is an absurdity. All that the phrase can really mean is that, under
certain circumstances the assertion contained in the law is not true;
and the just conclusion is, not that the order of nature is interrupted,
but that we have made a mistake in stating that order. A true natural
law is an universal rule, and, as such, admits of no exceptions.
Again, human laws have no meaning apart from the existence of human
society. Natural laws express the general course of nature, of which
human society forms only an insignificant fraction.
10. =Knowledge of Nature is the Guide of Practical Conduct.=
If nothing happens by chance, but everything in nature follows a
definite order, and if the laws of nature embody that which we have been
able to learn about the order of nature in accurate language, then it
becomes very important for us to know as many as we can of these laws of
nature, in order that we may guide our conduct by them.
Any man who should attempt to live in a country without reference to the
laws of that country would very soon find himself in trouble; and if he
were fined, imprisoned, or even hanged, sensible people would probably
consider that he had earned his fate by his folly.
In like manner, any one who tries to live upon the face of this earth
without attention to the laws of nature will live there for but a very
short time, most of which will be passed in exceeding discomfort; a
peculiarity of natural laws, as distinguished from those of human
enactment, being that they take effect without summons or prosecution.
In fact, nobody could live for half a day unless he attended to some of
the laws of nature; and thousands of us are dying daily, or living
miserably, because men have not yet been sufficiently zealous to learn
the code of nature.
It has already been seen that the practice of all our arts and
industries depends upon our knowing the properties of natural objects
which we can get hold of and put together; and though we may be able to
exert no direct control over the greater natural objects and the general
succession of causes and effects in nature, yet, if we know the
properties and powers of these objects, and the customary order of
events, we may elude that which is injurious to us, and profit by that
which is favourable.
Thus, though men can nowise alter the seasons or change the process of
growth in plants, yet having learned the order of nature in these
matters, they make arrangements for sowing and reaping accordingly; they
cannot make the wind blow, but when it does blow they take advantage of
its known powers and probable direction to sail ships and turn
windmills; they cannot arrest the lightning, but they can make it
harmless by means of conductors, the construction of which implies a
knowledge of some of the laws of that electricity, of which lightning is
one of the manifestations. Forewarned is forearmed, says the proverb;
and knowledge of the laws of nature is forewarning of that which we may
expect to happen, when we have to deal with natural objects.
11. =Science: the Knowledge of the Laws of Nature obtained by
Observation, Experiment, and Reasoning.=
No line can be drawn between common knowledge of things and scientific
knowledge; nor between common reasoning and scientific reasoning. In
strictness all accurate knowledge is =Science=; and all exact reasoning
is scientific reasoning. The method of =observation= and =experiment= by
which such great results are obtained in science, is identically the
same as that which is employed by every one, every day of his life, but
refined and rendered precise. If a child acquires a new toy, he observes
its character and experiments upon its properties; and we are all of us
constantly making observations and experiments upon one thing or
another.
But those who have never tried to observe accurately will be surprised
to find how difficult a business it is. There is not one person in a
hundred who can describe the commonest occurrence with even an approach
to accuracy. That is to say, either he will omit something which did
occur, and which is of importance; or he will imply or suggest the
occurrence of something which he did not actually observe, but which he
unconsciously infers must have happened. When two truthful witnesses
contradict one another in a court of justice, it usually turns out that
one or other, or sometimes both, are confounding their inferences from
what they saw with that which they actually saw. A swears that B picked
his pocket. It turns out that all that A really knows is that he felt a
hand in his pocket when B was close to him; and that B was not the
thief, but C, whom A did not observe. Untrained observers mix up
together their inferences from what they see with that which they
actually see in the most wonderful way; and even experienced and careful
observers are in constant danger of falling into the same error.
Scientific observation is such as is at once full, precise, and free
from unconscious inference.
Experiment is the observation of that which happens when we
intentionally bring natural objects together, or separate them, or in
any way change the conditions under which they are placed. Scientific
experiment, therefore, is scientific observation performed under
accurately known artificial conditions.
It is a matter of common observation that water sometimes freezes. The
observation becomes scientific when we ascertain under what exact
conditions the change of water into ice takes place. The commonest
experiments tell us that wood floats in water. Scientific experiment
shows that, in floating, it displaces its own weight of the water.
Scientific =reasoning= differs from ordinary reasoning in just the same
way as scientific observation and experiment differ from ordinary
observation and experiment—that is to say, it strives to be accurate;
and it is just as hard to reason accurately as it is to observe
accurately.
In scientific reasoning general rules are collected from the observation
of many particular cases; and, when these general rules are established,
conclusions are deduced from them, just as in every-day life. If a boy
says that “marbles are hard,” he has drawn a conclusion as to marbles in
general from the marbles he happens to have seen and felt, and has
reasoned in that mode which is technically termed =induction=. If he
declines to try to break a marble with his teeth, it is because he
consciously, or unconsciously, performs the converse operation of
=deduction= from the general rule “marbles are too hard to break with
one’s teeth.”
You will learn more about the process of reasoning when you study
=Logic=, which treats of that subject in full. At present, it is
sufficient to know that the laws of nature are the general rules
respecting the behaviour of natural objects, which have been collected
from innumerable observations and experiments; or, in other words, that
they are inductions from those observations and experiments. The
practical and theoretical results of science are the products of
deductive reasoning from these general rules.
Thus science and common sense are not opposed, as people sometimes fancy
them to be, but science is perfected common sense. Scientific reasoning
is simply very careful common reasoning, and common knowledge grows into
scientific knowledge as it becomes more and more exact and complete.
The way to science then lies through common knowledge; we must extend
that knowledge by careful observation and experiment, and learn how to
state the results of our investigations accurately, in general rules or
laws of nature; finally, we must learn how to reason accurately from
these rules, and thus arrive at rational explanations of natural
phenomena, which may suffice for our guidance in life.
II. MATERIAL OBJECTS.—A. MINERAL BODIES.
12. =The Natural Object Water.=
One of the commonest of common natural objects is =water=; everybody
uses it in one way or another every day; and consequently everybody
possesses a store of loose information—of common knowledge—about it.
But, in all probability, a great deal of this knowledge has never been
attended to by its possessor; and certainly, those who have never tried
to learn how much may be known about water, will be ignorant of a great
many of its powers and properties and of the laws of nature which it
illustrates; and consequently will be unable to account for many things
of which the explanation is very easy. So we may as well make a
beginning of science by studying water.
13. =A Tumbler of Water.=
Suppose we have a tumbler half-full of water. The tumbler is an
artificial object (§ 5); that is to say, certain natural objects have
been brought together and heated till they melted into glass, and this
glass has been shaped by a workman. The water, on the other hand, is a
natural object, which has come from some river, pond, or spring; or it
may be from a water-butt into which the rain which has fallen on the
roof of a house has flowed.
Now the water has a vast number of peculiarities. For example, it is
transparent, so that you can see through it; it feels cool; it will
quench thirst and dissolve sugar. But these are not the characters which
it is most convenient to begin with.
14. =Water occupies Space; it offers Resistance; it has Weight; and is
able to transfer Motion which it has acquired; it is therefore a form of
Matter.=
The water, we see, fills the cavity of the tumbler for half its height,
therefore it occupies that much =space=, or has that bulk or =volume=.
If you put the closed end of another tumbler of almost the same size
into the first, you will find that when it reaches the water, the latter
offers a resistance to its going down, and unless some of the water can
get out, the end of the second tumbler will not go in. Any one who falls
from a height into water will find that he receives a severe shock when
he reaches it. Water therefore offers =resistance=.
If the water is emptied out, the tumbler feels much lighter than it was
before; water, therefore, has =weight=.
And, finally, if you throw the water out of the tumbler at any slightly
supported object, the water hitting against it would knock it over. That
is to say, the water being put in motion is able to =transfer= that
motion to something else.
All these =phenomena=, as things which happen in nature are often
called, are effects of which water, under the conditions mentioned, is
the cause, and they may therefore be said to be properties (§ 4) of
water.
All things which occupy space, offer resistance, possess weight and
transfer motion to other things when they strike against them, are
termed =material substances= or =bodies=, or simply =matter=. Water,
therefore, is a kind, or form, of matter.
15. =Water is a liquid.=
You will easily observe that, though water occupies space, it has no
definite shape, but fits itself exactly to the figure of the vessel
which holds it. If the tumbler is cylindrical, the contour of the
surface of the water will be circular when the tumbler is held
vertically, and will change, without the least break or interruption, to
more and more of an oval when the tumbler is inclined, and whatever the
shape of the vessel into which you pour it, the sides of the water
always exactly fit against the sides of the vessel. If you put your
finger into the water you can move it in all directions with scarcely
any feeling of obstacle. If you pull your finger out there is no hole
left, the water on all sides rushing together to fill up the space that
was occupied by the finger. You cannot take up a handful of water, for
it runs away between your fingers, and you cannot raise it into a
permanent heap. All this shows that the parts of water move upon one
another with great ease. The same fact is illustrated if the tumbler is
inclined so that the level of the surface rises above the edge of the
tumbler on one side, and the water is therefore to some extent
unsupported by the tumbler at this point. The water then =flows= over in
a stream and falls to the ground, where it spreads out and runs to the
lowest accessible place, or gradually soaks up into crevices.
Nevertheless, although the parts of the water thus loosely slip and
slide upon one another, yet they hold together to a certain extent. If
the surface of the water is just touched with the finger, a little of it
will adhere; and if the finger is then slowly and carefully raised, the
adjacent water will be raised up into a slender column which acquires a
noticeable length before it breaks. So, in the early morning, after
heavy dew, you may see the water upon cabbage-leaves and blades of grass
in spherical drops, the parts of which similarly hold together.
Material substances, the parts of which are so movable that they fit
themselves exactly to the sides of any vessel which contains them, and
which flow when they are not supported, are called =fluids=, and fluids
the parts of which do not fly off from one another, but hold together as
those of water do, are called =liquids=.
Water therefore is a liquid.
16. =Water is almost incompressible.=
It has been seen that water, like every other material substance,
resists the intrusion of other matter into the place which it occupies.
But many things, though they resist, can be easily squeezed or
=compressed= into a smaller volume. This, however, is not the case with
water, which like other liquids, is almost =incompressible=; that is to
say, an immense pressure is needful to cause its volume to diminish to
any appreciable extent. It may seem strange that anything so apparently
yielding as water should yet be almost as difficult to squeeze as so
much iron; but the apparent yieldingness of water is due to the ease
with which it changes its shape; and, if water is prevented from
changing its shape, it is very difficult to drive its parts closer
together. It has been ascertained that if water is confined in a closed
space, a pressure amounting to fifteen pounds on the square inch
diminishes its volume by only 1/20000th part. Take a common syringe, and
having seen that the plug or =piston= fits the =cylinder= of the syringe
well, put the nozzle into water and draw the piston up. Then turn the
nozzle upward and push upon the piston till a little of the water
squirts out, so as to make sure that the cylinder contains nothing but
water. Now put your finger on the opening of the nozzle firmly, so as to
stop any water from passing out, and then try to push the piston down.
You will find that you cannot make it stir without great force; and, if
the piston moves appreciably, it will be because some of the water has
escaped by the sides of the piston. In fact, if the piston presented a
square inch of surface, and fitted accurately, and the column of water
in the cylinder were one inch long, it must be pressed down by a weight
of 30,000 pounds (about thirteen tons) to make it move one-tenth of an
inch.
17. =The Meaning of Weight.=
Let us next consider the property of weight. We say that anything has
weight when, on trying to lift it from the ground, or on holding it in
the hand, we have a feeling of effort. Or again, if anything which is
supported at a certain height above the ground, falls when the support
is taken away, we say that it has weight. Now the ground merely means
the surface of the earth; and, as all bodies which possess weight fall
directly towards the surface of the earth when they are not kept away
from it by some support, we may say that all bodies which have weight
tend to fall in this way. And it does not matter on what part of the
surface of the earth you make the experiment. Rain consists of drops of
water, and it does not matter whether we watch a shower in calm weather
here, or in New Zealand; the drops fall perpendicularly towards the
ground. But we know that the earth is a globe, and that New Zealand is
at our antipodes, or on the opposite side of the globe to England. Hence
if two showers are falling at the same time, one in New Zealand and one
here—the drops must be falling in opposite directions, towards one
another; that is, towards the centre of the earth which lies between
them. In fact, all bodies which have weight tend to fall towards the
centre of the earth—that is to say they fall in this way if there is
nothing to prevent them; and when we speak of weight we mean this
tendency to fall. To call anything heavy, is the same as saying that we
fully expect that, if there is nothing to support it, it will fall to
the ground; or that if we support it ourselves we shall be conscious of
effort.
