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Title: Liquid Drops and Globules, their Formation and Movements
Author: Darling, Chas. R.
Language: English
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LIQUID DROPS AND GLOBULES
_BY THE SAME AUTHOR._
————————————————
*PYROMETRY*
A Practical Treatise on the Measurement
of High Temperatures.
_With 60 Illustrations_, xii + 200 _pp._
_Crown 8vo, cloth_ (1911).
Price *5/-* net.
————————
*HEAT FOR ENGINEERS*
A Treatise on Heat, with special regard
to its Practical Applications.
_Second Edition Revised_, with 110 _Illustrations_,
xiv + 430 _pp._ _Demy 8vo, cloth_ (1912).
Price *12/6* net.
————————————————
_E. & F. N. SPON, Ltd., 57 Haymarket, London, S.W._
LIQUID DROPS AND
GLOBULES
Their Formation and Movements
THREE LECTURES DELIVERED
TO POPULAR AUDIENCES
BY
CHAS. R. DARLING
ASSOCIATE OF THE ROYAL COLLEGE OF SCIENCE, IRELAND; FELLOW OF THE
INSTITUTE
OF CHEMISTRY; FELLOW OF THE PHYSICAL SOCIETY, ETC.; LECTURER
IN PHYSICS AT THE CITY AND GUILDS OF LONDON
TECHNICAL COLLEGE, FINSBURY
WITH 43 ILLUSTRATIONS
_London_
E. & F. N. SPON, LIMITED, 57 HAYMARKET
_NEW YORK_
SPON & CHAMBERLAIN, 123 LIBERTY STREET
1914
CONTENTS
PAGE
_List of Illustrations_ . . . . . . . vii
_Preface_ . . . . . . . . . ix
_Lecture I._
Introduction . . . . . . . . . 1
General Properties of Liquids . . . . . . 2
Properties of the Surface Skin of Water . . . . 3
Elastic Skin of other Liquids-Minimum Thermometer . . . 5
Boundary Surface of two Liquids . . . . . . 6
Area of Stretched Surface . . . . . . . 7
Shape of detached Masses of Liquid . . . . . 8
Production of True Spheres of Liquids . . . . . 10
The Centrifugoscope . . . . . . . . 14
Effect of Temperature on Sphere of Orthotoluidine . . . 15
Other Examples of Equi-Density . . . . . . 17
Aniline Films or Skins . . . . . . . 19
Surface Tension . . . . . . . . 21
The “Diving” Drop . . . . . . . . 22
Formation of Falling Drops of Liquid . . . . . 24
Ascending or Inverted Drops . . . . . . 31
_Lecture II._
Automatic Aniline Drops . . . . . . . 33
Automatic Drops of other Liquids. . . . . . 37
Liquid Jets . . . . . . . . . 38
Liquid Columns . . . . . . . . 40
Communicating Drops . . . . . . . . 44
Combined Vapour and Liquid Drops . . . . . 47
Condensation of Drops from Vapour . . . . . 49
Liquid Clouds in Liquid Media . . . . . . 54
Overheated Drops . . . . . . . . 55
Floating Drops on Hot Surfaces . . . . . . 57
_Lecture III._
Spreading of Oil on the Surface of Water . . . . 60
Movements due to Solubility . . . . . . 63
Movements of Aniline Globules on a Water Surface . . . 63
Movements of Orthotoluidine and Xylidine 1-3-4 on a Water Surface 66
Production of Globules from Films . . . . . 68
Network formed from a Film . . . . . . 70
Quinoline Rings . . . . . . . . 71
Expanding Globules . . . . . . . . 71
Attraction between Floating Globules . . . . . 73
Analogies of Surface Tension Phenomena with Life . . . 75
_Conclusion_ . . . . . . . . . 76
_Appendix_
Apparatus and Materials required for Experiments on Drops
and Globules . . . . . . . . 78
_Index_ . . . . . . . . . . 81
LIST OF ILLUSTRATIONS
FIG. PAGE
1. Silver sheet floating on water . . . . . 4
2. Column and index of minimum thermometer . . . 6
3. Thread of golden syrup rising and forming a drop . . 8
4. Drops of different sizes resting on flat plate . . 10
5. Formation of a sphere of orthotoluidine . . . 12
6. Detached sphere floating under water . . . . 13
7. The centrifugoscope . . . . . . . 14
8. Aniline drops falling through cold water and ascending
through hot water . . . . . . 17
9. Aniline skins enveloping water . . . . . 20
10, 11, 12. The “diving” drop. Three stages . . . . 23
13. Apparatus for forming ascending or descending drops of liquids 27
14-20. Formation of a drop of orthotoluidine, showing the
droplet. Seven stages . . . . . 29-31
21, 22. Automatically formed aniline drops, showing the
formation of droplets from the neck . . . 34, 35
23-25. Jets of orthotoluidine discharged under water . . 39
26. Water stretched between a tube and a plate . . . 40
27-30. A liquid column stretched upwards by addition
of water until broken. Four stages . . . 43
31. A column of aceto-acetic ether in water . . . 44
32. Apparatus for communicating drops . . . . 45
33. Combined vapour and liquid drops . . . . 49
34. Spheroid of water on a hot plate . . . . 58
35. Forces acting on a floating globule . . . . 61
36. Aniline globules on a water surface . . . . 64
37. Orthotoluidine globules on a water surface . . . 66
38. Resolution of a floating skin into globules . . . 68
39. Network formed from a film of tar-oil . . . . 70
40. Quinoline rings and perforated plates . . . . 71
41. The expanding globule . . . . . . 72
42. The “devouring” globule. Five stages . . . 74
43. Photograph of one globule absorbing another . . . 75
PREFACE
The object of the present little volume is to reproduce in connected
form, an account of the many interesting phenomena associated with
liquid drops and globules. Much of the matter relates to experiments
devised by the author during the past four years, descriptions of which
have appeared in the _Proceedings of the Physical Society_; in the
columns of _Nature_ and _Knowledge_; and elsewhere. The exhibition of
these experiments at the conversazioni of the Royal Society and the
Royal Institution, and in the author’s lectures, has evoked such
interest as to suggest the present publication. It may be added that all
the experiments described may be repeated by any intelligent reader at a
trifling cost, no special manipulative skill being required.
The context maintains the form of the lectures delivered on this subject
by the author at various places, and the method of presentation is such
as may be followed by those who have not received a training in this
branch of science. It is hoped, in addition, that the book may prove of
some service to teachers of science and others interested in the
properties of liquids.
A number of the illustrations used have appeared in the pages of
_Knowledge_ in connexion with the author’s articles, and are here
reproduced by courtesy of the Editor. Other drawings have been provided
by Mr. W. Narbeth, to whom the author expresses his thanks.
CHAS. R. DARLING.
_City and Guilds Technical College,_
_Finsbury, 1914._
LIQUID DROPS AND GLOBULES
LECTURE I
*Introduction.*—In choosing a subject for a scientific discourse, it
would be difficult to find anything more familiar than a drop of liquid.
It might even appear, at first sight, that such a subject in itself
would be quite inadequate to furnish sufficient material for extended
observation. We shall find, however, that the closer study of a drop of
liquid brings into view many interesting phenomena, and provides
problems of great profundity. A drop of liquid is one of the commonest
things in nature; yet it is one of the most wonderful.
Apart from the liquids associated with animal or vegetable life, water
and petroleum are the only two which are found in abundance on the
earth; and it is highly probable that petroleum has been derived from
the remains of vegetable life. Many liquids are fabricated by living
organisms, such as turpentine, alcohol, olive oil, castor oil, and all
the numerous vegetable oils with which we are all familiar. But in
addition to these, there are many liquids produced in the laboratory of
the chemist, many of which are of great importance; for example, nitric
acid, sulphuric acid, and aniline. The progress of chemical science has
greatly enlarged the number of liquids available, and in our experiments
we shall frequently utilize these products of the chemist’s skill, for
they often possess properties not usually associated with the commoner
liquids.
*General Properties of Liquids.*—No scientific study can be pursued to
advantage unless the underlying principles be understood; and hence it
will be necessary, in the beginning, to refer to certain properties
possessed by all liquids, whatever their origin. The most prominent
characteristic of a liquid is _mobility_, or freedom of movement of its
parts. It is owing to this property that a liquid, when placed in a
vessel, flows in all directions until it reaches the sides; and it is
this same freedom of movement which enables water, gathering on the
hills, to flow under the pull of gravitation into the lowlands, and
finally to the sea. If we drop a small quantity of a strongly-coloured
fluid—such as ink—into a large volume of water, and stir the mixture for
a short time, the colour is evenly distributed throughout the whole mass
of water, because the freedom of movement of the particles enables the
different portions to intermingle readily. This property of mobility
distinguishes a liquid from a solid; for a solid maintains its own
shape, and its separate parts cannot be made to mix freely. Mobility,
however, is not possessed in equal degree by all liquids. Petrol, for
example, flows more freely than water, which in turn is more mobile than
glycerine or treacle. Sometimes a substance exhibits properties
intermediate between those of a solid and a liquid, as, for instance,
butter in hot weather. We shall not be concerned, however, with these
border-line substances, but shall confine our attention to well-defined
liquids.
There is another feature, however, common to all liquids, which has a
most important bearing on our subject. Every liquid is capable of
forming a boundary surface of its own; and this surface has the
properties of a stretched, elastic membrane. Herein a liquid differs
from a gas or vapour, either of which always completely fills the
containing vessel. You cannot have a bottle half full of a vapour or gas
only; if one-half of that already present be withdrawn, the remaining
half immediately expands and distributes itself evenly throughout the
bottle, which is thus always filled. But a liquid may be poured to any
height in a vessel, because it forms its own boundary at the top. Let us
now take a dish containing the commonest of all liquids, and in many
ways the most remarkable—water—and examine some of the properties of the
upper surface.
*Properties of the Surface Skin of Water.*—Here is a flat piece of thin
sheet silver, which, volume for volume, is 10½ times as heavy as water,
in which it might therefore be expected to sink if placed upon the
surface. I lower it gently, by means of a piece of cotton, until it just
reaches the top, and then let go the cotton. Instead of sinking, the
piece of silver floats on the surface; and moreover, a certain amount of
pressure may be applied to it without causing it to fall to the bottom
of the water. By alternately applying and relaxing the pressure we are
able, within small limits, to make the sheet of silver bob up and down
as if it were a piece of cork. If we look closely, we notice that the
water beneath the silver is at a lower level than the rest of the
surface, the dimple thus formed being visible at the edge of the
floating sheet (Fig. 1). If now I apply a greater pressure, the piece of
silver breaks through the surface and sinks rapidly to the bottom of the
vessel. Or, if instead I place a thick piece of silver, such as a
shilling, on the surface of the water, we find that this will not float,
but sinks immediately. All these results are in agreement with the
supposition that the surface layer of water possesses the properties of
a very thin elastic sheet. If we could obtain an extremely fine sheet of
stretched rubber, which would merely form a depression under the weight
of the thin piece of silver, but would break under the application of a
further pressure or the weight of a heavier sheet, the condition of the
water surface would then be realized. We may note in passing that a
sheet of metal resting on the surface of water is a phenomenon quite
distinct from the floating of an iron ship, or hollow metal vessel,
which sinks until it has displaced an amount of water equal in weight to
itself.
[Illustration: __Fig._ 1.—Silver sheet floating on water._]
We can now understand why a water-beetle is able to run across the
surface of a pond, without wetting its legs or running any risk of
sinking. Each of its legs produces a dimple in the surface, but the
pressure on any one leg is not sufficient to break through the skin. We
can imitate this by bringing the point of a lead pencil gently to the
surface of water, when a dimple is produced, but the skin is not
actually penetrated. On removing the pencil, the dimple immediately
disappears, just as the depression caused by pushing the finger into a
stretched sheet of indiarubber becomes straight immediately the finger
is removed.
*Elastic Skin of other Liquids—Minimum Thermometer.*—The possession of
an elastic skin at the surface is not confined to water, but is common
to all liquids. The strength of the skin varies with different liquids,
most of which are inferior to water in this respect. The surface of
petroleum, for example, is ruptured by a weight which a water surface
can readily sustain. But wherever we have a free liquid surface, we
shall always find this elastic layer at the boundary, and I will now
show, by the aid of lantern projection, an example in which the presence
of this layer is utilized. On the screen is shown the stem of a minimum
thermometer—that is, a thermometer intended to indicate the lowest
temperature reached during a given period. The liquid used in this
instrument is alcohol, and you will observe that the termination of the
column is curved (Fig. 2). In contact with the end of the column is a
thin piece of coloured glass, with rounded ends, which fits loosely in
the stem, and serves as an index. When I warm the bulb of the
thermometer, you notice that the end of the column moves forward, but
the index, round which the alcohol can flow freely, does not change its
position. On inclining the stem, the index slides to the end of the
column, but its rounded end does not penetrate the elastic skin at the
surface. I now pour cold water over the bulb, which causes the alcohol
to contract, and consequently the end of the column moves towards the
bulb. In doing so, it encounters the opposition of the index, which
endeavours to penetrate the surface; but we see that the elastic skin,
although somewhat flattened, is not pierced, but is strong enough to
push the index in front of it. And so the index is carried towards the
bulb, and its position indicates the lowest point attained by the end of
the column—that is, the minimum temperature. Obviously, a thermometer of
this kind must be mounted horizontally, to prevent the index falling by
its own weight.
[Illustration: __Fig._ 2.—Column and index of minimum thermometer._]
*Boundary Surface of two Liquids.*—So far we have been considering
surfaces bounded by air, or—in the case of the alcohol thermometer—by
vapour. It is possible, however, for the surface of one liquid to be
bounded by a second liquid, provided the two do not mix. We may, for
example, pour petroleum on to water, when the top of the water will be
in contact with the floating oil. If now we lower our piece of silver
foil through the petroleum, and allow it to reach the surface of the
water, we find that the elastic skin is still capable of sustaining the
weight; and thus we see that the elastic layer is present at the
junction of the two liquids. What is true of water and oil in this
respect also holds good for the boundary or interface of any two liquids
which do not mix. Evidently, if the two liquids intermingled, there
would be no definite boundary between them; and this would be the case
with water and alcohol, for example.
*Area of Stretched Surface.*—We will not at present discuss the nature
of the forces which give rise to this remarkable property of a liquid
surface, but will consider one of the effects. The tendency, as in the
case of all stretched membranes, will be to reduce the area of the
surface to a minimum. If we take a disc of stretched indiarubber and
place a weight upon it, we cause a depression which increases the area
of the surface. But on removing the weight, the disc immediately
flattens out, and the surface is restored to its original smallest
dimension. Now, in practice, the surface of a liquid is frequently
prevented from attaining the smallest possible area, owing to the
contrary action of superior forces; but the tendency is always manifest,
and when the opposing forces are absent or balanced the surface always
possesses the minimum size. A simple experiment will serve to illustrate
this point. I dip a glass rod into treacle or “golden syrup,” and
withdraw it with a small quantity of the syrup adhering to the end. I
then hold the rod with the smeared end downwards, and the syrup falls
from it slowly in the form of a long, tapered column. When the column
has become very thin, however, owing to the diminished supply of syrup
from the rod, we notice that it breaks across, and the upper portion
then shrinks upwards and remains attached to the rod in the form of a
small drop (Fig. 3). So long as the column was thick, the tendency of
the surface layer to reduce its area to the smallest dimensions was
overpowered by gravity; but when the column became thin, and
consequently less in weight, the elastic force of the outer surface was
strong enough to overcome gravitation, and the column was therefore
lifted, its area of surface growing less and less as it rose, until the
smallest area possible under the conditions was attained.
[Illustration: __Fig._ 3.—Thread of golden syrup rising and forming a
drop._]
*Shape of Detached Masses of Liquid.*—Let us now pay a little attention
to the small drop of syrup which remains hanging from the rod. It is in
contact with the glass at the top part only, and the lower portion is
only prevented from falling by the elastic skin around it, which
sustains the weight. We may compare it to a bladder full of liquid, in
which case also the weight is borne by the containing skin. Now suppose
we could separate the drop of syrup entirely from the rod; what shape
would it take? We know that its surface, if not prevented by outside
forces from doing so, would become of minimum area. Assuming such
extraneous forces to be absent or counterbalanced, what would then be
the shape of the drop? It would be an exact sphere. For a sphere has a
less surface-area in proportion to its volume than any other shape; and
hence a free drop of liquid, if its outline were determined solely by
its elastic skin, would be spherical. A numerical example will serve to
illustrate this property of a sphere. Supposing we construct three
closed vessels, each to contain 1 cubic foot, the first being a cube,
the second a cylinder of length equal to its diameter, and the third a
sphere. The areas of the surfaces would then be:—
Cube . . . . 6 square feet.
Cylinder . . . . 5·86 ,, ,,
Sphere . . . . 4·9 ,, ,,
──────────────────────────────────────────────────────
And whatever shape we make the vessel, it will always be found that the
spherical form possesses the least surface.
[Illustration: __Fig._ 4.—Drops of different sizes resting on flat
plate._]
Now let us examine some of the shapes which drops actually assume. I
take a glass plate covered with a thin layer of grease, which prevents
adhesion of water to the glass, and form upon it drops of water of
various sizes by the aid of a pipette. You see them projected on the
screen (Fig. 4). The larger drops are flattened above and below, but
possess rounded sides and resemble a teacake in shape. Those of
intermediate size are more globular, but still show signs of flattening;
whilst the very small ones, so far as the eye can judge, are spherical.
Evidently, the shape depends upon the size; and this calls for some
explanation. If we take a balloon of indiarubber filled with water, and
rest it on a table, the weight of the enclosed water will naturally tend
to stretch the balloon sideways, and so to flatten it. A smaller
balloon, made of rubber of the same strength, will not be stretched so
much, as the weight of the enclosed water would be less; and if the
balloon were very small, but still had walls of the same strength, the
weight of the enclosed water would be incompetent to produce any visible
distortion. It is evident, however, that so long as it is under the
influence of gravitation, even the smallest drop cannot be truly
spherical, but will be slightly flattened. The tendency of drops to
become spherical, however, is always present.
[Illustration: __Fig._ 5.—Formation of a sphere of orthotoluidine._]
*Production of True Spheres of Liquids.*—Now it is quite possible to
produce true spheres of liquid, even of large size, if we cancel the
effect of gravity; and we may obtain a hint as to how this may be
accomplished by considering the case of a soap-bubble, which, when
floating in air, is spherical in shape. Such a bubble is merely a skin
of liquid enclosing air; but being surrounded by air of the same
density, there is no tendency for the bubble to distort, nor would it
fall to the ground were it not for the weight of the extremely thin
skin. The downward pull of gravity on the air inside the bubble is
balanced by the buoyancy of the outside air; and hence the skin,
unhampered by any extraneous force, assumes and retains the spherical
form. And similarly, if we can arrange to surround a drop of liquid by a
medium of the same density, it will in turn become a sphere. Evidently
the medium used must not mix with the liquid composing the drop, as it
would then be impossible to establish a boundary surface between the
two. Plateau, many years ago, produced liquid spheres in this manner. He
prepared a mixture of alcohol and water exactly equal in density to
olive oil, and discharged the oil into the mixture, the buoyancy of
which exactly counteracted the effect of gravity on the oil, and hence
spheres were formed. The preparation of an alcohol-water mixture of
exactly correct density is a tedious process, and we are now able to
dispense with it and form true spheres in a more convenient way. There
is a liquid known as _orthotoluidine_, which possesses a beautiful red
colour, does not mix with water, and which has exactly the same density
as water when the temperature of both is 75° F. or 24° C. At this
temperature, therefore, if orthotoluidine be run into water, spheres
should be formed; and there is no reason why we should not be able to
make one as large as a cricket-ball, or even larger. I take a flat-sided
vessel for this experiment, in order that the appearance of the drop
will not be distorted as it would be in a beaker, and pour into it water
at 75° F. until it is about two-thirds full. I now take a pipette
containing a 3 per cent. solution of common salt, and discharge it at
the bottom of the water. Being heavier, the salt solution will remain
below the water, and will serve as a resting-place for the drop. The
orthotoluidine is contained in a vessel provided with a tap and wide
stem, which is now inserted in the water so that the end of the stem is
about 1 inch above the top of the salty layer. I now open the tap so as
to allow the orthotoluidine to flow out gradually; and we then see the
ball of liquid growing at the end of the stem (Fig. 5). By using a
graduated vessel, we can read off the quantity of orthotoluidine which
runs out, and thus measure the volume of the sphere formed. When the
lower part reaches the layer of salt solution, we raise the delivery
tube gently, and repeat this as needed during the growth of the sphere.
We have now run out 100 cubic centimetres, or about one-sixth of a pint,
and our sphere consequently has a diameter of 5¾ centimetres, or 2¼
inches. To set it free in the water we lift the delivery tube
rapidly—and there is the sphere floating in the water (Fig. 6). We could
have made it as much larger as we pleased, but the present sphere will
serve all our requirements.
[Illustration: __Fig._ 6.—The detached sphere floating under water._]
[Illustration: __Fig._ 7.—The Centrifugoscope._]
*The Centrifugoscope.*—I have here a toy, which we may suitably call the
centrifugoscope, which shows in a simple way the formation of spheres of
liquid in a medium of practically equal density. It consists of a large
glass bulb attached to a stem, about three-quarters full of water, the
remaining quarter being occupied by orthotoluidine. This liquid, being
slightly denser than water at the temperature of the room, rests on the
bottom of the bulb. When I hold the stem horizontally, and rotate
it—suddenly at first, and steadily afterwards—a number of fragments are
detached from the orthotoluidine, which immediately become spherical,
and rotate near the outer side of the bulb. The main mass of the red
liquid rises to the centre of the bulb, and rotates on its axis (Fig.
7), and we thus get an imitation of the solar system, with the planets
of various sizes revolving round the central mass; and even the
asteroids are represented by the numerous tiny spheres which are always
torn off from the main body of liquid along with the larger ones. When
the rotation ceases, the detached spheres sink, and after a short time
join the parent mass of orthotoluidine. We can therefore take this
simple apparatus at any time, and use it to show that a mass of liquid,
possessing a free surface all round, and unaffected by gravity,
automatically becomes a sphere. After all, this is only what we should
expect of an elastic skin filled with a free-flowing medium.