18. =Gravity and Gravitation.=
The word =gravity=, when it was first used, had exactly the same meaning
as weight; and a body which has weight is said to =gravitate= towards
the center of the earth. But gravity has now acquired a much wider sense
than weight. For an immense number of careful observations and
experiments have established the general rule, or law of nature, that
every material substance, tends to approach every other material
substance, just in the same way as a drop of rain falls towards the
earth; and, in fact, that any two portions of matter, whatever the
nature of that matter may be, will move towards one another if there is
nothing to prevent them from doing so.
To make this clear, let us suppose that the only material bodies in the
universe were two spherical drops of water, each a tenth of an inch in
diameter. Each of these drops would have the same bulk as the other, and
would be a quantity of matter exactly equivalent to the other. Then,
however great the distance which separated these two drops, they would
begin to approach one another; and, each moving with gradually
increasing swiftness, they would at length meet in a point exactly
half-way between the positions which they at first occupied. But if the
bulk of one drop were greater than that of the other drop, then the
larger would move more slowly, and the point of meeting would be by so
much nearer the larger drop. It follows that, if the one body of water
were as big as the earth and the other remained of its original size, no
bigger than a rain-drop—the motion of the large mass towards the small
one would be an inconceivably minute fraction of the total distance
travelled over. It would appear as if the large body were perfectly
still and drew the small body to itself.
This is just what happens when a single drop of water falls from a
cloud, say through a distance of a mile, to the earth. The earth really
moves towards it, just as it moves towards the earth, on the straight
line which joins the centres of the two. But the length of this line
which each travels over is =inversely proportional= to the quantity of
matter in each, that is to say is the less the bigger the quantity. So
that we have a rule-of-three sum. As the quantity of matter in the earth
is to that in a rain-drop, so is a mile to the distance travelled over
by the earth. And if any one worked out this sum, he would find that the
fourth term of the proportion would be an inconceivably minute fraction
of an inch. For all practical purposes, therefore, we may consider the
earth to be at rest in relation to all falling bodies, inasmuch as the
quantity of matter in any falling body is insignificant, in comparison
with that contained in the earth.
What is true of water is true, so far as we know, of all kinds of
matter, and we therefore say that it is a law of nature that all kinds
of matter possess gravity; that is to say, that of any two, each tends
to move towards the other, at a speed which is the slower the greater
the quantity of matter it contains in proportion to that which the other
contains; and this speed gradually becomes quicker as the two bodies
approach.
What is usually called the =law of gravitation= is a statement of the
same observed facts in another and more complete fashion. (See _Physics
Primer_.)
19. =The cause of Weight: Attraction: Force.=
We know nothing whatever of the reason why bodies possess weight. Bodies
do not fall on account of the law of gravitation (§ 9); nor does their
gravity explain why they fall. Gravity, as we have seen, is only a name
for weight, and the law of gravitation is only a statement of =how=
bodies approach one another, not =why= they do so.
It is often said that gravitation is =attraction=, and that bodies fall
to the earth because the earth attracts them. But the word “attract”
simply means to “draw towards,” and “attraction” means nothing but
“drawing towards;” and to say, when two bodies move towards one another,
that they are “drawn towards” one another, is simply to describe the
fact and makes us no whit wiser than we were before. On the contrary,
unless we take great care, it may make us a little less wise. For the
words “drawing towards” are so closely associated with ropes and hooks
and the act of pulling, that we are easily led to fancy the existence of
some analogous invisible machinery in the case of mutually attractive
bodies.
Again, gravitation is spoken of as a =force=; and as the word force is
in very common use, let us try to make out what we mean by it. A man is
said to exert force when he pushes or pulls anything so as either to
exert pressure upon it or to put it in motion. A wrestler’s force is
proved by his hug; a bowler’s force is shown by the swiftness of motion
of the ball.
Force, then, is the name which we give to that which causes or, in the
case of pressure, tends to cause, motion. The force of gravity therefore
means the cause of the pressure which we feel when bodies which possess
gravity are supported by our bodies, and the cause of their movement
towards the centre of the earth, when they are free to move. But it is
exactly about the cause of these phenomena that we know nothing
whatever.
A good deal of mischief is done by the inaccurate use of such words as
attraction and force, as if they were the names of things having an
existence apart from natural objects, and from the series of causes and
effects which are open to our observation; while they are, in reality,
merely the names of the unknown causes of certain phenomena. And it is
worth while to take pains to get clear ideas on this head at the outset
of the study of science.
Let us remember then that, so far as we know, it is a law of nature,
that any two material bodies, if they are free to move, approach one
another with gradually increasing swiftness; and that the space over
which each travels before the two meet, is inversely proportional to the
quantity of matter which it contains. =Attraction of gravitation= is a
name for this general fact; =weight= is the name for the fact in the
case of terrestrial bodies; =force= is a name which we give to the
unknown cause of the fact. The fact is that which it is important to
know. The names are of no great consequence so long as we recollect that
they are merely names and not things.
20. =The Weight of Water is Proportioned to its Bulk.=
We must next consider, not weight in general, but the weight of water.
We say that a tumbler full of water is heavier than an empty tumbler,
because the full tumbler gives us a greater feeling of effort when we
lift it than the empty tumbler does. The more water there is in the
tumbler the greater is the effort. A pail full of water requires still
more effort, though the empty pail feels quite light; and, when we come
to deal with a large tub full of water, we may be unable to stir it,
though the empty tub could be lifted with ease. Thus it seems that the
greater the bulk of water the more it weighs, and the less the bulk the
less it weighs. But then a single drop of water in the palm of the hand
seems to weigh nothing at all. However, this clearly cannot be, for the
drop falls to the ground readily, and therefore it must have weight.
Moreover, a few thousand drops would fill the tumbler, and if a thousand
drops weigh something, each drop must have a thousandth of that weight.
The fact is that our feeling of effort is a very rough measure of
weight, and does not enable us to compare small weights, or even to
perceive them if they are very small. To know anything accurately about
weight we must have recourse to an instrument which is contrived for the
purpose of measuring weights with precision.
21. =The Measuring of Weights. The Balance.=
Such an instrument is the _balance_ or _scales_, which you may see in
every grocer’s shop. It is composed of a beam which moves easily on a
pivot fixed to its middle, and which has a scale-pan attached to each
end. So long as both scale-pans are empty the beam is horizontal; but if
you put anything which has weight into one, that one goes down and the
other rises. If now you either pull or push the empty scale downwards,
the beam may be brought into the horizontal position again, and the
effort required to bring it into the horizontal position will be the
greater, the greater the weight of the body in the opposite scale. An
ounce in the one scale is easily raised by the pressure of a finger in
the other. A pound requires more effort; ten pounds needs putting out
the strength of the arm; to raise fifty pounds involves still more
exertion; while a couple of hundredweight will not be stirred by the
strongest push or pull upon the empty scale.
Suppose that, instead of pressing down the empty scale, you put
something that has weight into it; then, as soon as this weight is equal
to that in the other scale, the beam will become horizontal. In fact,
one scale has just as much tendency to move towards the centre of the
earth as the other has, and as neither can go down without pulling the
other up, they neutralise one another. It comes to the same thing, as if
two boys of equal strength were pulling one against the other; so long
as the pulls in opposite directions are equal, of course neither boy can
stir; while the smallest addition of strength to one enables him to pull
the other over.
22. =The Weight of the same Bulk or Volume of Water is Constant under
the same conditions. Mass. Density.=
Now let two graduated thin glass measures be put into the two scales,
and made to counterpoise one another exactly. Then, if even a single
drop of water is put into the one measure the scale will descend, if the
balance is a good one; showing that the drop has weight. If the measures
are graduated accurately, then whatever volume of water is put into one,
an exactly similar volume of the same water must be put into the other
to make the beam level. This obviously means that =the same volume of
water under the same circumstances always has the same weight=.
In § 18 it was said that bodies tend to move towards one another with a
relative velocity[1] which is inversely proportional to the quantity of
matter which they contain. But how are we to measure quantity of matter?
Is it to be estimated by the space which it occupies; that is, by its
volume? or are we to estimate the quantity of matter in a body by its
weight? You will soon learn that the volume of all bodies is constantly
changing in correspondence with the changes in the pressure exerted by
other bodies, but more especially in correspondence with the changes of
temperature to which they are subjected; while the weight of the same
body, at the same point on the earth’s surface, never alters. Hence we
may take the weight of a body as a measure of the quantity of matter
which it contains; and it follows that, for the same weight, the larger
the volume of a body the less matter it contains proportionally to its
volume, and the less the volume, the more matter it contains. The
proportion of its weight to its volume gives us the =density= of a body.
Footnote 1:
Velocity, or swiftness, is measured by the distance over which a body
travels, in a given time. Of two bodies, one of which travels through
one foot in a second, while the other travels through two feet, the
latter has the greater relative velocity.
Now what is true of water is true of all other bodies or material
substances. Suppose that one of the measures is emptied and replaced,
the beam may be brought to the horizontal position again by means of a
piece of lead cut to exactly the right size. The piece of lead will
thenceforth furnish an exactly corresponding or equivalent weight for so
much water; and pieces of iron or brass, which counterpoise the lead,
will also be equivalents of the weight of the water, or of the lead, or
of one another. But the pieces of lead, iron, or brass will obviously be
of much less volume or bulk than the water which they counterpoise. Here
it follows that the densities of these metals, or the quantity of matter
contained in the same volume, must be much greater than in the case of
water.
What are called =weights= in commerce are pieces of lead, or iron, or
brass exactly equivalent in weight to a certain bulk of water under
certain conditions. =An imperial gallon of water thus weighs ten pounds,
and therefore an imperial pint weighs a pound and a quarter.=
23. =Equal Volumes of Different Things under the same circumstances,
have Different Weights: the Density of Different Bodies is Different.=
The important fact which has just been alluded to must be considered
more fully. We have seen that an imperial pint measure gives us the
space which is taken up by as much water as weighs a pound and a
quarter; and this space is the bulk or =volume= of that weight of water.
But if you take an ordinary pound weight and a quarter-pound weight, and
put them into an imperial pint measure, you will find that instead of
filling it, they take up only a very small portion of the space in its
interior, or in other words, of its capacity. Thus the volume of a pound
and a quarter of lead, or of iron, or of brass, is very much less than
the volume of the same weight of water; that is to say, the metals are
=denser= than water; the same volume has greater mass or more gravity.
Or, to put the case in another way, fill the tumbler with which we began
half full of water, making a mark on the side exactly at the level of
the top of the water. Then place it in one scale of a balance, and
counterpoise it with weights in the other. Next, pour out the water, and
after drying the tumbler, fill it with fine sand carefully up to the
mark. The volume of sand will be equal to the volume of water. But now
the same weights will no longer counterpoise it, and you will have to
put more weights in the opposite scale. Volume for volume, therefore,
sand is heavier than water. Throw out the sand, and put in sawdust in
the same way, and you will find that a less weight than was necessary to
counterpoise the water counterpoises the sawdust. Volume for volume,
therefore, sawdust is lighter than water. Experiment in the same way
with spirit and oil, and they will be found to be lighter than water,
while treacle will be heavier, and quicksilver very much heavier than
water.
24. =The Meaning of Heavy and Light—Specific Gravity.=
We are in the habit of using the words =heavy= and =light= rather
carelessly. We call things that are easily lifted light, and things that
are hard to lift heavy. We say that sand, which is blown about by the
wind, is light, and that a block of wood is heavy, and yet we have just
seen that sand is heavier, bulk for bulk, than wood. In order to get rid
of this double meaning, the weight of a volume of any liquid or solid,
in proportion to the weight of the same volume of water at a known
temperature and pressure, is called its =specific gravity=. Water being
taken as 1, anything a volume of which is twice as heavy as the same
volume of water is said to have the specific gravity 2; if three times,
3; if four and a half times, 4·5, and so on. Thus the specific gravity
of any liquid or solid expresses its density in proportion to that of
water under the same conditions. Sawdust, oil, and spirit have a less
specific gravity than water, while treacle, sand, and quicksilver have a
greater specific gravity. In this sense, the former three substances are
=light=, while the latter three are =heavy=.
25. =Things of greater Specific Gravity than Water sink in Water; Things
of less Specific Gravity float.=
Here are two tumblers of water. Throw some sand into one and some
sawdust into the other. What happens? The sand sinks to the bottom, the
sawdust floats at the top. We may stir them up as we like, but the sand
will tumble to the bottom and the sawdust, as obstinately, rise to the
top. Thus that which is lighter than the water floats, and that which is
heavier (bulk for bulk) sinks. So, if we pour some oil into the water,
it floats, and if we pour some coloured spirit in carefully, it also
floats; while treacle and quicksilver sink to the bottom, just as the
iron-filings do.
We saw that the iron-filings sank, because iron is heavier than water.