*Effect of Temperature on Sphere of Orthotoluidine.*—I will now return
to the large sphere formed under water in the flat-sided vessel, and
direct your attention to an experiment which teaches an important
lesson. By placing a little ice on the top of the water, we are enabled
to cool the contents of the vessel, and we soon notice that the
red-coloured sphere becomes flattened on the top and below, and sinks a
short distance into the saline layer. Evidently the cooling action,
which has affected both liquids, has caused the orthotoluidine to become
denser than water. I now surround the vessel with warm water, and allow
the contents gradually to attain a temperature higher than 75° F. You
observe that the flattened drop changes in shape until it is again
spherical; and as the heating is continued elongates in a vertical
direction, and then rises to the surface, being now less dense than
water. So sensitive are these temperature effects that a difference of 1
degree on either side of 75° F. causes a perceptible departure from the
spherical shape in the case of a large drop. It therefore follows that
orthotoluidine may be either heavier or lighter than water, according to
temperature, and this fact admits of a simple explanation.
Orthotoluidine expands more than water on heating, and contracts more on
cooling. The effect of expansion is to decrease the density, and of
contraction to increase it; hence the reason why warm air rises through
cold air, and vice versa. Now if orthotoluidine and water, which are
equal in density at 75° F., expanded or contracted equally on heating
above or cooling below this temperature, their densities would always be
identical. But inasmuch as orthotoluidine increases in volume to a
greater extent than water on heating, and shrinks more on cooling, it
becomes lighter than water when both are hotter than 75° F., and heavier
when both are colder. We call the temperature when both are equal in
density the _equi-density temperature_. Here are some figures which show
how the densities of these two liquids diverge from a common value on
heating or cooling, and which establish the conclusions we have drawn:—
───────────────────────────────────────────────────────────────────
Temperature. Density.
Deg. F. Deg. C. Water. Orthotoluidine.
───────────────────────────────────────────────────────────────────
50 10 0·9997 1·009
59 15 0·9991 1·005
68 20 0·9982 1·001
Equal: 75 24 0·9973 0·997
86 30 0·9957 0·992
95 35 0·9940 0·988
104 40 0·9923 0·983
───────────────────────────────────────────────────────────────────
[Illustration: __Fig._ 8.—Aniline drops falling through cold water and
ascending through hot water._]
*Other Examples of Equi-Density.*—There are many other liquids which,
like orthotoluidine, may be heavier or lighter than water, according to
temperature, and I now wish to bring to your notice the remarkable
liquid _aniline_, which falls under this head. Aniline is an oily
liquid, which, unless specially purified, has a deep red colour. It
forms the basis of the beautiful and varied colouring materials known as
the aniline dyes, which we owe to the skill of the chemist. The
equi-density temperature of water and aniline is 147° F. or 64° C.; that
is, aniline will sink in water if both be colder than 147° F., and rise
to the surface if this temperature be exceeded. We may illustrate this
fact by a simple but striking experiment. Here are two tall beakers side
by side, and above them a cistern containing aniline (Fig. 8). The stem
of the cistern communicates with the two branches of a horizontal tube,
the termination of one branch being near the top of one of the beakers,
whilst the other branch is prolonged to the bottom of the second beaker,
and is curved upwards at the end. Both branches are provided with taps
to regulate the flow of liquid, and to commence with are full of
aniline. Cold water is poured into the beaker containing the shorter
branch until the end is submerged; and water nearly boiling is placed in
the second beaker to an equal height. I now open the taps, so that the
aniline may flow gradually into each beaker; and you notice that the
drops of aniline sink through the cold water and rise through the hot.
We have thus the same liquid descending and ascending simultaneously in
water, the only difference being that the water is cold on the one side
and hot on the other. Prolonging the delivery-tube to the bottom of the
beaker containing the hot water enables the rising drops to be observed
throughout the length of the column of water; and in addition enables
the cold aniline from the cistern to be warmed up on its way to the
outlet, so that by the time it escapes its temperature is practically
the same as that of the water. If this temperature exceed 147° F., the
drops will rise. We might, in this experiment, have used orthotoluidine
instead of aniline; or, indeed, any other liquid equal in density to
water at some temperature intermediate between those of the hot and cold
water—always provided that the liquid chosen did not mix with water.
Amongst such other liquids may be mentioned _anisol_; _butyl benzoate_;
and _aceto-acctic ether_; but none of these possess the fine colour of
aniline or its chemical relative orthotoluidine, and in addition are
more costly liquids. Besides these are a number of other liquids rarer
still, practically only known to the chemist, which behave in the same
way. These liquids are all carbon compounds, and more or less oily in
character. There is a simple rule which may be used to predict whether
any organic liquid will be both lighter and heavier than water,
according to temperature. Here it is: If the density of the liquid at
32° F. or 0° C. be not greater than 1·12, the liquid will become less
dense than water below 212° F. or 100° C., at which temperature water
boils. This rule is derived from a knowledge of the extent to which the
expansion of organic liquids in general exceeds that of water. I have
considered it necessary to enter at some length into this subject of
equi-density, as much that will follow involves a knowledge of this
physical relation between liquids.
*Aniline Films or Skins.*—We have previously concluded, largely from
circumstantial evidence, that a liquid drop is encased in a skin or what
is equivalent to a skin, and I propose now to show by experiments with
aniline how we can construct a drop, commencing with a skin of liquid.
Here is some aniline in a vessel, covered by water. I lower into the
aniline a circular frame of wire, which I then raise slowly into the
overlying water; and you observe that a film of aniline remains
stretched across the frame. By lifting the frame up and down in the
water the skin is stretched, forming a drop which is constricted near
the frame (Fig. 9). On lifting the wire more suddenly, the skin of
aniline closes in completely at the narrow part, and a sphere of water,
encased in an aniline skin, then falls through the water in the beaker,
and comes to rest on the aniline below—into which, however, it soon
merges. You were previously asked to regard a drop of liquid as being
similar to a filled soap-bubble; and this experiment realizes the terms
of the definition. And it requires only a little imagination to picture
a drop surrounded by its own skin instead of that of another liquid. It
is easy to make one of these enclosed water-drops by imitating the
blowing of a soap-bubble—using, however, water instead of air. In order
to do this I take a piece of glass tubing, open at both ends, and pass
it down the vessel, until it reaches the aniline. Water, in the
meantime, has entered the tube, to the same height as that at which it
stands in the vessel. On raising the tube gently, a skin of aniline
adheres to the end; and as we raise it still further, the water in the
tube, sinking so as to remain at the level in the vessel, expands the
skin into a sphere (Fig. 9)—the equivalent of a filled soap-bubble. On
withdrawing the tube gradually, the composite sphere is left hanging
from the surface of the water.
[Illustration: __Fig._ 9.—Aniline skins enveloping water._]
*Surface Tension.*—Before proceeding further, it will be advisable to
introduce and explain the term “surface tension.” We frequently use it,
without attaching to it any numerical value, to express the fact that
the free surface of a liquid is subjected to stretching forces, or is in
a state of tension; and thus we say that certain phenomena are “due to
surface tension.” But the physicist does not content himself with merely
observing occurrences; he tries also to measure, in definite units, the
quantities involved in the phenomena. And hence surface tension is
defined as the force tending to pull apart the two portions of the
surface on either side of a line 1 centimetre in length. That is, we
imagine a line 1 centimetre long on the surface of the liquid, dividing
the surface into two portions on opposite sides of the line, and we call
the force tending to pull these two portions away from each other the
surface tension. Experiments show that this force, in the case of cold
water, is equal to about 75 dynes, or nearly 8/100 of a gramme. If we
choose a line 1 inch long on the surface of water, the surface tension
is represented by about 3 1/6 grains. It is always necessary to specify
the length when assigning a value to the surface tension; and unless
otherwise stated a length of 1 centimetre is implied. The values for
different liquids vary considerably; and it is also necessary to note
that the figure for a given liquid depends upon the nature of the medium
by which it is bounded—whether, for example, the surface is in contact
with air or another liquid. The following table gives the values for
several liquids when the surfaces are in contact with air:—
──────────────────────────────────────────────────────────────────
Liquid. Tension at 15° C. (59° F.),
dynes per cm.
──────────────────────────────────────────────────────────────────
Water 75
Aniline 43
Olive Oil 32
Chloroform 27
Alcohol 25
──────────────────────────────────────────────────────────────────
When one liquid is bounded by another, the _interfacial_ tension, as it
is called, is generally less than when in contact with air. Thus the
value for water and olive oil is about 21 dynes per centimetre at 15° C.
We are now in a position to speak of surface tension _quantitatively_,
and shall frequently find it necessary to do so in order to explain
matters which will come under our notice later.
[Illustration: __Figs._ 10, 11 and 12.—The Diving Drop. Three stages._]
*The “Diving” Drop.*—In order to illustrate the tension at the boundary
surface of two liquids, I now show an experiment in which a drop is
forcibly projected downwards by the operation of this tension. I pour
some water into a narrow glass vessel, and float upon it a liquid called
_dimethyl-aniline_, so as to form a layer about 1 inch in depth. A glass
tube, open at both ends, is now passed down the floating liquid into the
water, and then raised gradually, with the result that a skin of water
adheres to the end, and is inflated by the upper liquid, forming a
sphere on the end of the tube (Fig. 10). On withdrawing the tube from
the upper surface, the sphere is detached and falls to the boundary
surface, where it rests for a few seconds, and is then suddenly shot
downwards into the water (Figs. 11 and 12). It then rises to the
interface; breaks through, and mingles with the floating liquid, thereby
losing its identity. Why should the drop, which is less dense than
water, dive below in this manner? The explanation is that the drop
(which consists of a skin of water filled with dimethyl-aniline), after
resting for a time on the joining surface, loses the under part of its
skin, which merges into the water below. The shape of the boundary of
the two liquids is thereby altered, the sides now being continuous with
the skin forming the upper part of the drop. This is an unstable shape;
and accordingly the boundary surface flattens to its normal condition,
and with such force as to cause the drop beneath it to dive into the
water, although the liquid is lighter than water and tends to float. The
result is the same as that which would occur if a marble were pressed on
to a stretched disc of rubber, and then released, when it would be
projected upwards owing to the straightening of the disc. I now repeat
the experiment, using paraffin oil instead of dimethyl-aniline; but in
this case the drop is only projected to a small depth, and the effect is
not so marked. The experiment furnishes conclusive evidence of the
existence of the interfacial tension.
*Formation of Falling Drops of Liquid.*—We will now direct our attention
to one of the most beautiful of natural phenomena—the growth and
partition of a drop of liquid. Let us observe, by the aid of the
lantern, this process in the case of water, falling in drops from the
end of a glass tube. The flow of water is controlled by a tap, and you
observe that the drop on the end gradually grows in size, then becomes
narrower near the end of the tube, and breaks across at this narrow
part, the separated drop falling to the ground. Another drop then grows
and breaks away; but the process is so rapid that the details cannot be
observed. None of you saw, for example, that each large drop after
severance was followed by a small droplet, formed from the narrowed
portion from which the main drop parted. But the small, secondary drop
is always present, and is called, in honour of its discoverer, Plateau’s
spherule. Nor did any of you observe that the large drop, immediately
after separation, became flattened at the top, nor were you able to
notice the changing shape of the narrow portion. To show all these
things it will be necessary to modify the experimental conditions.