Here is a piece of the thin tinned sheet-iron that they make tin boxes
of. What will happen if we drop it into the water? It is heavier than
water, bulk for bulk, and therefore it will sink as you see it does.
But now here is a “tin” canister made of this very same tinned
sheet-iron. We drop that into the water, and you see it does not sink at
all, but floats at the top as if it were made of cork. Here is a
perplexity. We were sure just now that iron is heavier than water, and
here is an iron box floating! Is this an exception to the law? Not at
all; for what we said was that a thing would float if it were lighter,
bulk for bulk, than water. Now let us weigh the tin box, and having
weighed it let us next try to find out how much the same bulk of water
weighs. This may be done very simply, for the walls of the box are very
thin, so that the inside of the box is very nearly as large as the whole
box. Consequently, if we fill the box with water, and then weigh the
water, we shall find out, very nearly, what is the weight of a bulk of
water as great as that of the box. But if we do this, we shall find that
the water which was contained in the box, weighs very much more than the
box does. So that, bulk for bulk, the box, although it is made of iron,
is really lighter than water, and that is why it floats.
You will all have heard of the iron ships which are now so common, and
you may have wondered how it is, that ships made of thick plates of iron
riveted together, and weighing many thousand tons, do not go to the
bottom. But they are nothing but our tin canisters on a great scale, and
they float because each ship weighs less than a quantity of water of the
same bulk does.
It is because of this property of water to bear up things lighter than
itself, and because of that other property of being easily moved which
the particles of water have, that the sea, and rivers, and canals, are
such great highways for mankind.
For there is nothing so heavy that it may not be made to float in water,
if the box which holds it is large enough to make the weight of the
whole less than the weight of the same bulk of water. And then, having
once got the weight to float, the particles of water are so easily
moved, that the force of the winds, or of oars, or of paddles, readily
causes it to slip through the water from one place to another.
26. =A Body which Floats in Water always occupies as much Space beneath
the level of the Surface of the Water as is equal to the Volume of Water
which weighs as much as that Body; in other words, it displaces its own
Weight of Water.=
A cubic inch of water weighs about 252 grains and a half. Suppose that
the tin box in the previous experiment was square, and had the bulk of
100 cubic inches, then the weight of a corresponding volume of water
would be 25,250 grains. If the box weighed 8,416 grains, just a third of
its bulk would be immersed; if 12,625 grains, half; if 16,832 grains, it
would sink two-thirds of its volume, and so on. Or, if, when the box is
floating, you make a mark upon its side at the exact level of the
surface of the water, the bulk of that portion of the box which lies
below the water-level can be ascertained. Suppose it to be thirty cubic
inches, then the weight of the box will be 30 × 252·5 or 7575 grains.
Hence it may be said that the immersed part of a floating body takes the
place of the water which it displaces, and, as it were, represents it.
If you press downwards upon the floating box, there is a feeling of
resistance as it descends, and when the pressure is taken off, the body
immediately rises again. Hence the water presses upwards against the
bottom of the floating body. But it also presses against the sides, for
if the sides of the box are very thin they will be driven in. If a thin
empty bottle is tightly corked and lowered into deep water the cork will
be driven in, or else the bottle will be crushed.
27. =Water presses in all Directions.=
Thus water presses in all directions upon things which are immersed in
it.
If a long wooden or metal pipe, placed vertically, has its lower end
stopped with a cork which does not fit very tightly, and water is poured
into the top of the tube, the water will at first fill the part of the
tube above the cork, and its weight will exert a certain =pressure= on
the cork. In fact, if the end of the tube is stopped by applying the
palm of the hand closely against it, the downward pressure of the water
will have to be overcome by a certain amount of effort. As the water
accumulates, this downward pressure will become greater and greater
until the hand is driven away, or the cork is forced out, and the water
falls to the ground. The pressure in this case is the same as the weight
of the water, and the cork would have been driven out equally well by a
rod of lead of the same weight.
Suppose the tube to be square, and that the inside of the square
measures exactly one inch each way. Then an inch of height of the tube
will hold exactly one cubic inch of water. Since one cubic inch of water
weighs 252 grains and a half, as much water as will fill the tube about
two feet three inches and a half high, will weigh a pound (7,000
grains), and fifteen pounds of water will fill such a tube between
thirty-three and thirty-four feet high. And these respective weights
measure the pressure of two columns of water, one twenty-seven and a
half inches high, and the other nearly thirty-four feet high, on a
square inch of the surface on which they rest.
The specific gravity (§ 24) of lead is 11·45; in other words it is about
eleven and a half times denser than water. Therefore if a bar of lead
cut square and one inch in the side, and rather less than 1/11th of the
height of a column of water, is slipped into the tube in place of the
water, it will exert the same pressure on the bottom.
And now comes a difference between the lead and the water, which depends
on the fluidity of the latter. The lead exerts no pressure on the sides
of the tube, but the water does. If a small hole is cut in the side of
the tube close to the bottom, and stopped with a cork, the lead will not
press upon the cork. But if the column of water is high enough the cork
will be driven out with as much force as before, so that the water
presses just as much sideways as downwards. It is easy to satisfy
oneself of this by inserting a long glass tube, with its lower end bent
at right angles and fitted with a cork, into the side of the wooden
pipe. The water will at once rise in the tube to the same height as it
has in the pipe. Whence it is obvious that the pressure of the water on
any point of the side is exactly equal to the vertical pressure at that
point; for the pressure outwards is exactly balanced by that of the
vertical column in the tube inwards. The water in a watering-pot always
stands at the same level in the can and in the spout.
If a glass tube is bent into the shape of a =U=, and water is poured
into it, the water will always stand at the same level in the two legs
of the tube, whatever the shape of the bend may be, or the relative
capacities of the two legs, or the inclination of the tube.
And this must needs be so, for the force with which the water tends to
flow out of the one half of the arrangement depends on the vertical
height[2] of the surface of the water above the aperture of exit; so
that any column of equal vertical height must balance it.
Footnote 2:
Vertical height is the height measured along a line drawn from the
surface of the water perpendicularly to the surface of the earth. A
plumb-line is a string to one end of which a weight is attached and
thus hangs suspended. If the other end of the line is brought opposite
the surface of the water the direction of the string answers to the
line of vertical height.
That a column of water will stand at exactly the same level as any other
with which it communicates, may be seen still more simply by placing a
glass tube, open at each end, in a basin of water. However the tube may
be inclined or bent, whether its lower end is wide or narrow, the column
of water inside it will be at exactly the same level as the water
outside it. Yet, of course, the rigid glass walls of the tube cut off
all communication between the column of water inside it and the rest,
except at the bottom.
In a well-ordered town, water is supplied to every house and can be
drawn from taps placed in the highest stories. These are fed by pipes
which lead from a cistern at the top of the house. This water is brought
from a large pipe, or =main=, in the street, by a smaller house-pipe,
which is often made to twist about in various directions before it
reaches the cistern at the top of the house into which it delivers the
water. If you followed the main, you would find that it took a long
course up and down, beneath the pavement of the streets, until at last
it reached the water-works. Here you would find that the main was
connected with a reservoir; and either this reservoir is at a greater
height than any of the cisterns into which the water is delivered, or
there is some means of pumping the water from it to that height on its
way to the main. Thus the reservoir, the main, and the house-pipe form
one immense =U=-tube, and the water in the house-pipe tends to rise to
the same level as that of the water in the reservoir, and hence flows
into the cistern when the supply-pipe is open.
28. =The Transference of Motion by Moving Water: the Momentum of Moving
Water.=
Suppose a wooden vat with a horizontal tap, the sectional area[3] of the
tube of which is one square inch, inserted close to the bottom, to be
filled with water up to 100 inches above the tap. Then supposing the tap
to be shut, the pressure upon its sectional area will be 25,250 grains,
or rather more than three pounds and a half—and there is the same
pressure on every square inch of the bottom of the vat.
Footnote 3:
The sectional area of a tube is the surface occupied by its cavity
when it is cut across. It would be represented by the surface of a
piece of cardboard, like the wad of a gun, just large enough to go
into the tube.
If the tap is now turned, the water nearest to it being unsupported on
its outer side, the pressure on the inner side sets it in motion, and it
flows out in a stream. At first the stream shoots out violently and the
water is carried to a long distance. That is to say, the weight of the
column of water 100 inches high acts as a force, or cause of motion,
upon the water nearest the tap, and this water is forced out with a
velocity depending on that force in a horizontal direction. Now suppose
that you take a common toy cup-and-ball and bring the ball into the way
of the stream of water. The stream will at once strike the ball and
drive it in the same direction as that which it is itself taking. The
power which the moving water has of transferring or communicating motion
to a body which is at rest, but free to move as the ball is, is due to
its =momentum=. The greater the mass of the stream and the more rapidly
it moves, the more motion will it communicate to the ball, or the
heavier the ball it will move. Close to the mouth of the tap the
direction of the stream is horizontal; but it very soon begins to bend
downwards, and describing a rapid curve, comes to the ground. It does
this for just the same reasons that a stone thrown horizontally
describes a curve; and at length strikes the ground; and, in fact, the
stream may be regarded as so much water thrown horizontally.
These reasons are two: firstly, as soon as the water has left the tap it
is an unsupported heavy body; and, as such, it begins to fall to the
ground. Secondly, the momentum of the water is continually being
diminished by the resistance of the air through which it passes. For,
although the air which surrounds us is so thin and movable a body that
we ordinarily take no notice of it—the fact that it offers resistance to
bodies which move through it is easily observed; as, for example, in
using a fan. The water has to overcome this resistance, and its momentum
is proportionally diminished.
If, when the water leaves the tap, the air and gravitation were alike
abolished, the water, keeping its momentum, would travel for ever in the
same direction.
As the water runs out, it will be observed that the velocity of the
stream becomes less and the curve which it describes sharper, so that it
comes to the ground sooner; and finally, when the vat is nearly empty,
the stream falls nearly vertically downwards. The reason of this is that
the level of the top of the water is gradually lowered; consequently,
the height of the column which presses on the water close to the tap is
gradually lessened, and therefore its weight is diminished. But this
weight or pressure is the cause of the motion of the water, and as the
cause diminishes the effect of that cause must diminish. Therefore the
momentum of the water is gradually lessened and it is carried less and
less far horizontally in the time which it takes to fall to the ground:
until finally, it acquires no appreciable horizontal motion at all, and
so falls vertically downwards from the mouth of the tap.
29. =The Energy of Moving Water.=
If a short pipe bent at right angles like the letter =L= is fitted by
one arm on to the end of the tap, while the other is turned vertically
upwards, and the vat is full as before; when the tap is turned, the
water will shoot up into the air, and after rising for a certain
distance will stop, and then fall. In fact we shall have a fountain.
Observe the difference between the vertical jet of water and the
horizontal jet. If we leave the resistance of the air out of
consideration, the water in the horizontal jet has no obstacle to
overcome; and it might go on for ever, if its weight did not gradually
cause its path to become more and more bent towards the earth, against
which it eventually strikes.
When the jet is vertical the case is altered. The water thrown up
vertically constantly tends to fall down vertically, as any other heavy
body would do, and its momentum has to overcome the obstacle of its
gravity. Any given portion of the water is, in fact, acted upon by two
opposite tendencies, momentum urging it up, and gravity pulling it down.
Now if two equal tendencies exactly oppose one another, the body upon
which they act does not move at all; while, if one is stronger than the
other, the body moves in the direction of the stronger.
Thus a portion of water which has just left the spout shoots up, because
the velocity with which it is impelled upwards is sufficient to carry it
through a greater space in a given time, say a second, than that through
which its gravity would, in the same time, impel it downwards.
But the distance which the water will travel during this second will be
the difference between the distance which it would have ascended if
there had been no gravity forcing it down, and the distance which it
would have descended if there had been no momentum driving it up; and,
at the end of the second, the rate of its motion upwards, or its
velocity, will be proportionally slower. Thus, at the end of the first
second, the water has spent a certain portion of its momentum in
overcoming its gravity. And as there is nothing to make good the loss,
it would, if left to itself, travel more slowly, or over a less
distance, in the second second than it tended to do in the first. But
though the momentum of the water is diminished, its gravity, weight, or
tendency to fall downwards, for a given distance in a second, remains
exactly what it was, and operates in the course of the second second to
exactly the same extent as in the first. Hence, at the end of the second
second, the distance through which the water travels upwards is still
smaller, and its velocity is still more diminished. It is obvious that,
however great the disproportion between momentum and gravity to start
with, gravity must gain the day in the long run under these
circumstances. The store of momentum will be used up; and, after a
momentary rest, the water, reduced to the condition of a body without
support, will begin to be carried downwards by the unopposed action of
gravity.
The case is similar to that of a boy sculling a boat, the bows of which
are suddenly seized and the boat thrust violently backwards by a strong
man. The boat will go stern-foremost rapidly, at first, but every stroke
of the boy’s oar at the stern will retard its backward motion; until, at
length, the stock of momentum conferred upon it by the man’s thrust will
be completely exhausted in working against the boy, and the boat, after
a momentary rest, will resume its onward course. The distance to which
the boat will be propelled backwards will evidently depend upon the
amount of muscular power which the man, as it were, suddenly capitalizes
in the boat, and which the boat then slowly pays out.
We call people who possess much muscular or other power energetic; and
we estimate their =energy= by the obstacles they overcome, or, in other
words, by the =work= they do. In the present illustration the man’s
energy would be measured by the distance to which the boat was propelled
before it stopped.
It is easy to transfer this conception of energy, as the power of doing
work, to inanimate things; and thus when a body in motion overcomes any
kind of obstacles in its way, parting with its momentum and more or less
coming to rest in the process, we say that it has =energy= and that it
does =work=.
The energy of moving water is thus measured by the intensity of the
opposing forces which it can overcome multiplied into the distance which
it can travel before that energy is exhausted; that is to say, by the
work it does before it is itself reduced to a state of rest. In the case
under consideration, the energy by which gravity is overcome, for a
greater or less time, depends upon the velocity of the stream; and this
again depends upon the height of the water in the vat above the tap.
Just as the energy of the horizontal stream diminished as the level of
the water became lower, so does the energy of the vertical stream
diminish. Hence, as the vat empties, the jet becomes shorter and
shorter, until at last it sinks down to nothing.
The energy of moving water makes it, under some circumstances, one of
the most destructive of natural agents; and, under others, one of the
most useful servants of man. A stream is water falling down hill with a
velocity depending upon the inclination of its bed. As it falls it
acquires momentum and, hence, energy; and thus a mountain stream,
suddenly swollen by rain or melting snow, will tear away masses of rock
and sweep everything before it. Nothing can look softer or more harmless
than a calm sea, but if the wind sweeping over its surface puts the
water in motion, it strikes upon the shore with terrific force; and its
energy is expended in throwing up great waves, which lift vast blocks or
drive masses of shingle up the beach.
In all kinds of watermills it is the energy of more or less rapidly
falling water which is turned to account. The water is made to flow
against buckets or floats attached to the circumference of a wheel. Each
bucket or float is therefore an obstacle to which the water transfers
some of its own motion; it moves away and thus makes the wheel to which
it is fastened turn. But the turning of the wheel brings a new obstacle
in the way of the stream. This is treated in the same fashion, and the
wheel turns still further, thus introducing another obstacle in the way
of the stream upon which the same effect is produced. Thus each float,
or bucket, is a means by which some of the momentum of the stream is, as
it were, caught and transferred to the water-wheel, which consequently
turns round with a certain velocity.
But this water-wheel is now a mass of matter in motion, and therefore
itself contains a store of energy or power doing work. If a cord with a
weight at the end of it were fastened to the axle of the wheel, the cord
would be wound upon the axle, and the weight could be raised, or, in
other words, so much work would be done by the turning of the wheel and
we should thus have a rough measure of the amount of energy which had
been given up by the stream to the wheel.
The machinery of the mill is simply a set of contrivances for
transferring the energy stored up in the water-wheel to the place in
which work has to be done. In a flour-mill, for example, a series of
wheels carries it from the water-wheel to the grindstones, which it sets
in motion.
30. =The Properties of Water are Constant.=
If, whenever there is a shower, you catch some rain-water, you will find
that it possesses all the properties which have been described. It will
be found to be an almost incompressible liquid, an imperial pint of
which weighs about a pound and a quarter. It would make no difference if
the rain-water were collected in Africa or in New Zealand; or if it had
been obtained centuries ago and kept bottled up ever since. And there is
every reason to believe that rain-water will have exactly the same
properties a hundred or a thousand years hence. So far as the properties
of rain-water are concerned =the order of nature is constant=.
This, however, is by no means the same thing as saying that the
properties of water are always the same. In fact the properties of the
substance, water, vary immensely according to the conditions to which it
is exposed; but, under the same conditions, they are the same, so that
we may still say that so far as water is concerned, the order of nature
is constant.
31. =Increase of Heat at first causes Water to Increase in Volume.=
It has been seen that a certain weight of water always has the same
volume under the same conditions. The most important of these conditions
is the heat or cold to which it is exposed. Water which has stood for
some time in a warm room becomes less in volume, or =contracts=, if it
is taken into a cool place; while its volume increases, or it =expands=,
if it is made hot. The same thing is true of quicksilver, of spirit, and
of liquids in general. A =thermometer= is simply a small flask—the
bulb—with a long and narrow neck—the tube—filled with as much mercury or
spirit as will rise a short distance into the neck. If the liquid in the
bulb is warmed, its volume is increased and it overflows into the tube,
increasing the height of the column of liquid in the tube. If, on the
other hand, the liquid in the bulb is cooled, its volume is diminished;
and, as it shrinks, the column of liquid in the tube flows back into the
bulb, and the level of the top of the column is lowered.
If a mark is made on the tube, or on a scale fixed to it, at the point
which the liquid reaches when the bulb is placed in boiling water; and
another mark at the point to which it sinks when the bulb is in melting
ice; and the space between the two marks is divided into 180 equal
parts, each of these parts is what is called a “degree” in the
thermometers ordinarily used in this country (called Fahrenheit). And if
the boiling-point is counted as 212° the freezing-point must be 32° (212
- 32 = 180). With the same amount of heat the fluid in the tube always
stands at the same degree, and hence the instrument measures
=temperature=.
That hot water is lighter than cold is easily seen when a bath is filled
from two taps, one of hot and one of cold water, which run at the same
time. Unless care is taken to stir the water, the top of the bath will
be very much hotter than the bottom. Thus, an imperial pint of water
weighs a pound and a quarter only at a certain temperature or degree of
warmth, namely at 62°; if it is made hotter its volume increases, and
therefore its specific gravity diminishes.
It was for this reason that in § 22 the weight of the same volume of
water was said to be constant =under the same conditions=; and, of
course, the same qualification must be borne in mind when we speak of
the weight of a cubic inch of water being about 252 and a half grains.
Its weight is in fact 252·45 grains only when Fahrenheit’s thermometer
stands at 62°—but as this is the temperature of ordinary mild weather,
and the expansion or contraction of water for a degree about this
temperature amounts to less than 1/3000th of its volume, the weight of a
cubic inch may for all practical purposes be taken as 252 and a half
grains.
32. =Increase of Heat at length causes Water to become Steam.=
Thus a change is effected in the properties of water by heating it ever
so little. If it is more strongly heated a still greater change takes
place. You know what happens when a saucepan containing water is put on
the fire. The water gets hotter and hotter, then it begins to simmer,
and finally, when it reaches 212°, it boils away into steam, which
passes into the air and disappears. If the boiling is carried on long
enough all the water vanishes. It looks at first as if the water had
been destroyed by the heat. In reality, however, not a particle of water
has been destroyed. It has merely changed its state. The heat has
altered it from the state of liquid water into that of gaseous water,
=vapour= or =steam=.
Try the same experiment with a tea-kettle instead of a saucepan, but
only put a little water in the tea-kettle, and shut the lid well down.
Then, as soon as the water begins to boil, the steam will shoot out of
the spout in a jet; and this will go on as long as any water remains in
the kettle.
The steam, as it comes out of the spout, is so hot that it will scald
you if you put your finger in it. But you may satisfy yourself that it
is very hot, without scalding your fingers, by holding a stick of
sealing-wax in it. The wax will soften, just as if you held it before
the fire. Moreover, if you look through the steam, just where it leaves
the spout, you will see that it is quite transparent; it is only at some
little distance from the spout that it loses its transparency, changes
into a white opaque cloud, and rapidly vanishes in the air.
33. =The taking away of Heat from Steam causes the steam to change into
Hot Water.=
Now take a cold spoon, or a cold plate, and hold it against the jet of
steam, for a moment or two. When you take it away, you will find that it
is quite wet, being covered with drops of warm water, and, moreover, the
cold spoon, or plate, has become warm. And if you fit a long cold metal
pipe to the nozzle of the tea-kettle, you will find that no steam at all
issues from the end of the pipe, but only water, while the pipe becomes
warmed.
Thus the heat passes from the fire into the saucepan, or kettle, and
thence to the water which they contain; the water gets hotter and
hotter, and, when it has taken in a certain quantity of heat, it becomes
steam, or vapour of water. When the steam comes against the cold plate,
or passes through the cold pipe, it gives up the heat it has taken in to
the plate, or the metal of the pipe. They carry off the heat which kept
the water in a condition of a =vapour=, and so it passes back into the
condition of =liquid=.
Thus steam and water are two conditions of the same thing, =water=; they
are effects of the quantity of heat which the water has taken in.
34. =When Water is changed into Steam, its Volume becomes about 1,700
times greater that it was at first.=
If you could measure and weigh the water in your kettle to begin with,
and then measure and weigh all the steam into which the heat of the fire
changes it, you would find that the bulk of the steam was nearly 1,700
times as great as the bulk of the water, though the weight of the steam
would be exactly the same as that of the water. If you had a small
square cup like a die, the inside measure of which was exactly one inch
each way, it would hold one cubic inch of water. If this cup full of
water were heated till all the water was turned into steam, the steam
would nearly occupy a cubic foot; since there are 1,728 cubic inches in
a cubic foot. A cubic inch of water weighs 252½ grains, and the steam
into which it is converted has just the same weight. Thus we may say
that steam is water expanded by heat into a vapour which is of 1,700
times less specific gravity than water. On the other hand, a pint of
steam allowed to cool, becomes converted into a quantity of water, which
measures only 1/1700th of a pint, though it weighs just as much as the
whole pint of steam did. The steam, therefore, is =condensed= to a
1/1700th of its volume of water.
The power with which water expands when it is converted into steam is
very great. If you were to stop up the nozzle of the tea-kettle, the
steam, inside the kettle, in trying to expand, would burst open the lid;
and if you were to fasten down the lid, it would pretty soon burst the
kettle itself. You sometimes hear of the strong boilers of steam-engines
being burst in this way.
35. =Gases or Elastic Fluids. Air.=
Here is a glass flask with a long neck and an open mouth. If we pour
water in at the mouth until it rises to the lip we say that the flask is
full of water. If we now pour the water out we say that the flask is
empty. But is it empty? Press the flask mouth downwards into a glass jar
full of water. If the flask were empty there would be no reason why the
water should not enter the neck of the flask and stand at the same
height inside the neck as it does outside. If you take an “empty” glass
tube open at each end and press it down into the water, the water inside
and the water outside will stand at the same level. But if you put your
finger on the upper end of the tube so as to convert it into a closed
vessel, the water will enter the lower end only a little way. So with
the flask, the water enters the neck only a little way. Hence there is
something inside the “empty” tube and in the “empty” flask; something
which is material, because it occupies space and offers resistance. In
fact the flask is full of that form of matter which is termed =air=, a
thick coat of which surrounds the earth as the =atmosphere=. Air has
weight, as you will learn more fully by and by; and that air in motion
can transfer that motion to other bodies you are taught by the effects
of the winds, which are merely air in motion.
Air therefore has all the characters of a material substance. Moreover
it is a fluid, for it fits itself exactly to the shape of any vessel
which contains it; its parts are very easily moved, or we should feel
its resistance every time we move a limb; that it “flows” is seen in
every breeze and every time you use a pair of bellows, when the air is
driven in a stream out of the nozzle; and it presses on all sides
anything contained in it.
But though air is a fluid it is not a liquid. In the first place it is
very compressible. We saw that the water entered a little way into the
tube or the neck of the flask in the preceding experiment. The reason of
this is that the water compresses the air into a smaller volume. A bag
full of air, such as a common air-cushion, can be squeezed till the air
in its interior occupies a much smaller volume; and, if you treat a
syringe full of air in the same way as the syringe full of water was
treated, you will find, if the piston fits well, that it can be driven
down some distance and then springs back again. Air in fact is not only
a compressible, but it is an =elastic= fluid or =gas=. Heat expands air
just as it expands water, but the expansion of air for the same degree
of heat is much greater.
36. =Steam is an Elastic Fluid or Gas.=
In all the properties which have been mentioned water in the form of
steam is an elastic fluid or gas like air.
If a little water is placed in the flask mentioned in the preceding
section all the “empty” part of the space will contain air. If the flask
is now made hot the water will at length boil, bubbles of steam forming
in the water and breaking at its surface. By degrees, the air, which at
first lay above the water, will be driven out; and if the whole flask is
kept hot, the “empty” part of it will be full of the gaseous water,
which is transparent and colourless like air. The steam flows out of the
mouth of the flask still a clear and colourless gas; but it soon cools
and becomes condensed as a cloud of small particles of fluid water.