Mr. H. G. Wells, in one of his short stories, describes the wonderful
effects of a dose of a peculiarly potent drug, called by him the
“Accelerator.” While its influence lasted, all the perceptions were
speeded up to a remarkable degree, so that occurrences which normally
appeared to be rapid seemed absurdly slow. A cyclist, for example,
although travelling at his best pace, scarcely appeared to be making any
movement; and a falling body looked as if it were stationary. Now if we
could come into possession of some of this marvellous compound, and take
the prescribed quantity, we should then be able to examine all that
happens when a drop forms and falls at our leisure. But it is not
necessary to resort to such means as this to render the process visible
to the eye. We could, for example, take a number of photographs
succeeding each other by very minute intervals of time—a kind of moving
picture—from which the details might be gleaned by examining the
individual photographs. This procedure, however, would be troublesome;
and evidently the simplest plan, if it could be accomplished, would be
to draw out the time taken by a drop in forming and falling. And our
previous experiments indicate how this may be done, as we shall see when
we have considered the forces at work on the escaping liquid.
A liquid issuing from a tube is pulled downwards by the force of
gravitation, and therefore is always tending to fall. At first, when the
drop is small, the action of gravity is overcome by the surface tension
of the liquid; but as the drop grows in size and increases in weight, a
point arrives at which the surface tension is overpowered. Then
commences the formation of a neck, which grows narrower under the
stretching force exerted by the weight of the drop, until rupture takes
place. Now if we wish to make the process more gradual, it will be
necessary to reduce the effect of gravity, as we cannot increase the
surface tension. We have already seen how this may be done in connexion
with liquid spheres—indeed, we were able to cancel the influence of
gravity entirely, by surrounding the working liquid by a second liquid
of exactly equal density. We require now, however, to allow the downward
pull of the drop ultimately to overcome the surface tension, and we must
therefore form the drop in a less dense liquid. If this surrounding
liquid be only slightly less dense, we should be able to produce a very
large drop; and if we make its growth slow we may observe the whole
process of formation and separation with the unaided eye.
[Illustration: __Fig._ 13.—Apparatus for forming ascending or descending
drops of liquids._]
Now it so happens that we have to hand two liquids which, without any
preparation, fulfil our requirements. Orthotoluidine, at temperatures
below 75° F. or 24° C., is denser than water of equal temperature. At
75° F. their densities are identical; and as the ordinary temperature of
a room lies between 60° and 70° F., water, under the prevailing
conditions, will be slightly the less dense of the two, and will
therefore form a suitable medium in which to form a large drop of
orthotoluidine. I therefore run this red-coloured liquid into water from
a funnel controlled by a tap (Fig. 13), and in order to make a large
drop the end of the stem is widened to a diameter of 1½ inches. It is
best, when starting, to place the end of the stem in contact with the
surface of the water, as the first quantity of orthotoluidine which runs
down then spreads over the surface and attaches itself to the rim of the
widened end of the stem. The tap is regulated so that the liquid flows
out slowly, and we may now watch the formation of the drop. At first it
is nearly hemispherical in shape; gradually, as you see, it becomes more
elongated; now the part near the top commences to narrow, forming a
neck, which, under the growing weight of the lower portion, is stretched
until it breaks, setting the large drop free (Figs. 14 to 18). And then
follows the droplet; very small by comparison with the big drop, but
plainly visible (Figs. 19 and 20). The graceful outline of the drop at
all stages of the formation must appeal to all who possess an eye for
beauty in form; free-flowing curves that no artist could surpass,
changing continuously until the process is complete.
Slow as was the formation of this drop, it was still too rapid to enable
you to trace the origin of the droplet. It came, as it always does come,
from the drawn-out neck. When the large drop is severed, the mass of
liquid clinging to the delivery-tube shrinks upwards, as the downward
pull upon it is now relieved. The result of this shrinkage—which, as
usual, reduces the area of surface to the minimum possible—is to cut off
the elongated neck, at its upper part, thus leaving free a
spindle-shaped column of liquid. This column immediately contracts,
owing to its surface tension, until its surface is a minimum—that is, it
becomes practically a sphere; and this constitutes the droplet. In a
later experiment, in which the formation is slower still, and the liquid
more viscous, the origin of the droplet will be plainly seen, and the
correctness of the description verified. The recoil due to the
liberation of the stretching force after rupture of the neck was visible
on the top of the large drop, and also on the bottom of the portion of
liquid which remained attached to the tube, both of which were
momentarily flattened (Figs. 19 and 20) before assuming their final
rounded shape. This is exactly what we should expect to happen if a
filled skin of indiarubber were stretched until it gave way at the
narrowest part.
[Illustration: __Fig._ 14._]
As a variation on the two liquids just used, I now take the yellow
liquid _nitrobenzene_, and run it into nitric acid (or other suitable
medium) of specific gravity 1·2, and you observe the same sequence of
events as in the previous experiment, even to the details. Very rapid
photography shows that the breaking away of a drop of water from the end
of a tube in air is in all respects identical with what we have just
seen on a large scale.
[Illustration: __Figs._ 14 to 20.—Formation of a drop of orthotoluidine,
showing the droplet. Seven stages._]
*Ascending or Inverted Drops.*—If we discharge orthotoluidine into water
when both are hotter than 75° F., the former liquid will rise, as its
density is now less than that of water. If, therefore, I take a funnel
with the stem bent into a parallel branch, so as to discharge upwards
(A, Fig. 13) and raise the temperature of both liquids above 75° F., we
see that the drop gradually grows towards the top of the water, finally
breaking away and giving rise to the droplet. Everything, in fact, was
the same as in the case of a falling drop, except that the direction was
reversed. A slight rise in temperature has thus turned the whole process
topsy-turvy, but the action is really the same in both cases. When, on
heating, the water acquired the greater density, its buoyancy overcame
the pull of gravitation on the orthotoluidine, and accordingly the drop
was pushed upwards, the result being the same as when it was pulled
downwards. An inverted drop may always be obtained by discharging a
light liquid into a heavier one, e.g. olive oil into water, or water
into any of the liquids mentioned on p. 19, below the equi-density
temperature.
LECTURE II
*Automatic Aniline Drops.*—In the foregoing experiments the drop was
enlarged until it broke away by feeding it with liquid; but it is
possible to arrange that the formation shall be quite automatic. The
experiment, as we shall see, is extremely simple, and yet it contains an
element of surprise. Into a beaker containing water nearly boiling I
pour a considerable quantity of aniline, which at first breaks up into a
large number of drops. After a short time, however, all the aniline
floats to the surface, having been warmed by contact with the water to a
temperature higher than that of equi-density (147° F., or 64° C.)—which
is exactly what we should expect to happen. There it remains for a brief
period in the form of a large mass with the lower portion curved in
outline. Soon, however, we observe the centre of the mass sinking in the
water, and taking on the now familiar outline of a falling drop.
Gradually, it narrows at the neck and breaks away; but as aniline is a
viscous liquid, the neck in this case is long and therefore easily seen.
The large drop breaks away and falls to the bottom of the beaker, its
upper surface rising and falling for some time owing to the recoil of
its skin after separation, finally becoming permanently convex.
Immediately after the large drop has parted, the upper mass shrinks
upwards, spreading out further on the surface of the water, with the
result that the long neck is severed at the top, its own weight
assisting the breakage. Now follows the resolution of the detached neck
into two or more spheres, usually a large and a small (Fig. 22). And
now, to those who view the experiment for the first time, comes the
surprise. The large drop, which was more or less flattened when it came
to rest at the bottom of the beaker, becomes more and more rounded, and
finally spherical. Then, unaided, it rises to the top and mingles itself
with the aniline which remained on the surface. After a brief interval a
second drop falls, imitating the performance of the first one; and, like
its predecessor, rises to the surface, after remaining for a short time
at the bottom of the vessel. And so long as we keep the temperature a
few degrees above that of equi-density, the process of partition and
reunion goes on indefinitely. The action is automatic and continuous,
and the large size of the drop and of the neck, and the slowness of the
procedure, enables us to follow with ease every stage in the formation
of a parting drop.
[Illustration: __Fig._ 21._]
[Illustration: __Figs._ 21 and 22.—Automatically formed aniline drops,
showing the formation of droplets from the neck._]
And now as to the explanation of this curious performance. When the
aniline reaches the surface, and spreads out, it cools by contact with
the air more rapidly than the water below. As it cools, its density
increases, and soon becomes greater than that of the water, in which it
then attempts to sink. The forces of surface tension prevent the whole
of the aniline from falling—the water surface can sustain a certain
weight of the liquid—but the surplus weight cannot be held, and
therefore breaks away. But when the detached drop reaches the bottom of
the vessel, it is warmed up again; and when its temperature rises above
that of equi-density it floats up to the top. And so the cycle of
operations becomes continuous, owing to cooling taking place at the top
and heating at the bottom.
Perpetual motion, you might suggest. Nothing of the kind. Perpetual
motion means the continuous performance of work without any supply of
energy; it does not mean merely continuous movement. A steam-engine
works so long as it is provided with steam, and an electric motor so
long as it is fed with electricity; but both stop when the supply of
energy is withdrawn. So with our aniline drop, which derives its energy
from the heat of the water, and which comes to rest immediately the
temperature falls below 147° F. or 64° C. But in order that the process
of separation and reunion may continue, the cooling at the top is quite
as necessary as the heating at the bottom. Our aniline drop is in
essence a heat-engine—although it does no external work—and like all
heat-engines possesses a source from which heat is derived, and a sink
into which heat at a lower temperature is rejected. We might, with
certain stipulations, work out an indicator diagram for our liquid
engine, but that would be straying too far from our present subject.
*Automatic Drops of other Liquids.*—Liquids which possess a low
equi-density temperature with water do not form automatic drops like
aniline, as the rate of cooling at the surface is too slow, and hence
the floating mass of liquid does not attain a density in excess of that
of the water beneath. Aceto-acetic ether, however, behaves like aniline,
if the temperature of the water be maintained at about 170° F. (77° C.),
but as this liquid is fairly soluble in hot water further quantities
must be added during the progress of the experiment. Results equal to
those obtained with aniline, however, may be secured by using
nitrobenzene in nitric acid of specific gravity 1·2 at 59° F. (15° C.),
the acid being heated to 185° F. (85° C.); and here you see the yellow
drop performing its alternate ascents and descents exactly as in the
case of aniline and water. Other examples might be given; but we may
take it as a general rule that when the equi-density temperature of the
liquid and medium is above 125° F. (52° C.), the phenomenon of the
automatic drop may usually be observed when the temperature is raised by
30° F. (17° C.), above this point.
*Liquid Jets.*—So far we have been observing the formation of single
drops, growing slowly at the end of a tube, or breaking away from a
large mass of the floating liquid. If, however, we accelerate the speed
at which the liquid escapes, the drop has no time to form at the outlet,
and a jet is then formed. We are all familiar with a jet of water
escaping from a tap; it consists of an unbroken column of the liquid up
to a certain distance, depending upon the pressure, but the lower part
is broken up into a large number of drops, which break away from the
column at a definite distance from the tap. There are many remarkable
features about jets which I do not intend to discuss here, as it is only
intended to consider the manner in which the drops at the end are
formed. To observe this procedure, it is necessary again to resort to
our method of slowing down the rate of formation, by allowing the liquid
to flow into a medium only slightly inferior in density. For this
purpose, orthotoluidine falling into water at the ordinary room
temperature is eminently satisfactory; and we see on the screen the
projection of a pipe, with its end under water, placed so that a jet of
orthotoluidine may be discharged vertically downwards from a stoppered
funnel. I open the tap slightly at first, and we then merely form a
single drop at the end. Now it is opened more widely, and you observe
that the drop breaks away some distance below the outlet, being rapidly
succeeded by another and another (Fig. 23). On still further opening the
tap the drops form at a still greater distance from the end of the pipe,
and succeed each other more rapidly, so that quite a number appear in
view at any given moment (Figs. 24 and 25). Notice how the drop is
distorted by breaking away from the stream of liquid, and how it
gradually recovers its spherical shape during its fall through the
water. Finally, I increase the discharge to such an extent that the
formation of the terminal drops is so rapid as to be no longer visible
to the eye, but appears like the turmoil observed at the end of a jet of
water escaping into air.