Steam is lighter than air, and hence it rises in the air, just as bodies
which are lighter than water rise in water.
37. =Gases and Vapours.=
Air is as much a gas in the coldest winter as it is in the hottest
summer. But air can be liquefied by exposing it to a very low
temperature, while, at the same time, it is subjected to an extremely
great pressure. Thus, the difference between gases like air, which are
condensed with extreme difficulty, and gases like steam, which are
condensed easily, is only one of degree. Nevertheless there is a certain
convenience in distinguishing those gases, which, like steam, are easily
condensed as =vapours=. In what we ordinarily call steam, all the water
of which it is composed remains gaseous only at and above the
temperature of boiling water (212° Fahrenheit). Cooled ever so little
below this point, most of it becomes condensed into hot liquid water.
However, it must be recollected that though that particular form of
gaseous water which we call steam exists only at and above the
temperature of boiling water, yet water is capable of existing in the
gaseous state down to the freezing-point.
Suppose that when our boiling flask contained nothing but water and
steam, the mouth were stopped and the lamp removed. Then, so long as the
temperature of the whole remained at that of boiling water, every cubic
inch of steam above the water in the flask would weigh about ⅐th of a
grain, since 100 cubic inches weigh about 15 grains. Suppose the
capacity of the flask, exclusively of the fluid water in it, to be 100
cubic inches. Then, to begin with, the gaseous water which it contains
will weigh 15 grains. If the flask is now allowed to cool, more and more
of the gaseous water condenses into the fluid state; but, even down to
the freezing-point, some water will remain in the gaseous state and will
fill that part of the flask which is unoccupied by the fluid water. At
blood-heat (98°) the gaseous water weighs only about a grain, though it
still occupies 100 cubic inches; at the ordinary temperature of the air
it weighs not more than ⅓rd of a grain; while, at the freezing-point,
its weight is only ⅛th of a grain. But inasmuch as there is less and
less actual weight of water in the same volume of gaseous water as the
temperature falls, it follows that the density, or specific gravity, of
the gaseous water must be less the lower the temperature. Moreover,
while, at the boiling-point, gaseous water or steam resists compression
with exactly the same force as air does, the lower the temperature the
more easily compressible is the gaseous water.
Suppose an elastic bag were to be tied on to the nozzle of a kettle full
of boiling water. If the bag were kept as hot as the boiling water it
would become fully distended, and maintain its shape in spite of the
pressure of the air upon all sides of it. If the bag were taken away it
would retain its shape so long as it was kept as hot as boiling water;
but, if it were allowed to cool, it would gradually become flattened by
the outside air squeezing up the less and less resisting gaseous water
of the lower temperatures. Hence, when the stopped flask has been
allowed to cool, the air rushes in with great violence if it is opened.
38. =The Evaporation of Water at ordinary Temperatures.=
If some water is poured into a saucer and is allowed to stand even in a
cool room or in the open air, you know that it sooner or later
disappears. Wet clothes hung on a line soon dry—that is to say, the
water clinging to them disappears or =evaporates=. The disappearance of
the water under these circumstances results from the property just
mentioned. In fact, it becomes gaseous water of the density appropriate
to the temperature, and as such mixes with the air as any other gas
would do. And as the sea, lakes, and rivers, are constantly giving off
gaseous water into the air in proportion to the temperature, it is not
wonderful that the atmosphere always contains gaseous water.
Air is said to be moist when the weight of water in a given quantity,
say 100 cubic inches, is as much, or nearly as much, as can exist in the
state of gas at the temperature. Under these circumstances, if the
temperature is lowered even a very little, some of the gaseous water is
converted into liquid water. We see this in hot moist weather, when the
outside of a tumbler of fresh drawn cold spring water immediately
becomes bedewed. The gaseous water in immediate contact with the
tumbler, in fact, is cooled down below the point at which it can all
exist as gas, and the superfluity is deposited as dew. In such days wet
clothes do not dry well, because there is, already, nearly as much
gaseous water in the atmosphere as the amount of heat marked by the
thermometer can maintain in that state.
39. =When Hot Water is cooled, it Contracts to begin with, but after a
time Expands.=
We have now seen what a wonderful change is brought about by heating
water. At first, it expands gradually and slightly; but, when it reaches
the boiling-point, it suddenly expands enormously, and is no longer a
liquid, but a gas.
On the other hand, if warm water is allowed to cool, it gradually
contracts till it reaches the ordinary temperature of the air in mild
weather; but, if the weather is very cold, or if the water is cooled
artificially, it goes on contracting only down to a certain temperature
(39°), and then begins to expand again. In this peculiarity water is
unlike all other bodies which are fluid at ordinary temperatures. Hence
the temperature of 39° is that at which pure water has its greatest
density or specific gravity, and water at this temperature is heavier,
bulk for bulk, than the same water at any other temperature. Therefore
if water at the top of a vessel is cooled down to this temperature, it
falls to the bottom, and if the water at the bottom of a vessel is
cooled below this temperature it rises to the top.
40. =Water cooled still further becomes the transparent brittle solid
Ice.=
Our tumbler of water, if put out of doors on a cold winter’s night,
would gradually cool until it assumed a temperature of 39° throughout.
Cooling below this temperature, the water so cooled would gradually
accumulate at the surface by reason of its less density, and its
temperature would fall till the thermometer placed in it marked 32°. As
soon as this upper water cooled ever so little below 32°, a film like
glass would form on its surface by the conversion of the coldest fluid
water into solid water or =ice=. And if all the water cooled down to the
same degree it would all gradually change into the same kind of
substance.
In this condition water is solid. It occupies space, offers resistance,
has weight and transmits motion as the water did, but if you shake it
out of the tumbler in a cold place it retains its form without the least
change. If you press it, it proves to be exceedingly hard and
unyielding; and, if the pressure is increased, it becomes crushed and
breaks like glass. It may thus be crushed to powder, and the ice powder
can be formed into heaps as if it were sand.
Just as any quantity of steam has exactly the same weight as the water
which was converted into it by heat; so the ice has exactly the same
weight as the water which has been converted into it by taking away
heat.
41. =Ice has less Specific Gravity than the Water from which it was
formed.=
But though the ice in the tumbler has the same weight as the water had,
it has not the same volume. The expansion which began at 39° goes on,
and when water passes into the solid state its volume is about 1/11th
greater than it was at 39°. Taking water at this temperature as 1·0, ice
has a specific gravity of 0·916.
But although water in freezing expands only to this small amount, it
resembles steam in the tremendous force with which it expands. If you
fill a hollow iron shell quite full of water, screw down the opening
tight, and then put it in a cold place where the water may freeze, the
water as it freezes will burst the iron walls of the shell. You know
that when the winter is severe, the pipes by which water is brought to a
house often burst. This is because the water in them freezes, and, being
unable to get out of the pipe, bursts it, just as you may burst a jacket
that is too tight for you by stretching yourself. Among the bare
hill-tops, or on the face of cliffs exposed to the weather, the
strongest and hardest rocks are every winter split and broken, just as
if quarrymen had been at work at them. In the summer the rain-water gets
into the little cracks and rifts in the stone and lodges there. Then the
winter comes with its cold and freezes the water. And the water bursts
the rocks asunder just as it bursts our waterpipes.
42. =Hoar Frost is the Gaseous Water which exists in the Atmosphere,
condensed and converted into Ice Crystals.=
In the winter-time you often notice, on a clear sharp night, that the
tops of the houses and the trees are covered with a white powder called
=hoar frost=; and, on the windows of the room when you wake up, you see
most beautiful figures, like delicate plants. Take a little of the
hoar-frost, or scrape off some of the stuff that makes the window look
like ground glass, and you find that it melts in your hand and turns to
water. It is in fact ice. And if you look at the figures on the window
pane with a magnifying glass you will see that they are made up bits of
ice which have a definite shape, and are arranged in a regular pattern.
Each of these definitely shaped bits of ice has been formed in the
following way. The air in the room is much warmer than that outside, and
there is mixed with it nearly as much water, derived from the breath and
the evaporation of moist surfaces, as can maintain itself in the gaseous
state at the temperature. The windowpanes, being thin, are cooled by the
outside air, and of course the gaseous water inside the room, when it
comes in contact with the cold windowpanes, becomes condensed on them
into fine drops of cold water. The panes becoming colder and colder,
these minute drops at last freeze, and the water not only becomes solid,
but it =crystallises=; that is to say, the little solid masses take on
more or less regular geometrical forms with flat faces, inclined to one
another at constant angles, so that they resemble bits of glass cut
according to particular fixed patterns. All ice is in fact crystalline,
but in ice which has been formed from thick sheets of water, the
crystals are so packed together that they cannot be distinguished
separately.
43. =When Ice is warmed it begins to change back into Water as soon as
the Temperature reaches 32°.=
A lump of ice brought out of the open air in very cold weather may have
a temperature of 30°, or 20°, or lower. If such a lump is brought into a
warm room it gradually becomes warmer, but remains unchanged otherwise,
until it has risen to 32°. Then it begins to melt, and remains at 32° as
long as it is melting; and the water which proceeds from it is at first
also at 32°.
If you were to throw a lump of ice into the middle of a hot fire, so
long as a particle of ice remained as such, it would have a temperature
of 32° and no more. This is a fact exactly parallel to that which is
observed when water is raised to the boiling-point. So long as any of
the water remains unconverted into steam it becomes no hotter. Moreover
the steam itself is at first at 212°.
44. =Ice the solid, Water the liquid, and Steam the gas, are three
states of one natural object; the Condition of each State being a
certain Amount of Heat.=
Ice, liquid water, and steam, are three things as unlike as any three
things can well be. What do we mean then by saying that they are states
of one substance, water?
What we really mean is that if we take a given quantity of water, say a
cubic inch, and change it first into ice and then into steam, there is
something which remains identically the same through all these changes.
This something is, in the first place, the weight of the material
substance. The water weighs 252½ grains, the ice into which it is
converted weighs 252½ grains, and the steam produced from it weighs 252½
grains. In the second place, the same force would cause the ice, the
water, and the steam, to move with the same rapidity; and, when set in
motion, they would produce the same effect upon anything movable against
which they struck.
In the third place, when you study chemistry, you will learn that the
ice, the steam, and the liquid water, would yield the same weight of the
same two gases, =oxygen= and =hydrogen=, and nothing else. Every one
cubic inch of water, 1,700 cubic inches of steam, and 1/111 cubic inch
of ice, yield 281/18 grains of hydrogen, with 2248/18 grains of oxygen,
and nothing else. (See § 50.)
As there is not the slightest difference in weight between a given
quantity of water and the ice, or the steam, into which it may be
converted, it is clear that the heat which is added to or taken from the
water to give rise to these several states, can possess no weight. If
then heat is a material body, it must be devoid of weight—and hence, in
former times, heat was called an =imponderable= substance. It was
thought to be a kind of fluid, called =caloric=, which had no weight,
and which drove the particles of bodies asunder, when it entered them as
they were heated, and let them come together as it left and they grew
cool.
45. =The Phenomena of Heat are the Effects of a rapid Motion of the
Particles of Matter.=
This much, however, is certain: that heat can be caused by motion. Every
boy knows that a metal button may be made quite hot by rubbing it. A
skilful smith will hammer a piece of iron red hot. The axles of wheels
become red hot by rubbing against their bearings, if they are not
properly lubricated; and even two pieces of ice may be melted by the
heat evolved when they are rubbed together. And there are abundant other
reasons, as you will find when you study physics, for the belief that
the sensation we call heat, and all the phenomena which we ascribe to
heat, are the effects of the rapid motion of matter.
However, a quiescent body may be made hot without exhibiting the least
appearance of motion. The surface of the water in a tumbler at 100° is
just as unruffled as that of the same water at 32°. What, then, is meant
by saying that heat is a kind of motion, and that the greater the heat
in any body the greater the amount of motion in that body?
The answer to this question is that the motion which causes the
phenomena of heat, is not a visible motion of the whole mass of the hot
body, but a motion of the individual =particles= of which it is
composed. And each particle moves, not straight forward, but backwards
and forwards in the same space, so that its motion may be roughly
compared to that of a pendulum, or to that of the balance-wheel of a
watch. It is in fact a sort of =vibratory= movement; each vibration
taking place through a very short distance and with extreme rapidity.
The sensation of heat is caused by the vibratory movements of the
particles of matter, just as sound is so caused. The prongs of a
tuning-fork which has been struck, certainly vibrate, for you can see
them do so if the note is low. If you now put your ear at one end of a
long piece of timber and the handle of the vibrating tuning-fork is
placed upon the other end, the vibratory motion of the tuning-fork will
be communicated to the particles of the wood and will be loudly heard.
All the time the sound is heard the particles of the wood are vibrating.
Nevertheless, the wood as a whole does not move, but its particles swing
backwards and forwards through such a minute space that their motion is
imperceptible.
But what are these =particles= of matter which by their vibration give
rise to the phenomena of heat?
46. =The Structure of Water.=
We have seen that pure water is perfectly clear and transparent. The
naked eye can discern no difference between one part and another. In
other words, it has no visible texture or =structure=. It does not
follow that it really possesses none, however, for there are many things
which seem to be the same throughout, or =homogeneous=, which yet show
structure if they are examined with a magnifying glass. Thus the surface
of a sheet of fine white paper looks perfectly even and smooth to the
eye; but a magnifying glass of no great power will show the minute woody
fibres of which it is made up; while, under a powerful microscope, the
paper looks like a coarse matting.
But if we put a small drop of water on a slide, such as is used for
microscopic objects, and cover it over with a thin glass so as to spread
it out into a film, perhaps not more than 1/10000th of an inch thick, it
may be examined with the very highest magnifying powers we can command,
and yet it looks as completely homogeneous and shows as little evidence
of being made up of separate parts as before. However, this is still no
proof that the water is not made up of little parts, or particles,
distinctly separated from one another. It may merely mean that the
particles are so extremely small that they cannot be distinguished even
by microscopes which magnify four or five thousand diameters.
It is certain that solid bodies may be divided into particles so minute
that the best microscopes show no trace of them. Common gum-mastic
cannot be dissolved by water, but it readily dissolves in strong spirit
or alcohol, and mastic varnish is an alcoholic solution of gum-mastic.
If you add water to mastic varnish, the alcohol takes away the water and
the mastic falls out, or =precipitates=, as a curdy solid composed of
very visible whitish particles. But if a drop of the varnish is added to
a good deal, say half a pint, of water and well stirred at the same
time, the mastic, though it is still precipitated as a solid, is in a
state of extremely minute division. No separate solid particles of
mastic are visible to the naked eye, but the water assumes a faint milky
tinge.
This milkiness arises from the presence of solid particles of mastic
diffused through the water; and yet, if the experiment has been properly
managed, a drop of the fluid may be spread out as before and examined
with the highest magnifying powers, and nothing can be seen of such
particles. So far as vision goes it might be a drop of pure water. Now
our best microscopes are able to show us anything solid which has a
diameter of 1/100000th of an inch, quite distinctly; and probably solid
opaque particles of much smaller size would make themselves apparent as
a turbidity or cloudiness. The particles of mastic must be therefore so
much smaller than this that they remain invisible. Hence it follows that
if water were made up of separate particles, or droplets, 1/1000000th of
an inch in diameter, and thus had the structure of a mass of very fine
shot, no microscope that has yet been constructed would enable us to see
even a trace of that structure. We could not obtain any direct evidence
of it.
47. =Suppositions or Hypotheses; their Uses and their Value.=
When our means of observation of any natural fact fail to carry us
beyond a certain point, it is perfectly legitimate, and often extremely
useful, to make a supposition as to what we should see, if we could
carry direct observation a step further. A supposition of this kind is
what is called a =hypothesis=, and the value of any hypothesis depends
upon the extent to which reasoning upon the assumption that it is true,
enables us to explain or account for the phenomena with which it is
concerned.
Thus, if a person is standing close behind you, and you suddenly feel a
blow on your back, you have no direct evidence of the cause of the blow;
and if you two were alone, you could not possibly obtain any; but you
immediately suppose that this person has struck you. Now that is a
hypothesis, and it is a legitimate hypothesis, first, because it
explains the fact; and, secondly, because no other explanation is
probable; probable meaning in accordance with the ordinary course of
nature. If your companion declared that you fancied you felt a blow, or
that some invisible spirit struck you, you would probably decline to
accept his explanation of the fact. You would say that both the
hypotheses by which he professed to explain the phenomenon were
extremely improbable; or in other words, that in the ordinary course of
nature fancies of this kind do not occur, nor spirits strike blows. In
fact, his hypotheses would be illegitimate, and yours would be
legitimate; and, in all probability, you would act upon your own. In
daily life, nine-tenths of our actions are based upon suppositions or
hypotheses, and our success or failure in practical affairs depends upon
the legitimacy of these hypotheses. You believe a man on the hypothesis
that he is always truthful; you give him pecuniary credit on the
hypothesis that he is solvent.
Thus, everybody invents, and, indeed, is compelled to invent, hypotheses
in order to account for phenomena of the cause of which he has no direct
evidence; and they are just as legitimate and necessary in science as in
common life. Only the scientific reasoner must be careful to remember
that which is sometimes forgotten in daily life, that a hypothesis must
be regarded as a means and not as an end; that we may cherish it so long
as it helps us to explain the order of nature; but that we are bound to
throw it away without hesitation as soon as it is shown to be
inconsistent with any part of that order.
48. =The Hypothesis that Water is composed of Separate Particles
(Molecules).=
It has been pointed out that we cannot see, and indeed that there is not
much hope of our ever being able to see, the separate particles of
water, even if water is composed of such particles. But it is perfectly
legitimate to suppose that water is made up of such particles, if that
hypothesis will enable us to explain the properties of water.
Let us suppose then that any portion of fluid water is really composed
of a prodigious number of particles less (and probably much less) than a
millionth of an inch in diameter. We may call these particles
=molecules=.[4]
Footnote 4:
Diminutive of _moles_, a mass.
We are justified, in accordance with the general properties of matter (§
18), in supposing that these molecules tend to approach one another. But
the fact that water is slightly compressible justifies the supposition
that its molecules are not in actual contact, but that they are
separated by interspaces, just as the motes in the air of a dusty room
are so separated.
What is it that keeps the molecules apart? We have seen that great
mechanical pressure brings them but slightly nearer to one another;
hence there is an equivalent resistance of some kind which keeps them
apart. This resistance must have the same origin as the sensation which
we know as heat, for it has been seen that diminution of heat diminishes
the bulk of water; that is, allows the molecules to come closer
together; that is, diminishes their tendency to keep asunder. Increase
of heat, on the other hand, increases the volume of water; that is to
say, drives the molecules further apart, or increases their tendency to
keep asunder.
Suppose we call the cause of the tendency of the molecules of water to
come together an =attractive force=; and the cause of their keeping
apart, which manifests itself to us as the sensation of heat and is, as
we have seen, in all probability, a rapid vibratory or whirling motion
of the molecules, a =repulsive force=; then, in the liquid state, these
forces are so adjusted that the molecules are quite free to move, and
yet hold together.
By adding heat the repulsive force is increased, until the molecules are
about twelve times as far apart as they were in each direction; while
the attractive force is overcome, and the molecules fly off in all
directions as soon as they are unconfined. On the other hand, by taking
heat away, the repulsive force is diminished, until the molecules become
inseparable and the water assumes the solid form.
It is probable that the expansion of fluid water, at a temperature below
39°, depends upon the molecules taking up a peculiar arrangement as they
approach one another. If sixteen men are formed into a column, four
deep, and each man a foot from the other, the same men may stand closer
together and yet form a hollow square, which occupies a larger space.
That the molecules of water do take up a particular order in assuming
the solid condition, is shown by the crystalline form of ice. Each
crystal of hoar-frost owes its shape to the arrangement of its
molecules, according to a definite geometrical pattern.
Thus the hypothesis that water is composed of separate molecules, is
useful, for it helps us to some extent to explain the properties of
water. And, when you study physics and learn the laws of motion, you
will find that there is no end to the number of the truths established
by observation and experiment, which can be explained by this
hypothesis. Hence it may fairly be adopted and employed as a means of
picturing to ourselves the order of nature, so long as no facts are
discovered which are inconsistent with it.
49. =All Matter is probably made up either of Molecules or of Atoms.=
The same reasons which lead to the adoption of the hypothesis that water
is composed of separate particles justify its extension to all forms of
matter whatever.
The metal =mercury= or =quicksilver=, for instance, may be supposed to
be made up of distinct particles of mercury of extreme minuteness, and
according to the temperature, these associate themselves in the solid
(frozen mercury), liquid (ordinary quicksilver), or gaseous form (vapour
of mercury). To whatever treatment pure mercury may be subjected, we
cannot get anything but mercury out of it. The particles of mercury have
never been broken up. Hence they are generally termed =atoms=, or
particles that cannot be divided; and mercury is said to be an
=element=, or a substance which is not compounded of any other
substances.
Here is a case in which it is very useful to distinguish between fact
and hypothesis. The matter of fact is that, up to the present time, no
one has been able to get out of pure mercury anything but pure mercury.
The statement that mercury is a simple substance, and therefore never
can be broken up into any other substances, is a hypothesis which future
observation and experiment may or may not confirm.
A hundred and fifty years ago it was universally believed that water was
as much an element as mercury. But water is now well known to be a
compound. In fact, as has already been said, the particles of water may
be very readily broken up or =decomposed= (in what way, you will learn
when you study chemistry) into two totally distinct substances, =oxygen=
and =hydrogen=, which are gaseous at all known temperatures, though by
combining vast pressure with extreme cold they have recently been
liquefied. Each of these gases, according to our hypothesis, consists of
particles, and since these can by no known means be further broken up,
they are considered to be =atoms= like those of mercury.
Nine parts by weight of pure water always yield eight of oxygen and one
of hydrogen. The hypothetical particle, or molecule of water, therefore,
must be composed of atoms of oxygen and hydrogen having this relative
weight; and chemists have grounds for believing that one atom of oxygen
and two atoms of hydrogen exist in each molecule of water. If this be
so, the structure of water must be more complicated than we thought at
first; and each particle of water (the molecule) must be a system
composed of three separate atoms.
50. =Elementary Bodies are neither destroyed nor is their Quantity
increased in Nature.=
It has been seen that when a cubic inch of water is dissipated by heat,
it is not destroyed, but that it merely changes its form from the fluid
to the gaseous state, while its weight remains unaltered. If the same
cubic inch of water is decomposed into oxygen and hydrogen gases, the
water is indeed destroyed, but the matter of which it consisted remains
unchanged in weight. If the water weighed 252·5 grains, the oxygen gas
will weigh 224·45 grains and the hydrogen gas will weigh 28·05 grains.
And nothing that man has been able to do has affected the weight of a
given quantity of either of these gases. So far as we know, elementary
bodies retain their weight under all circumstances, and can be traced by
it whatever shape they may take. If this is true it follows that, in the
order of nature, matter is =indestructible=: the quantity of it neither
increases nor diminishes.
Hence it follows that natural things and artificial things resemble one
another in one respect. It is true of both that the matter of which they
are composed is never destroyed and never increased; and therefore the
order of events in nature as much consists in the joining together and
putting apart of natural bodies by natural agencies, as the order of
events in the artificial world consists in the joining together and the
putting apart of natural bodies by human agencies.
51. =Simple Mixture.=
In order to learn the manner in which water may be broken up into its
elements or decomposed, you must turn to the Primer on Chemistry. But as
a preliminary to the study of that science, it may be useful to consider
some simple cases of composition and decomposition which are exemplified
by water.
If half a pint of water, coloured by putting a little ink into it, is
added to the same quantity of clean water, the two will readily mingle;
the total quantity of water will be a pint; and its colour will be just
half as dark as that of the coloured half-pint. This is a case of simple
=mixture=. The volume of the mixture equals the sum of the volumes of
the things mixed, and there is no change in the properties of these
things. So when water evaporates, the gaseous water or vapour mixes with
the air in the same way, the molecules of the one body dispersing
themselves between the molecules of the other until there is the same
proportion of each everywhere. In like manner, sand and sugar may be
(and unfortunately often are) mixed, without any change in the
properties of either, or in the space which they primitively occupied.
On the other hand, oil and water will not mix, however much you may stir
the two together; and the oil, being the lighter, rises to the top as
soon as the fluid is quiet. Nor will quicksilver and water mix, but the
quicksilver, being very much heavier than the water, rushes to the
bottom of the vessel into which the two are put. Neither will sand nor
iron filings mix with water; as heavier bodies, they also sink to the
bottom. Nor does powdered ice, though it is water in another shape, mix
with ice cold water; as a lighter body it floats at the top.
52. =Mixture followed by Increase of Density; Alcohol and Water.=
Strong spirit, or =alcohol=, is a clear transparent fluid which looks
like water, but is a very different substance. For example, it boils at
a much lower temperature, it burns with a blue flame, it has
intoxicating properties, and, like oil, it is very much lighter than
water. Hence, if coloured spirit is poured gently upon the surface of
water the spirit rests upon the water. Suppose, now, that we take a tall
measure graduated into ten equal parts. Fill the lower five with water,
and then, very gently, pour in the strongest alcohol, coloured in some
way, until the tenth mark is reached. We shall have five volumes of
water below, and an equal quantity, or five volumes, of coloured alcohol
above. Where the two are in contact, the colour will be diffused into
the water for a short distance, but not far, showing that only a slight
mixture is taking place. This, however, is not because the two fluids
mingle with difficulty; for, with slight stirring, they mix completely,
and you have a fluid the colour of which is about half as intense as
that of the alcohol, and many of the other properties of which are
intermediate between those of pure alcohol and those of pure water.