[Illustration: __Figs._ 23, 24, 25.—Jets of Orthotoluidine, discharged
under water._]
[Illustration: __Fig._ 26.—Water stretched between a tube and a plate._]
*Liquid Columns.*—A simple experiment will suffice to illustrate what is
meant by a liquid column. Here is a drop of water hanging from the end
of a glass tube. I place it in the lantern and obtain a magnified image
on the screen, and then bring up a flat plate of glass until it just
touches the suspended drop. As soon as contact is established, the water
spreads outwards over the plate, causing the drop to contract in
diameter at or near its middle part, so that its outline resembles that
of a capstan (Fig. 26). The end of the glass tube is now connected to
the plate by a column of water of curved outline, which is quite stable
if undisturbed. If, however, I gradually raise the tube, or lower the
plate, the narrow part of the column becomes still narrower, and finally
breaks across. In the same way we may produce columns of other liquids;
those obtained with viscous liquids such as glycerine being capable of
stretching to a greater extent than water, but showing the same general
characteristics. A liquid column, then, is in reality a supported drop,
and the severance effected by lowering the support is similar to that
which occurs when a pendent drop breaks away owing to its weight.
In our previous experiments we have seen that in order to produce large
drops of a given liquid, the surroundings should be of nearly the same
density, so as largely to diminish the effective weight of the suspended
mass. We might therefore expect that large columns of liquid could be
produced under similar conditions; and our conjecture is correct. We
may, for example, use the apparatus by means of which large drops of
orthotoluidine were formed (Fig. 13), using a shallow layer of water, so
that the lower end of the drop would come into contact with the bottom
of the vessel before the breaking stage was reached, and thus produce,
on a large scale, the same result as that we have just achieved by
allowing a hanging drop of water to touch a glass plate. This method,
however, restricts the diameter of the top of the column to that of the
delivery tube, and in this respect the shape is strained. The remedy for
this is to hang the drop from the surface of the water, when a degree of
freedom is conferred upon the upper part, which enables the column to
assume a greater variety of shapes. In order to show how this may be
accomplished, I pour a small quantity of water into the rounded end of a
wide test-tube, which is now seen projected on the screen, and then pour
gently down the side a quantity of _ethyl benzoate_, a liquid slightly
denser than water. You observe that the liquid spreads out on the
surface of the water, forming a hanging drop which at first is nearly
hemispherical in shape; but as I continue to add the liquid the drop
grows in size downwards, and finally reaches the bottom of the tube.
There is our liquid column (Fig. 27), which has formed itself in its own
way, free from the restriction imposed by a delivery tube. Notice the
graceful curved outline, produced by a beautiful balance between the
forces of surface tension and gravitation; and notice also how the
outline may be varied by the gradual addition of water, which causes the
upper surface to rise, and thus stretches the column (Fig. 28). The
middle becomes more and more narrow (Fig. 29), and finally breaks
across, leaving a portion of the former column hanging from the surface,
and the remainder, in rounded form (Fig. 30), at the bottom of the tube.
And, as usual, the partition was accompanied by the formation of a small
droplet.
[Illustration: __Figs._ 27, 28, 29, 30.—A liquid column stretched
upwards until broken by addition of water. Four stages._]
It is possible, by using other liquids, and different diameters of
vessels, to produce columns of a large variety of outlines. Some liquids
spread over a greater area on the surface of water than others, and
therefore produce columns with wider tops. Here we see a column of
orthotoluidine, which has a top diameter of 2 inches; and here again, in
contrast, is a column of aceto-acetic ether, the surface diameter of
which is only ½ inch (Fig. 31). Other liquids, such as aniline, give an
intermediate result. The lower diameter is determined by the width of
the vessel; and hence we are able to produce an almost endless number of
shapes. It is interesting to note how workers in glass and pottery have
unconsciously imitated these shapes; and I have here a variety of
articles which simulate the outlines of one or other of the liquid
columns you have just seen. It is possible that designers in these
branches of industry might obtain useful ideas from a study of liquid
columns, which present an almost limitless field for the practical
observation of curved forms.
[Illustration: __Fig._ 31.—A column of aceto-acetic ether in water._]
*Communicating Drops.*—There is a well-known experiment, which some of
you may have seen, in which two soap-bubbles are blown on separate
tubes, and are then placed in communication internally. If the bubbles
are exactly equal in size, no alteration takes place in either; but if
unequal, the smaller bubble shrinks, and forces the air in its interior
into the larger one, which therefore increases in size. Finally, the
small bubble is resolved into a slightly-curved skin which covers the
end of the tube on which it was originally blown. It is evident from
this experiment that the pressure per unit area exerted by the surface
of a bubble on the air inside is greater in a small than in a large
bubble. The internal pressure may be proved to vary inversely as the
radius of the bubble; thus by halving the radius we double the pressure
due to the elastic surface, and so on. The reciprocal of the radius of a
sphere is called its _curvature_, and we may therefore state that the
pressure exerted by the walls of the bubble on the interior vary
directly as the curvature.
[Illustration: __Fig._ 32.—Apparatus for communicating drops, with
extensions of unequal length attached._]
We have already seen that a drop of liquid possesses an elastic surface,
and is practically the same thing as a soap-bubble filled with liquid
instead of air. We might therefore expect the same results if two
suspended drops of liquid were placed in communication as those observed
in the case of soap-bubbles. And our reasoning is correct, as we may now
demonstrate. The apparatus consists (Fig. 32) of two parallel tubes,
each provided with a tap, and communicating with a cross-branch at the
top, which contains a reservoir to hold the liquid used. About half-way
down the parallel tubes a cross-piece, provided with a tap, is placed.
We commence by filling the whole of the system with the liquid under
trial, and the parallel tubes equal in length. Drops are then formed at
the ends of each vertical tube by opening the taps on these in turn, and
closing after suitable drops have been formed. Then, by opening the tap
on the horizontal cross-piece, we place the drops in communication and
watch the result.
I have chosen orthotoluidine as the liquid, and by placing the ends of
the vertical tubes under water—which at the temperature of the room is
slightly less dense than orthotoluidine—I am able to form much larger
drops than would be possible in air. You now see a small and a large
drop projected on the screen; and I now open the cross-tap, so that they
may communicate. Notice how the little drop shrinks until it forms
merely a slightly-curved prominence at the end of its tube. It attains a
position of rest when the curvature of this prominence is equal to that
of the now enlarged drop which has swallowed up the contents of the
smaller one. So far the result is identical with that obtained with
soap-bubbles; but we can extend the experiment in such a way as to
reverse the process, and make the little drop absorb the big one. In
order to do this I fasten an extension to one of the tubes, and form a
small drop deep down in the water, and a larger one on the unextended
branch near the top. When I open the communicating top, the system
becomes a kind of siphon, the orthotoluidine tending to flow out of the
end of the longer tube. The tendency of the large drop to siphon over is
opposed by the superior pressure exerted by the skin of the smaller
drop; but the former now prevails, and the big drop gradually shrinks
and the little one is observed to grow larger. It is possible by
regulating the depth at which the smaller drop is placed, to balance the
two tendencies, so that the superior pressure due to the lesser drop is
equalled by the extra downward pressure due to the greater length of the
column of which it forms the terminus. Both pressures are numerically
very small, but are still of sufficient magnitude to cause a flow of
liquid in one or other direction when not exactly in equilibrium. In the
case of communicating soap-bubbles, containing air and surrounded by
air, locating the small bubble at a lower level would not reverse the
direction of flow, which we succeeded in accomplishing with liquid drops
formed in a medium of slightly inferior density.
[Illustration: __Fig._ 33.—Combined drops of vapour and liquid._]
*Combined Vapour and Liquid Drops.*—All liquids when heated give off
vapour, the amount increasing as the temperature rises. The vapour
formed in the lower part of the vessel in which the liquid is heated
rises in the form of bubbles, which may condense again if the upper part
of the liquid be cold. When the liquid becomes hot throughout, however,
the vapour bubbles reach the surface and break, allowing the contents to
escape into the air above. Everyone who has watched a liquid boiling
will be familiar with this process, but it should be remembered that a
liquid may give off large quantities of vapour without actually boiling.
A dish of cold water, if exposed to the air, will gradually evaporate
away; whilst other liquids, such as petrol and alcohol, will disappear
rapidly under the same circumstances—and hence are called “volatile”
liquids.
The formation of vapour and its subsequent escape at the surface of the
liquid, enable us to produce a very novel kind of drop; if, instead of
allowing the bubbles to escape into air, we cause them to enter a second
liquid. Here, for example, is a coloured layer of chloroform¹ at the
bottom of a beaker, with a column of water above. I project the image of
the beaker on the screen, and then heat it below. The chloroform vapour
escapes in bubbles; but notice that each bubble carries with it a
quantity of liquid, torn off, as it were, at the moment of separation.
The vapour bubbles and their liquid appendages vary in size, but some of
them, you observe, have an average density about equal to that of the
water, and float about like weighted balloons. Some rise nearly to the
surface, where the water is coldest; and then the vapour partially
condenses, with the result that its lifting power is diminished, and
hence the drops sink into the lower part of the beaker. But the water is
warmer in this region, and consequently the vapour bubble increases in
size and lifting power until again able to lift its load to the surface.
So the composite drops go up and down, until finally they reach the
surface, when the vapour passes into the air, and the suspended liquid
falls back to the mass at the bottom of the beaker. Notice that the drop
of liquid attached to each bubble is elongated vertically. This is
because chloroform is a much denser liquid than water (Fig. 33). There
is a practical lesson to be drawn from this experiment. Whenever a
bubble of vapour breaks through the surface of a liquid, it tends to
carry with it some of the liquid, which is dragged mechanically into the
space above. In our experiment the space was occupied by water, which
enabled the bubble to detach a much greater weight than would be
possible if the surface of escape had been covered by air, which is far
less buoyant than water. But even when the bubbles escape into air, tiny
quantities of liquid are detached; so that steam from boiling water, for
example, is never entirely free from liquid. All users of steam are well
acquainted with this fact.
¹ Mono-brom-benzene is better than chloroform for this experiment,
but is more costly. It may be coloured with indigo. Chloroform may
be coloured with iodine.
*Condensation of Drops from Vapour,—Mists, Fogs and Raindrops.*—The
atmosphere is the great laboratory for the manufacture of drops. It is
continually receiving water in the form of vapour from the surface of
the sea, from lakes, from running water, and even from snow and ice. All
this vapour is ultimately turned into drops, and returned again to the
surface, and to this never-ceasing exchange all the phenomena connected
with the precipitation of moisture are due. The atmosphere is only
capable of holding a certain quantity of water in the form of vapour,
and the lower the temperature the less the capacity for invisible
moisture. When fully charged, the atmosphere is said to be “saturated”—a
condition realized on the small scale by air in a corked bottle
containing some water, which evaporates until the air can hold no more.