Thus far, nothing further than simple mixture, as when coloured water
was added to pure water, seems to have occurred; but, in reality,
something more has happened. In the first place, the mixture is a good
deal warmer than either of its components; that is to say, =heat= has
been =generated=. In the second place, if you measure the volume of the
whole fluid after it has cooled, it no longer stands at the mark =ten=,
but distinctly lower, or about =nine and three-quarters=. As the volume
of the mixture is less than the sum of the volumes of its two
components, it follows that the =density= of the mixture must be
=greater= than a density midway between that of the water and that of
the alcohol. In other words, the molecules in the mixture do not occupy
the same space as they did when they were separate. The result is the
same as if the ten volumes had been compressed until they occupied only
nine and three-quarters; so that the effect is a contraction similar to
that which would be brought about by taking away heat from the mixture.
In fact, as we have seen, the mixture gives out a quantity of heat.
There is another respect in which the mixture is unlike both its
constituents. It both =boils= and =freezes= at a much =lower=
temperature than =water= does, and at a =higher= temperature than
=alcohol= does. In fact pure alcohol has not yet been frozen. If the
molecules of the alcohol were merely diffused among those of the water
as water is diffused through wet sand, they ought to pass into the
gaseous state at the same temperature as that at which alcohol boils;
and it would then be very easy to separate alcohol from water by
distillation. But the fact is not so; alcohol cannot be obtained free
from water by distillation unless something which holds water very
strongly, such as quicklime, is added, so as to keep all the water back
when the fluid is heated.
Thus alcohol and water, mingled together, give rise to a fluid which is
not a mere mixture, the properties of which are known if we know the
properties of its components; it is, in strictness, a new body, in which
the molecules of the water and those of the alcohol affect one another
to a certain extent and modify the pre-existing properties of each.
This effect of different bodies upon one another becomes much more
manifest when water is brought into contact with certain solids.
53. =Solution; Water Dissolves Salt.=
If a spoonful of salt is put into a tumbler of cold water and the water
is stirred, the salt swiftly vanishes from view; and, after a time, so
far as our sense of vision goes, the water appears to be just what it
was before. But if the water in the tumbler at first weighed five ounces
and the salt weighed two ounces, the water in the tumbler will now weigh
seven ounces; the water will now taste salt, the salt is said to be
=dissolved=, and the =solution= is called =brine=. Moreover, the
solution is said to be =saturated=, for if you put more salt in it will
remain unchanged. Water, in fact, will dissolve two-fifths of its weight
of salt and no more. If the brine thus formed is put into a wide dish,
so that the water may evaporate; or if it is heated and the water boiled
away; as fast as the water diminishes, a quantity of salt, equal to
two-fifths of the water which is converted into steam, returns to the
solid state and falls to the bottom of the vessel. And when all the
water is driven off, the salt which remains will have exactly the
weight, and all the other properties which it had before it was
dissolved by the water.
Thus, contact with water has had a very singular effect upon the salt.
It appears to have changed one of the properties of the salt, namely,
its =solidity=, but to have left all the rest unaltered. We saw just now
that powdered ice does not mix with ice-cold water, but that the
fragments of ice remain solid. The moment, however, that the temperature
rises, the =cohesion=, or sticking together of the molecules, which is
the characteristic of the solid state, comes to an end; they become
loose and free to move, and they mingle with the surrounding water. Or
we may say that the ties which held the molecules of the solid together
are dissolved, so that the solid water becomes fluid.
The resemblance of this process to the dissolving of salt in water is so
obvious that, in common language, it is often said that a lump of salt
or of sugar =melts= away in water; but if you try to make salt fluid by
heat, you will have to expose it to a very high temperature, so that the
conversion of salt from the solid state into the liquid state by
solution in cold water is obviously a very different process from
liquefaction by heat. Nevertheless the result is the same so far as the
condition of the salt is concerned. The cohesion between its molecules
is destroyed, and they distribute themselves evenly among the molecules
of the water, just as the molecules of steam distribute themselves among
the molecules of air. And, when you study chemistry, you will learn how
it may be proved that the smallest drop of the solution of salt contains
exactly the same proportion of salt as the whole does.
If brine is allowed to evaporate slowly, the molecules of the salt
arrange themselves, as the water leaves them, in beautifully regular
cubical crystals. You may see them form easily enough if you watch a
drop of brine gradually dry up under a microscope. The salt crystals
contain nothing but salt. If they are heated till they become red-hot
they pass into the fluid state; and when still further heated, the fluid
salt becomes a vapour or gas and, as such, flies off into the air, or
=volatilizes=.
Thus we see that when salt and water are brought into contact, the salt
undergoes a certain amount of change, while the water does not remain
wholly unchanged. For brine no longer boils at 212°, but requires a
considerably higher temperature. The salt, as it were, holds the water
back, and prevents it from assuming the gaseous state under the same
conditions as if it were pure, just as, in the previous case, the water
held the alcohol back; or we may say that the force of heat which drives
the molecules of liquid water apart, when steam is formed, has a greater
resistance to overcome when salt is dissolved in the water. And just as
the presence of alcohol lowers the freezing point of the water with
which it is mixed, so does the presence of salt lower the freezing point
of water. Sea water, which is a weak brine, begins to freeze at about
27°; and the ice which is formed is quite pure, while the remainder of
the sea water becomes richer in salt.
If we mean by attraction that which opposes any force which tends to
separate bodies, then we may say that the molecules of salt and those of
water attract one another. And such attraction between molecules of
matter of different kinds is called =chemical attraction=.
54. =Quicklime and Water: Plaster of Paris and Water: Combination.=
Quicklime is a substance obtained by heating chalk or limestone to
redness. When pure, it is a white hard solid which can be made to pass
into the liquid and gaseous states only at enormously high temperatures.
If a lump of fresh quicklime be placed in a saucer and about one-third
of its weight of water poured upon it, there will be a great turmoil,
heat will be evolved, the water will disappear, and the lime will
crumble down into a soft white powder. This operation is what
bricklayers call =slaking= lime. And if no more water has been added
than the proportion mentioned, the pure white powder which results will
be solid and dry, the water having, to all appearance, vanished.
In the solution of salt we saw a solid become fluid under the influence
of water; in the slaking of lime the fluid water enters into the
structure of a solid. If more water is added, this solid dissolves or
becomes liquid, as the salt did, and the solution is called =limewater=.
By carefully managed evaporation of the water the lime may be recovered
in the form of crystals, just as the salt was recovered. But there is
this difference, that the salt crystals contain no water, while the lime
crystals not only contain water, but contain exactly the same proportion
as exists in slaked lime, that is to say, 18 parts water to 56 parts
lime.
The water thus bound up with the lime into a new solid holds on so
firmly to the lime that it requires a red heat to separate the two. The
lime and the water are said to be =chemically combined=; and as the
proportion of lime and water in slaked lime, or lime crystals, is always
the same, they are said to be combined in =definite proportions=; and
the slaked lime receives the special name of =hydrate of lime=.
=Gypsum= or =Plaster of Paris= is a dry white powder. If mixed with a
little water it does not slake after the fashion of quick lime, but the
mixture soon =sets= or becomes hard; and, at the same time, the greater
part of the water disappears. In fact, it has combined with the plaster
of Paris and forms part of another hydrate, in which, when the
superfluous moisture dries, not a trace of water is to be seen. It is
this property which is taken advantage of when plaster of Paris is used
in making casts and moulds. The fluid plaster is poured over and round
the body to be cast; as a fluid, it applies itself conveniently to all
the inequalities of its surface; and, when it sets, it retains the shape
which it has thus acquired. Set plaster of Paris may be perfectly dry;
but it nevertheless contains between ⅐ and ⅛ of its weight of water,
fixed and forming an integral part of the solid hydrate. And if the set
plaster is strongly heated, the combined water is driven off and it
returns to its original state.
Gypsum is found abundantly in nature, in the shape of beautiful
transparent crystals which are called =selenite=. These crystals have
the same composition as set plaster, that is to say, they are hydrates.
A thin flake of such a crystal viewed with the highest powers of the
microscope appears perfectly homogeneous. Nevertheless, there is good
reason for the conclusion that it consists of molecules of water and
molecules of gypsum which hold together so strongly that they form a
hard brittle glassy solid. Moreover, the molecules of the hydrate itself
hold together more strongly in some directions than in others. It is
very easy to split the crystals lengthwise; while much more force is
needed to cut them crosswise and then they do not split, but break.
Glauber’s salts and Epsom salts are other examples of solids which
dissolve in water and separate in the crystalline form as the water
evaporates; and which, like lime and gypsum, combine with a definite
proportion of water to form crystalline compounds. In fact, each of
these glassy brittle solids contains more than half its weight of water.
Thus we see that two bodies, of which water is one, may combine together
to give rise to something different from either. And we are thus led to
the science of =chemistry=, which tells us exactly how bodies combine,
what comes of their combination, and how compounds may be separated into
their constituents.
55. =Mineral bodies may take on definite shapes and grow, or increase in
size, by the addition of like parts.=
Water and all the other natural bodies which have hitherto been
mentioned, are what are called =mineral bodies=, although, in common
use, the term mineral is usually restricted to ores and metals. Now we
have repeatedly had occasion to remark that, under certain
circumstances, not only water, but many other mineral bodies, assume
regular shapes. The most familiar example is that of the beautiful
imitation of leaves and foliage which is presented by the ice which
forms on a window in winter. But we have also seen that common salt,
lime, gypsum, Glauber’s salts and Epsom salts, also assume the
crystalline form as they or their compounds with water are deposited
from their solutions. And if a drop of solution of Glauber’s salts or of
Saltpetre, is allowed to evaporate under the microscope, a wonderful
spectacle will be presented. As the salt assumes the solid state, the
crystals suddenly appear in the field of view as needles and plates
disposed in beautiful patterns, which rival those of hoar frost, though
they are quite different from them. In fact, as you will learn if you
study =crystallography=, every crystallizable substance has its proper
crystalline forms and never departs from certain strictly related
geometrical figures.
A crystal of any of these substances will =grow= if placed under proper
conditions. Thus, if a crystal of common salt is hung by a thread in a
saturated solution of salt, which is exposed to the air, so as to allow
the water to evaporate slowly, the molecules of the salt which is left
behind and can no longer be held in solution, deposit themselves on the
crystal in regular order and increase its size without changing its
form. And, in this way, the small crystal may =grow= to a great size.
The large crystals of sugar candy, which consist of sugar and water
deposited from a strong syrup or saturated solution of sugar, grow in
the same fashion, upon threads suspended in the evaporating syrup. In
this mode of growth you will observe that the enlargement is effected by
addition to the outside of the growing body; and moreover the matter
which is added, namely, the salt or the sugar, already exists as salt in
the brine or as sugar in the syrup.
B. LIVING BODIES.
56. =The Wheat Plant and the substances of which it is composed.=
Every one has seen a cornfield. If you pluck up one of the innumerable
=wheat plants= which are fixed in the soil of the field, about harvest
time, you will find that it consists of a stem which ends in a =root= at
one end and an =ear= at the other, and that blades or =leaves= are
attached to the sides of the stem. The ear contains a multitude of oval
grains which are the =seeds= of the wheat plant. You know that when
these seeds are cleared from the =husk= or =bran= in which they are
enveloped, they are ground into fine powder in mills, and that this
powder is the =flour= of which bread is made. If a handful of flour
mixed with a little cold water is tied up in a coarse cloth bag, and the
bag is then put in a large vessel of water and well kneaded with the
hands, it will become pasty, while the water will become white. If this
water is poured away into another vessel, and the kneading process
continued with some fresh water, the same thing will happen. But if the
operation is repeated the paste will become more and more sticky, while
the water will be rendered less and less white, and at last will remain
colourless. The sticky substance which is thus obtained by itself is
called =gluten=; in commerce it is the substance known as =maccaroni=.
If the water in which the flour has thus been washed is allowed to stand
for a few hours, a white sediment will be found at the bottom of the
vessel, while the fluid above will be clear and may be poured off. This
white sediment consists of minute grains of =starch=, each of which,
examined with the microscope, will be found to have a concentrically
laminated structure. If the fluid from which the starch was deposited is
now boiled it will become turbid, just as white of egg diluted with
water does when it is boiled, and eventually a whitish lumpy substance
will collect at the bottom of the vessel. This substance is called
=vegetable albumin=.