The maximum weight of vapour that can be held by 1 cubic metre of air at
different temperatures is shown in the table:—
──────────────────────────────────────────────────────────────────
Temperature. Weight of water vapour
(grammes) required to
Deg. C. Deg. F. saturate 1 cubic metre.
──────────────────────────────────────────────────────────────────
0 32 4·8
5 41 6·8
10 50 9·3
15 59 12·7
20 68 17·1
25 77 22·8
30 86 30·0
35 95 39·2
40 104 50·6
──────────────────────────────────────────────────────────────────
It will be seen from the table that air on a warm day in summer, with a
temperature of 77° F., can hold nearly five times as much moisture as
air at the freezing point, or 32° F. The amount actually present,
however, is usually below the maximum, and is recorded for
meteorological purposes as a percentage of the maximum. Thus if the
“relative humidity” at 77° F. were 70 per cent., it would imply that the
weight of moisture in 1 cubic metre was 70 per cent. of 22·8 grammes;
that is, nearly 16 grammes. If 1 cubic metre of air at 77° F.,
containing 16 grammes of moisture, were cooled to 50° F., a quantity of
water equal to (16-9·3) = 6·7 grammes would separate out, as the maximum
content at the lower temperature is 9·3 grammes. Precipitation would
commence at 66° F., at which temperature 1 cubic metre is saturated by
16 grammes. And similarly for all states of the atmosphere with respect
to moisture, cooling to a sufficient extent causes deposition of water
to commence immediately below the saturation temperature, and the colder
the air becomes afterwards the greater the amount which settles out. The
temperature at which deposition commences is called the “dew point.”
Whenever atmospheric moisture assumes the liquid form, drops are
invariably formed. These may vary in size, from the tiny spheres which
form a mist to the large raindrops which accompany a thunderstorm. But
in every instance it is necessary that the air shall be cooled below its
saturation point before the separation can commence; and keeping this
fact in mind we can now proceed to demonstrate the production of mists
and fogs. Here is a large flask containing some water, fitted with a
cork through which is passed a glass tube provided with a tap. I pump
some air into it with a bicycle pump, and then close the tap. As excess
of water is present, the enclosed air will be saturated. Now a
compressed gas, on expanding into the atmosphere, does work, and is
therefore cooled; and consequently if I open the tap the air in the
flask will be cooled, and as it was already saturated the result of
cooling will be to cause some of the moisture to liquefy. Accordingly,
when I open the tap, the interior of the flask immediately becomes
filled with mist. If we examine the mist in a strong light by the aid of
a magnifying glass, we observe that it consists of myriads of tiny
spheres of water, which float in the air, and only subside very
gradually, owing to the friction between their surfaces and the
surrounding air preventing a rapid fall. The smaller the sphere, the
greater the area of surface in proportion to mass, and therefore the
slower its fall. And so in nature, the mists are formed by the cooling
of the atmosphere by contact with the surface, until, after the
saturation point is reached, the surplus moisture settles out in the
form of tiny spheres, which float near the surface, and are dissipated
when the sun warms up the ground and the misty air, and thus enables the
water again to be held as vapour.
Fogs, like mists, are composed of small spheres of water condensed from
the atmosphere by cooling; but in these unwelcome visitors the region of
cooling extends to a higher level, and the lowering of temperature is
due to other causes than contact with the cold surface of the earth. In
our populous cities, the density of the fogs is accentuated by the
presence of large quantities of solid particles in the atmosphere, which
arise from the smoke from coal fires, and the abrasion of the roads by
traffic. We can make a city fog in our flask. I blow in some tobacco
smoke, and then pump in air as before. You will notice that the smoke,
which is now disseminated through the air in the flask, is scarcely
visible; but now, on opening the tap, the interior becomes much darker
than was the case in our previous experiment. We have produced a genuine
yellow fog, that is, a dense mist coloured by smoke. When we have
learned how to abolish smoke, and how to prevent dust arising from the
streets, our worst fogs will be reduced to dense mists, such as are now
met with on the sea or on land remote from large centres of habitation.
There is one feature common to the spheres which compose a mist or fog,
or indeed to any kind of drop resulting from the condensation of
moisture in the atmosphere. As shown by the deeply interesting
researches of Aitken and others, each separate sphere forms round a core
or nucleus, which is usually a small speck of dust, and hence an
atmosphere charged with solid particles lends itself to the formation of
dense fogs immediately the temperature falls below the dew-point. But
dust particles are not indispensable to the production of condensed
spheres, for it has been shown that the extremely small bodies we call
“ions,” which are electrically charged atoms, can act as centres round
which the water will collect; and much atmospheric condensation at high
elevations is probably due to the aid of ions.² Near the surface,
however, where dust is ever present, condensation round the innumerable
specks or motes is the rule. Here, for example, is a jet of steam
escaping into air, forming a white cloud composed of a multitude of
small spheres of condensed water. If now I allow the steam to enter a
large flask containing air from which the dust has been removed by
filtration through cotton wool, no cloud is formed in the interior, but
instead condensation takes place at the end of the jet, from which large
drops fall, and on the cold sides of the flask. The cloud we see in
dusty air is entirely absent, and the effect of solid particles in the
process of condensation is thus shown in a striking manner. Clouds are
masses of thick mist floating at varying heights in the atmosphere. On
sinking into a warmer layer of dry air the particles of which clouds are
composed will evaporate and vanish from sight. If the condensation
continue, however, the spheres will grow in size until the friction of
the atmosphere is unable to arrest their fall; and then we have rain.
And whether the precipitation be very gentle, and composed of small
drops falling slowly, as in a “Scotch mist,” or in the form of
rapid-falling large drops such as accompany a thunderstorm, the
processes at work are identical. Every particle of a mist or cloud, and
every raindrop, is formed round a nucleus, and owes its spherical shape
to the tension at the surface.
² Mr. C. T. R. Wilson has recently devised an apparatus for making
visible the tracks of ionizing rays, by the condensation of water
vapour round the freshly liberated ions.
*Liquid Clouds in Liquid Media.*—Just as the excess of moisture is
precipitated from saturated air when the temperature falls, so is the
excess of one liquid dissolved in another thrown down by cooling below
the saturation temperature. Moreover, a liquid when precipitated in a
second liquid appears in the form of myriads of small spheres, which
have the appearance of a dense cloud. Here is some boiling water to
which an excess of aniline has been added, so that the water has
dissolved as much aniline as it can hold. Aniline dissolves more freely
in hot water than in cold, so that if I remove the flame, and allow the
beaker to cool, the surplus of dissolved aniline will settle out.
Cooling takes place most rapidly at the surface, and you observe white
streaks falling from the top into the interior, where they are warmed up
and disappear. Soon, however, the cooling spreads throughout; and now
the streaks become permanent, and the water becomes opaque, owing to the
thick white cloud of precipitated aniline. The absence of the red colour
characteristic of aniline is due to the extremely fine state of division
assumed in the process. If left for some hours, the white cloud sinks
through the water to the bottom of the beaker, where the small particles
coalesce and form large drops, leaving the overlying water quite
transparent. The process is quite analogous to the precipitation of
moisture from the atmosphere in the form of small spheres, which, if
undisturbed, would gradually sink to the ground and leave the air clear.
*Overheated Drops.*—The temperature at which a liquid boils, under
normal conditions, depends only upon the pressure on its surface. Thus
water boiling in air, when the pressure is 76 centimetres or 29·92
inches of mercury, corresponding to 14·7 pounds per square inch,
possesses a temperature of 100° C. or 212° F. At higher elevations,
where the pressure is less, the boiling point is lower; thus Tyndall
observed that on the summit of the Finsteraarhorn (14,000 feet) water
boiled at 86° C. or 187° F. Conversely, under increased pressure, the
boiling point rises; so that at a pressure of 35 pounds per square inch
water does not boil until the temperature reaches 125° C. or 257° F.
There are certain abnormal conditions, however, under which the boiling
point of a liquid may be raised considerably without any increase in the
pressure at the surface; and it is then said to be “over-heated.” Dufour
showed that when drops of water are floating in another liquid of the
same density, they may become greatly overheated, and if very small in
size may attain a temperature of 150° C. or 302° F., or even higher,
before bursting into steam. In order to provide a medium in which water
drops would float at these temperatures, Dufour made a mixture of
linseed oil and oil of cloves, which possessed the necessary
equi-density temperature with water. To demonstrate this curious
phenomenon, I take a mixture of 4 volumes of ethyl benzoate and 1 volume
of aniline, which at 125° C. or 257° F. is exactly equal in density to
water at the same temperature. I add to the mixture two or three cubic
centimetres of freshly-boiled water, the temperature being maintained at
125° C. by surrounding the vessel with glycerine heated by a flame. At
first the water sinks, but on attaining the temperature of the mixture
it breaks up with some violence, forming spheres of various sizes which
remain suspended in the mixture. Any portion of the water which has
reached the surface boils vigorously, and escapes in the form of steam;
and some of the larger spheres may be observed to be giving off steam,
which rises to the surface. Most of the spheres, however, remain
perfectly tranquil, in spite of the fact that the water of which they
are composed is many degrees above its normal boiling point. If I
penetrate one of these spheres with a wire, you notice that it breaks up
immediately, with a rapid generation of steam. A complete explanation of
this abnormal condition of water is difficult to follow, as a number of
factors are involved. One of the contributory causes—though possibly a
minor one—is the opposition offered to the liberation of steam by the
tension at the surface of the spheres.
[Illustration: __Fig._ 34.—Spheroid of water on a hot plate._]
*Floating Drops on Hot Surfaces.*—If a liquid be allowed to fall in
small quantity on to a very hot solid, it does not spread out over the
surface, but forms into drops which run about and gradually evaporate.
By careful procedure, we may form a very large, flattened drop on a hot
surface, and on investigation we shall notice some remarkable facts. I
take a plate of aluminium, with a dimple in the centre, and make it very
hot by means of a burner. You see the upper surface of this plate
projected on the screen. I now allow water to fall on the plate drop by
drop, and you hear a hissing noise produced when each drop strikes the
plate. The separate drops gather together in the depression at the
centre of the plate, forming a very large flattened globule. You might
have expected the water to boil vigorously, but no signs of ebullition
are visible; and what is more remarkable, the temperature of the drop,
in spite of its surroundings, is actually less than the ordinary boiling
point. Notice now how the drop has commenced to rotate, and has been set
into vibration, causing the edges to become scalloped (Fig. 34). The
drop, although not actually boiling, is giving off vapour rapidly, and
therefore gradually diminishes in size. And now I want to prove that the
drop is not really touching the plate, but floating above it. To do this
I make an electric circuit containing a cell and galvanometer, and
connect one terminal to the plate and place the other in the drop. No
movement is shown on the galvanometer, as would be the case if the drop
touched the plate and thus completed the electric circuit. And at close
range we can actually see a gap between the drop and the plate, so that
the evidence is conclusive. If now I remove the flame—leaving the
electric circuit intact—and allow the plate to cool, we notice after a
time that the globule flattens out suddenly and touches the plate, as
shown by the deflection of the galvanometer; and simultaneously a large
cloud of steam arises, due to the rapid boiling which occurs immediately
contact is made.
What we have seen in the case of water is shown by most liquids when
presented to a surface possessing a temperature much higher than the
boiling point of the liquid. A liquid held up in this manner above a hot
surface is said to be in the _spheroidal state_, to distinguish it from
the flat state usually assumed by spreading when contact occurs between
the liquid and the surface. It is doubtful whether any satisfactory
explanation of the spheroidal state has ever been given. Evidently, the
layer of vapour between the plate and the drop must exert a considerable
upward pressure in order to sustain the drop, but the exact origin of
this pressure is difficult to trace.
LECTURE III
*Spreading of Oil on the Surface of Water.*—If a small drop of oil be
placed on the surface of water it will be observed to spread immediately
until it forms a thin layer covering the surface. If a further addition
of the oil be made, globules will be formed, which, as you now see upon
the screen, remain floating on the surface. The spreading of the oil in
all directions from the place on which the small quantity of oil was
dropped is due to the superior surface tension of water, which pulls the
oil outwards. The surface tension of the oil opposes that of the water,
and would prevent the drop from spreading were it not overcome by a
greater force. The result is the same as would be observed if the centre
or any other part of a stretched rubber disc were weakened; the weak
part would be stretched in all directions, and the rest of the disc
would shrink towards the sides. When the oil has spread out, however,
and contaminated, as it were, the surface of the water, the surface
tension is reduced, and is not sufficiently strong to stretch out a
further quantity of oil, which, if added, remains in the form of a
floating globule.
[Illustration: __Fig._ 35.—Forces acting on a floating globule._]
Let us study the forces at work on the floating globule a little more
closely. Its upper surface is in contact with air, and the surface
tension tends, as usual, to reduce the area to a minimum. The top of the
globule is not flat, but curved (Fig. 35), and its surface meets that of
the water at an angle; and the counter-pull exerted against the
stretching-pull of the water surface is not horizontal, but inclined in
the direction of the angle of contact, as shown by the line B. The under
part of the globule is also curved, and meets the water surface from
below at an angle; and here also is exerted a pull in opposition to that
of the water surface, different in magnitude to the force at the upper
surface, but also directed at the angle of contact as shown by the line
C. This tension at the joining surface of two liquids is called the
“interfacial” tension, to distinguish it from that of a surface in
contact with air. Acting against these two tensions is that of the
water, which is directed horizontally along the surface, as shown by the
line A. The lines A, B, and C indicate the forces acting at a single
point; but the same forces are at work at every point round the circle
of contact of the globule and the surface of the water. And therefore
the tendency on the part of the water tension is to cause the globule to
spread out in all directions, whereas the other two tensions tend to
prevent any enlargement of its surface. The result depends upon the
magnitudes and directions of the conflicting forces. We can imagine a
kind of tug-of-war taking place, in which one contestant, A, is opposed
to two others, B and C, all pulling in the directions indicated in Fig.
35. Although A is single-handed, he has the advantage of a straight
pull, whereas B and C can only exert their strength at an angle, and the
larger the angle the more they are handicapped. If A be more powerful
than B and C, the globule will spread; but the result of the spreading
is to diminish the angles at which the pulls of B and C are inclined to
the surface, and hence their effective opposition to A will be
increased. Moreover, the spreading of the liquid diminishes the surface
tension of the water—that is, weakens A—and hence it becomes possible
for B and C to prevail and draw back the surface of the globule which A
had previously stretched. If, in spite of these disabilities, A should
still be the stronger, the globule will be stretched until it covers the
whole surface; whereas if B and C overcome A, the globule will shrink,
increasing the angles at which B and C operate, and therefore reducing
their effective pulls, until their combined strength is equal to that of
A, when the globule will remain at rest. Bearing these facts in mind, we
can understand why a small drop of oil placed on a clean water surface
spreads across; for in this case A is stronger than B and C combined.
But when the surface of the water is covered with a layer of oil, A is
weakened, and can no longer overcome the opposing pulls of B and C.
Hence a further drop of oil poured on to the surface remains in the form
of a globule.
*Movements due to Solubility.*—When small fragments of camphor are
placed on the surface of water some remarkable movements are seen.³ The
bits of camphor move about with great rapidity over the surface, and
generally, in addition, show a rapid rotary motion. The explanation
usually given is that the camphor dissolves in the water at the points
of contact forming a solution which possesses a less surface tension
than pure water. This solution is in consequence stretched by the
tension of the rest of the surface, and the camphor floating on its
solution is therefore made to move in the direction of the line along
which the stretching force happens to be the greatest. But the camphor
continues to dissolve wherever it goes, and is therefore continuously
pulled about as a result of this interplay of tensions. Touching the
surface with a wire which has been dipped in oil immediately arrests the
movements, owing to the tension of the water being diminished to such an
extent by the skin of oil that it is no longer competent to stretch the
part on which the camphor floats. No doubt this explanation is correct
so far as it goes, but it is highly probable that when the floating
substance dissolves, other forces are called into action in addition to
the tensions.
³ These movements were first recorded by Romieu in 1748 and were
ascribed by him to electricity.
[Illustration: __Fig._ 36.—Aniline globules on a water surface._]
*Movements of Aniline Globules on a Water Surface.*—If we allow a small
quantity of aniline to run on to the surface of water, it forms itself
into a number of floating globules. I now project on the screen a water
surface on which a little aniline has been poured, and we are thus
enabled to watch the movements which occur. All the globules appear to
be twitching or shuddering; and if you observe closely you will notice
the surface of each globule stretching and recoiling alternately. The
recoil is accompanied by the projection of tiny globules from the rim,
which becomes scalloped when the globule is stretched. The small
globules thrown off appear to be formed from the protuberances at the
edge (Fig. 36), and after leaving the main globule they spread out over
the surface, or dissolve. This process continues for a long time,
gradually diminishing in vigour, until small stationary globules are
left floating on the surface, which is now covered with a skin of
aniline. This action is in striking contrast to the tranquil formation
of floating globules of oil, and calls for some special comment.
Let us recall again the three forces at work at the edge of a floating
globule (Fig. 35). The surface tension of the water, acting
horizontally, tends to stretch the globule, and is successful
momentarily in overcoming the opposing tensions, each of which pulls at
an angle to the surface. Enlargement of the upper surface of the
globule, however, reduces the angles at which the tensions B and C act,
and in consequence their effective strength is increased. The spreading
of the aniline over the water surface diminishes the pull A, which B and
C combined now overcome, and hence the surface of the globule shrinks
again. For some unexplained reason both the stretching and recoil of the
globule occur suddenly, there being an interval of repose between each,
and these jerky movements result in small portions of the rim being
detached, each of which forms a separate small globule. The aniline
which spreads over the surface of the water dissolves, and the water
tension A, which had been enfeebled by the presence of the aniline skin,
recovers its former strength, and again stretches the globule; and so
the whole process is repeated. When the surface of the water becomes
permanently covered with a skin, which occurs when the top layer is
saturated with aniline, the globule remains at rest, and has such a
shape that the tensions B and C act at angles which enable them just to
balance the weakened pull of A. Why the edge of the globule becomes
indented during the movements, and why these movements are spasmodic
instead of gradual, has not been clearly made out. It is interesting to
recall that a spheroid of liquid on a hot plate also possesses a
scalloped edge, and it may be that the two phenomena have something in
common.
[Illustration: __Fig._ 37.—Orthotoluidine globules on a water surface._]
*Movements of Orthotoluidine and Xylidine 1-3-4 on a Water Surface.*—We
will now observe, by the aid of the lantern, movements of globules more
striking, and certainly more puzzling, than those of aniline. I place on
the surface of the water a quantity of a special sample of
orthotoluidine, and you see that immediately a number of globules are
formed which are endowed with remarkable activity. They become indented
at one side, and then dart across the surface at a great speed, usually
breaking into two as a result of the violent action (Fig. 37). Then
follows a short period of rest, when suddenly, as if in response to a
signal, all the larger globules again become indented, forming shapes
like kidneys, and again shoot across the surface, breaking up into
smaller globules. Notice that the very small globules remain at rest; it
is only those above a certain size that display this remarkable
activity. A film of the liquid forms on the water, and the action
gradually becomes more intermittent, ceasing altogether when a skin is
well established, and the large globules have sub-divided into very
small ones. My sample of orthotoluidine is somewhat unique, as other
specimens of the liquid, obtained from the same and other sources, do
not show the same lively characteristics. As in the case of camphor,
touching the surface with a drop of oil arrests the movements
immediately. The organic liquid _xylidine_ 1-3-4, however, exhibits the
same movements, as you now see on the screen; and, if anything, is even
more active than the orthotoluidine previously shown. It may be added
that occasional samples of aniline show similar movements, but of less
intensity.
Now if I am asked to explain these extraordinary movements, I am bound
to confess my inability to do so at present. Why should the globules
become indented on one side only? The two tensions acting at the edge in
opposition to the water tension are at work all round the globule, and
it is not easy to see why they should prevail to such a marked degree at
one spot only. The movement across the surface, if we followed our
previous explanations, would be due to the superior pull of the water
tension behind the globule, opposite the indented part; although to look
at it would seem as if some single force produced the indentation and
pushed the globule along bodily. Are there local weaknesses in the
tension of the water, and, if so, why should such weak spots form
simultaneously near each globule, causing each to move at the same
moment? Any explanation we may give as to the origin of the cavity in
the side of the globule does not suffice to account for the intermittent
character of the movement, and its simultaneous occurrence over the
whole surface. We must therefore leave the problem at present, and trust
to future investigation to provide a solution.
[Illustration: __FIG. 38._—Resolution of a floating skin into
globules._]
*Production of Globules from Films.*—When a film of oil spreads over a
water surface it sometimes remains as such indefinitely. Certain other
liquids, however, form films which after a short interval break up into
globules, and the process of transition is at once striking and
beautiful. In order to show it, I project a water surface on the screen,
and pour on to it a very small quantity of _dimethyl-aniline_—an oily
liquid related to but distinct from ordinary aniline. It spreads out
into a film of irregular outline, which floats quietly for a short time.
Soon, however, indentations are formed at the edges, which penetrate the
film, and from the sides of the indentations branches spread which in
turn become branched; and shortly the whole film becomes ramified,
resembling a mass of coral, or, to use a more homely illustration, a
jig-saw puzzle (Fig. 38). The various branches join in numerous places,
cutting off small islands from the film; and immediately each island
becomes circular in outline—and the resolution into globules is
complete. We have witnessed one of the beauty-sights of Nature.
The same method of globule formation is shown by nitro-benzol and
_quinoline_, and as the action is more gradual in the case of the latter
substance, I show it in order that we may study the process in greater
detail. Notice the formation of the indentations and their subsequent
branching; and also that holes form in the skin from which branchings
also proceed. In this instance the film is broken up in sections, but
the action continues until nothing but globules remain on the surface.⁴
It is not easy to see why the canals of water penetrate the film and
split it up into small sections, nor why entry takes place at certain
points on the edge in preference to others. Some orderly interplay of
forces, not yet properly understood, gives rise to the action; and a
satisfactory explanation has yet to be given.
⁴ The breaking-up of films on the surface of water was first noticed
by Tomlinson about 50 years ago. He used essential oils, and called
the patterns “cohesion figures.”