Besides the albumin, the gluten, and the starch, other substances, about
which this rough method of analysis gives us no information, are
contained in the wheat grain. For example, there is woody matter or
=cellulose=, and a certain quantity of =sugar= and =fat=. It would be
possible to obtain a substance similar to albumin, starch, saccharine
and fatty matters, and cellulose, by treating the stem, leaves, and root
in a similar fashion, but the cellulose would be in far larger
proportion. =Straw=, in fact, which consists of the dry stem and leaves
of the wheat plant, is almost wholly made up of cellulose. Besides this,
however, it contains a certain proportion of mineral bodies, among them,
pure flint or =silica=; and, if you should ever see a wheat rick burnt,
you will find more or less of this silica, in a glassy condition, in the
embers. In the living plant, all these bodies are combined with a large
proportion of water, or are dissolved, or suspended in that fluid. The
relative quantity of water is much greater in the stem and leaves than
in the seed.
57. =The Common Fowl and the Substances of which it is Composed.=
Everybody has seen a common fowl. It is an active creature which runs
about and sometimes flies. It has a body covered with feathers, provided
with two wings and two legs, and ending at one end in a neck terminated
by a head with a beak, between the two parts of which the mouth is
placed. The hen lays =eggs=, each of which is enclosed in a hard shell.
If you break an egg the contents flow out and are seen to consist of the
colourless glairy “white” and the yellow “yolk.” If the white is
collected by itself in water and then heated it becomes turbid, forming
a white solid, very similar to the vegetable albumin, which is called
=animal albumin=.
If the yolk is beaten up with water, no starch nor cellulose is obtained
from it, but there will be plenty of fatty and some saccharine matter,
besides substances more or less similar to albumin and gluten.
The feathers of the fowl are chiefly composed of horn; if they are
stripped off and the body is boiled for a long time, the water will be
found to contain a quantity of =gelatin=, which sets into a jelly as it
cools; and the body will fall to pieces, the bones and the flesh
separating from one another. The bones consist almost entirely of a
substance which yields gelatin when it is boiled in water, impregnated
with a large quantity of salts of lime, just as the wood of the wheat
stem is impregnated with silica. The flesh, on the other hand, will
contain albumin, and some other substances which are very similar to
albumin, termed =fibrin= and =syntonin=.
In the living bird, all these bodies are united with a great quantity of
water, or dissolved, or suspended in water; and it must be remembered
that there are sundry other constituents of the fowl’s body and of the
egg, which are left unmentioned, as of no present importance.
58. =Certain Constituents of the Body are very similar in the Wheat
Plant and in the Fowl.=
The wheat plant contains neither horn, nor gelatin, and the fowl
contains neither starch, nor cellulose; but the albumin of the plant is
very similar to that of the animal, and the fibrin and syntonin of the
animal are bodies closely allied to both albumin and gluten.
That there is a close likeness between all these bodies is obvious from
the fact that when any of them is strongly heated, or allowed to
putrefy, it gives off the same sort of disagreeable smell; and careful
chemical analysis has shown that they are, in fact, all composed of the
elements =Carbon=, =Hydrogen=, =Oxygen=, and =Nitrogen=, combined in
very nearly the same proportions. Indeed, =charcoal=, which is impure
carbon, might be obtained by strongly heating either a handful of corn,
or a piece of fowl’s flesh, in a vessel from which the air is excluded
so as to keep the corn or the flesh from burning. And if the vessel were
a still, so that the products of this =destructive distillation=, as it
is called, could be condensed and collected, we should find water and
ammonia, in some shape or other, in the receiver. Now ammonia is a
compound of the elementary bodies, nitrogen and hydrogen; therefore (§
50) both nitrogen and hydrogen must have been contained in the bodies
from which it is derived.
It is certain, then, that very similar nitrogenous compounds form a
large part of the bodies of both the wheat plant and the fowl, and these
bodies are called =proteids=.
59. =Proteid Substances are met with in Nature only in Animals and
Plants; and Animals and Plants always contain Proteids.=
It is a very remarkable fact that not only are such substances as
albumin, gluten, fibrin and syntonin, known exclusively as products of
animal and vegetable bodies; but that every animal and every plant, at
all periods of its existence, contains one or other of them, though, in
other respects, the composition of living bodies may vary indefinitely.
Thus, some plants contain neither starch nor cellulose, while these
substances are found in some animals; while many animals contain no
horny matter and no gelatin-yielding substance. So that the matter which
appears to be the =essential= foundation of both the animal and the
plant is the =proteid= united with =water=; though it is probable that,
in all animals and plants, these are associated with more or less
=fatty= and =amyloid= (or starchy and saccharine) substances, and with
very small quantities of certain mineral bodies, of which the most
important appear to be =phosphorus=, =iron=, =lime=, =and potash=.
Thus there is a substance composed of water + proteids + fat + amyloids
+ mineral matters which is found in all animals and plants; and, when
these are alive, this substance is termed =protoplasm=.
60. =What is meant by the word Living?=
The wheat plant in the field is said to be a =living= thing; the fowl
running about the farm-yard is also said to be a =living= thing. If the
plant is plucked up, and if the fowl is knocked on the head, they soon
die and become =dead= things. Both the fowl and the wheat plant, as we
have seen, are composed of the same elements as those which enter into
the composition of mineral matter, though united into compounds which do
not exist in the mineral world. Why then do we distinguish this matter
when it takes the shape of a wheat plant, or a fowl, as =living matter=?
61. =The Living Plant increases in size, by adding to the Substances
which compose its Body, like Substances; these, however, are not derived
from without, but are manufactured within the Body of the Plant from
simpler Materials.=
In the spring, a wheat-field is covered with small green plants. These
grow taller and taller until they attain many times the size which they
had when they first appeared; and they produce the heads of flowers
which eventually change into ears of corn.
In so far as this is a process of growth, accompanied by the assumption
of a definite form, it might be compared with the growth of a crystal of
salt in brine: but, on closer examination, it turns out to be something
very different. For the crystal of salt grows by taking to itself the
salt contained in the brine, which is added to its exterior; whereas the
plant grows by addition to its interior: and there is not a trace of the
characteristic compounds of the plant’s body, albumin, gluten, starch,
or cellulose, or fat, in the soil, or in the water, or in the air.
Yet the plant creates nothing (§ 50) and, therefore, the matter of the
proteids and amyloids and fats which it contains must be supplied to it,
and simply manufactured, or combined in new fashions, in the body of the
plant.
It is easy to see, in a general way, what the raw materials are which
the plant works up, for the plant gets nothing but the materials
supplied to it by the atmosphere and by the soil. The atmosphere
contains oxygen and nitrogen, a little carbonic acid gas, a minute
quantity of ammoniacal salts, and a variable proportion of water. The
soil contains clay and sand (silica), lime, iron, potash, phosphorus,
sulphur, ammoniacal salts, and other matters which are of no importance.
Thus, between them, the soil and the atmosphere contain all the
elementary bodies which we find in the plant: but the plant has to
separate them and join them together afresh.
Moreover, the new matter, by the addition of which the plant grows, is
not applied to its outer surface, but is manufactured in its interior;
and the new molecules are diffused among the old ones.
62. =The Living Plant, after it has grown up, detaches part of its
Substance, which has the Power of developing into a similar Plant, as a
Seed.=
The grain of wheat is a part of the flower of the wheat plant, which,
when it becomes ripe, is easily separated. It contains a minute and
rudimentary plant; and, when it is sown, this gradually grows, or
becomes =developed= into, the perfect plant, with its stem, roots,
leaves and flowers, which again give rise to similar seeds. No mineral
body runs through a regular series of changes of form and size and then
gives off parts of its substance which take the same course. Mineral
bodies present no such =development= and give off no seeds or =germs=.
They do not reproduce their kind.
63. =The Living Animal increases in Size by adding to the Substances
which compose its Body, like Substances; these, however, are chiefly
derived directly from other Animals or from Plants.=
The fowl in the farm-yard is incessantly pecking about and swallowing
now a grain of corn, and now a fly or a worm. In fact, it is feeding,
and, as every one knows, would soon die if not supplied with food. It is
also a matter of every day knowledge that it would not be of much use to
give a fowl the soil of a cornfield, with plenty of air and water, to
eat.
In this respect, the fowl is like all other animals; it cannot
manufacture the proteid materials of its body, but it has to take them
ready made, or in a condition which requires but very slight
modification, by devouring the bodies either of other animals or of
plants. The animal or vegetable substances devoured are taken into the
animal’s stomach; they are there digested or dissolved; and thus they
are fitted to be distributed to all parts of the fowl’s own body, and
applied to its maintenance and growth.
64. =The Living Animal, after it has grown up, detaches part of its
Substance, which has the Power of growing into a similar Animal, as an
Egg.=
The fowl’s egg is formed in the body of the hen, and is, in fact, part
of her body inclosed in a shell and detached. It contains a minute
rudiment of a fowl; and when it is kept at a proper temperature by the
hen’s sitting upon it, or otherwise, for three weeks, this rudiment
grows or develops, at the expense of the materials contained in the yolk
and the white, into a small bird, the chick, which is then hatched and
grows into a fowl. The animal, therefore, is produced by the development
of a germ in the same way as the plant; and, in this respect, all plants
and all animals agree with one another and differ from all mineral
matter.
65. =Living Bodies differ from Mineral Bodies in their Essential
Composition, in the manner of their Growth, and in the fact that they
are reproduced by Germs.=
Thus there is a very broad distinction between mineral matter and living
matter. The elements of living matter are identical with those of
mineral bodies; and the fundamental laws of matter and motion apply as
much to living matter as to mineral matter; but every living body is, as
it were, a complicated piece of mechanism which “goes,” or lives, only
under certain conditions. The germ contained in the fowl’s egg requires
nothing but a supply of warmth, within certain narrow limits of
temperature, to build the molecules of the egg into the body of the
chick. And the process of development of the egg, like that of the seed,
is neither more nor less mysterious than that, in virtue of which, the
molecules of water, when it is cooled down to the freezing-point, build
themselves up into regular crystals.
The further study of living bodies leads to the province of =Biology=,
of which there are two great divisions—=Botany=, which deals with
plants, and =Zoology=, which treats of animals.
Each of these divisions has its subdivisions—such as =Morphology=, which
treats of the form, structure, and development of living beings, and
=Physiology=, which explains their actions or functions, besides others.
III. IMMATERIAL OBJECTS.
66. =Mental Phenomena.=
Material objects are all either not living, that is to say, mineral
bodies, or they are living bodies. Everything which occupies space,
offers resistance, has weight and transfers motion, belongs to one or
other of these two great provinces of nature. The sciences of Astronomy,
Mineralogy, Physics, and Chemistry deal with the former, while Biology,
with its two divisions of Zoology and Botany, treats of the latter. But
natural knowledge is not exhausted by this catalogue of its topics. In
the very first paragraph of this Primer, in fact, we had occasion to
draw a distinction between =Things=, or material objects, and
=Sensations=; and a moment’s reflection is sufficient to convince you
that sensations are not material objects. A smell takes up no space and
has no weight; and to speak of a pound or of a cubic foot of sound, or
of brightness, is, on the face of the matter, an absurdity. Pleasure is
said metaphorically to be fugitive, but you cannot imagine a pleasure as
a thing in motion.
What we call our =Emotions= are in like manner devoid of all the
characters of material bodies. Love and hatred, for example, cannot for
a moment be conceived to have shape, or weight, or momentum. And when,
in reasoning, we think, our =Thoughts= have the same lack of the
qualities of material things.
Sensations, emotions, and thoughts, thus constitute a peculiar group of
natural phenomena, which are termed =mental=.
67. =The order of Mental Phenomena: Psychology.=
A definite order obtains among mental phenomena, just as among material
phenomena; and there is no more chance, nor any accident, nor uncaused
event, in the one series than there is in the other. Moreover, there is
a connection of cause and effect between certain material phenomena and
certain mental phenomena. Thus, for example, certain sensations are
always produced by the influence of particular material bodies on our
organs of sense. The prick of a pin gives pain, feathers feel soft,
chalk looks white, and so on. The study of mental phenomena, of the
order in which they succeed one another, and of the relations of cause
and effect which obtain between them and material phenomena, is the
province of the science of =Psychology=.
All the phenomena of nature are either material or immaterial, physical
or mental; and there is no science, except such as consists in the
knowledge of one or other of these groups of natural objects, and of the
relations which obtain between them.
THE END.
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TRANSCRIBER’S NOTES
1. Silently corrected typographical errors and variations in spelling.
2. Retained anachronistic, non-standard, and uncertain spellings as
printed.
3. Footnotes have been re-indexed using numbers.
4. Enclosed italics font in _underscores_.
5. Enclosed bold font in =equals=.
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