*Network formed from a Film.*—A further example of the breaking up of a
film is furnished by certain oils derived from coal-tar, the result in
this case being the formation of a network or cellular structure. I
place on the surface of water in a glass dish a small quantity of
tar-oil, and project it on the screen. It spreads out at first into a
thin film, which, by reflected light, shows a gorgeous display of
colours. After a short time, little holes make an appearance in the
film, and these holes gradually increase in size until the whole of the
film is honeycombed (Fig. 39), the oil having been heaped up into the
walls which divide the separate compartments. Here again the accepted
views on surface tension do not appear competent to explain the action.
It appears to be the case that most films on the surface of water show
this tendency to perforation, which may be due to inequalities in the
thickness of the film, or in the distribution of the strain to which it
is subjected.⁵
⁵ An interesting discussion on cellular structures of this type may
be found in _Nature_, April 16 to June 11, 1914.
[Illustration: __Fig._ 39.—Network formed from a film of tar-oil on the
surface of water._]
[Illustration: __Fig._ 40.—Quinoline rings and perforated plates._]
*Quinoline Rings.*—Reference has already been made to the breaking-up of
a quinoline film into globules. But if we examine the surface about half
an hour after the formation of these globules, we find that each has
been perforated in the centre, forming a ring or annulus (Fig. 40). Some
of the larger globules have undergone perforation in several places,
forming honeycombed plates. These rings and plates, which you now see
projected on the screen, remain unchanged, and apparently represent the
final stage of equilibrium under the action of the various forces.
Quinoline, so far as observations have been made, appears to be unique
in respect to the formation of stable rings from globules.
[Illustration: __Fig._ 41.—The expanding globule._]
*Expanding Globules.*—I now wish to show, by an experiment, how
sensitive a floating globule is to disturbances in the existing
tensions, which maintain it at rest. On the screen is projected a
globule of dimethyl-aniline, floating tranquilly on the surface of
water. I now allow a small drop of quinoline to fall upon it, and
immediately it spreads out over the surface, forming a hole in its
centre (Fig. 41), after which it gradually resumes its former shape.
Sometimes the action is so violent that the globule is split up into
several portions, which, however, join together again after a short
time. In order to explain this action, we must again refer to the three
tensions operating on the globule (Fig. 35). When in equilibrium, A is
balanced by the joint pull of B and C; and hence if either of the latter
be weakened, A will predominate and stretch the globule. In our
experiment it is the interfacial tension, C, which has been diminished
in strength, as we may now prove by a second experiment. In this
instance I float on the water surface a globule of lubricating oil, with
which quinoline does not readily mix, and which does not act so
immediately as dimethyl-aniline. On allowing the drop of quinoline to
fall into it, no action is observed until the drop has rested on the
junction of the oil and water for a short time; but when it has
penetrated the interface the oil globule suddenly spreads over the water
surface, and with such violence as to detach several portions from the
main globule. Merely touching the upper surface of the oil with a rod
moistened with quinoline has no effect, and hence the result is due to
the weakening of the interfacial tension. A similar effect is obtained
when quinoline is dropped into a globule of aniline, and may be obtained
with various other liquids.
*Attraction between Floating Globules.—The “Devouring” Globule.* When
globules of different liquids are floating on the same water surface, a
tendency to coalesce is sometimes noticed, but is by no means general. I
will show one example which possesses striking features, showing as it
does the remarkable results which may be brought about by surface
forces. First of all, we form a number of active orthotoluidine globules
on the surface of a dish of water, which you see wriggling about in
their characteristic fashion. After their activity has subsided
somewhat, I float on to the surface a large globule of dimethyl-aniline.
Attraction of some kind is at once apparent, for the nearest globule of
orthotoluidine immediately approaches the intruder. And now comes the
process of absorption. The large globule of dimethyl-aniline develops a
protuberance in the direction of its victim (Figs. 42 and 43), and the
small globule of orthotoluidine coalesces with this “feeler,” which then
shrinks back into the large globule, conveying with it the entangled
orthotoluidine. This, however, by no means satisfies the devouring
globule, as a second victim is at once appropriated in the same manner;
and you will notice a nibbling process at work round the edges
continuously, which is due to the absorption of the smaller globules of
orthotoluidine. The action continues until the whole of the surface has
been cleared of orthotoluidine, after which the large globule floats
tranquilly in the centre of the vessel, apparently resting after its
heavy meal. The interaction of the forces which gives rise to this
phenomenon is difficult to fathom; there are no doubt several tensions,
constantly changing in magnitude, which in the result cause the liquids
of the large and small globules to intermingle. Separate globules of a
single liquid sometimes unite in this manner, but this is not common, it
being more usual for the scattered units to remain apart.
[Illustration: __Fig._ 42.—The “devouring” globule. Five stages._]
[Illustration: __Fig._ 43.—Photograph of one globule absorbing
another._]
*Analogies of Surface Tension Phenomena with Life.*—When we watch the
movements of globules on the surface of water, the resemblance to the
antics of the lower forms of life immediately occurs to our minds. Now I
do not intend here to intrude any opinion on the much-discussed subject
of the Origin of Life, but merely to point out that certain phenomena,
usually supposed to be associated only with living things, may result
from the interplay of surface tensions. In our experiments we have
witnessed expansive and contractile motion (aniline globules on water);
movement of translation, of a very vigorous kind (xylidine and
orthotoluidine globules); incorporation of external matter, or feeding
(dimethyl-aniline absorbing orthotoluidine)—we are getting quite
familiar with these long names now—, splitting up of masses, or division
(skins of quinoline, etc., breaking up into branched portions, and
sub-division of large globules); and formation of cellular structure
(tar-oil on water). And the conclusion we may legitimately draw is this:
that mechanical forces may account for many observed phenomena in
connexion with life which formerly were attributed to the action of
“vital” forces. Modern biological research all points in the same
direction, and it seems probable that the operations of the animate and
inanimate are controlled by the same forces. But the mystery of Life
still remains.
*Conclusion.*—I have endeavoured in these lectures to bring to your
notice some of the remarkable results which may be produced by the use
of water and a few other liquids, and the scientific conclusions which
may be drawn from them. It may be that the phenomena we have considered
have little or no commercial application; but science has other uses in
addition to its fruitful alliance with commerce. The study of the
methods by which Nature achieves her ends stimulates the imagination and
quickens the perceptions, and is therefore of the highest educational
value. It is a great scientific achievement to run a railway to the
summit of the Jungfrau, but we should not envy the mental condition of
the individual to whom that glorious mountain appealed only through the
railway dividends. And I trust that we shall never become so imbued with
the industrial aspects of science, as to lessen our appreciation of the
works of Nature, whether manifested in the snow-clad peak or the equally
wonderful drop of water.
APPENDIX
Apparatus and Materials required for Experiments on Drops and Globules.
*Vessels.*—For direct observation of liquid spheres, large drops, etc.,
beakers about 6 inches in height and 4 inches in diameter are suitable.
It must be remembered, however, that a beaker containing water behaves
like a cylindrical lens, and hence objects in the interior appear
distorted in shape. In order to observe the true dimensions, flat-sided
vessels must be used, in which the faces are of uniform thickness. Glass
battery-vessels, which are made of a single piece of glass, have sides
of irregular thickness, and are not to be recommended. A useful form of
vessel is one in which the bottom and edges are made of copper, the
sides being formed of windows of plate glass cemented to the copper
framework. Water may be boiled in such a vessel without danger to the
glass, starting with cold water; it is not advisable to pour hot water
into the cold vessel, however, as the glass may crack. Suitable
dimensions for a vessel of this kind are 6 inches high, and 4 inches in
width and thickness. A beaker containing water, in which drops are
formed may be placed in this square vessel, and surrounded by water,
when distortion will be absent; and the whole of the contents may be
kept hot—as required, for example, with the automatic aniline drop. It
is best to conduct the experiments in beakers immersed as described, as
the materials used may then be easily recovered without having to clean
out the flat vessel.
For the formation of liquid columns, test-tubes, of diameter 1 to 2
inches, or small beakers, may be used. Test-tubes provided with a foot,
which will stand upright, are most satisfactory; and the true shape may
be seen by immersing the test-tube or beaker in water in a flat-sided
vessel of the form described above. The effect of heat on the shape of
the column may be observed by warming the water in the vessel. The
centrifugoscope (Fig. 7) and the apparatus depicted in Figs. 8, 13, and
32, may be procured from the makers, Messrs. A. Gallenkamp & Co., Sun
Street, E.C.
Experiments with skins and globules may be conducted in beakers of about
4 inches diameter, or in small porcelain photographic dishes. If
intended for lantern projection shallow cells, with a bottom of plate
glass, are necessary, and may be obtained from dealers in scientific
apparatus.
*Materials.*—Sufficient quantities of the various liquids used may be
procured from dealers in chemicals at a small cost. Aniline and
orthotoluidine, which figure largely in the experiments, should be
obtained in the “commercial” form, which is the cheapest and most
suitable. The remaining liquids should be of the variety described as
“pure” in the catalogues. When used for the formation of films, they
should be kept in bottles in which the glass stopper is prolonged into a
tapered rod, which dips into the liquid, and which, on removal, carries
a convenient quantity of liquid to drop on to the water surface.
Accessories such as glass rods, plates, tubing of various diameters,
thin copper wire, and an aluminium plate for the spheroidal state, can
be obtained from any dealer in apparatus; and the same applies to
clamp-stands for holding funnels, etc.
*Water.*—Ordinary tap-water suffices for all the experiments described,
and for work with films and globules is superior to distilled water,
which often possesses a surface so greasy as to retard or even entirely
prevent the desired result. All experiments conducted on the surface of
water should be performed in a clean vessel which has been rinsed out
several times with tap-water before filling.
*Lantern Projection.*—In demonstrating the phenomena to an audience, a
lantern may be used to advantage. It should be of the type now
procurable, which is arranged for the projection of experiments
conducted either in a horizontal or vertical position, by the use of the
electric arc or other suitable source of light. Flat-sided vessels are
essential for the successful projection of views of objects in a
vertical position; and for showing globules, etc., on the surface of
water, better definition is secured if cells with plate-glass bottoms
are used instead of vessels made of a single piece of glass. The author
has generally used a “Kershaw” lantern for lecture purposes, with quite
satisfactory results. This lantern may also be adapted for projecting
solid objects by reflected light—as, for example, a hot plate on which a
spheroid of water is floating (Fig. 34). The contrivance known as the
“Mirrorscope” may also be used, with slight modification, for producing
a magnified image of solid objects on the screen.
INDEX
A PAGE
Aceto-acetic ether, automatic drops of, . . . 37
” columns of, . . . . . . 44
Aniline, automatic drops of, . . . . 33
” equi-density temperature of, . . . 17
” films or skins, . . . . . 19
” globules, movements of, . . . 63
Anisol, . . . . . . . . 19
Area of stretched surfaces, . . . . . 7
B
Boundary surface of two liquids, . . . . 6
Butyl benzoate, . . . . . . . 19
C
Camphor, movements of on the surface of water, . 63
Centrifugoscope, . . . . . . 14
Chloroform, composite drops of, . . . . 48
D
Dimethyl-aniline, skin of on water, . . . 68
“Diving” drop, . . . . . . . 22
Droplet, formation of, . . . . 28, 34
Drops of liquid, apparatus for, . . . . 27
” ” automatic, . . . 33, 37
” ” combined with vapour, . . 47
” ” communicating, . . . 44
” ” condensation of from vapour, . 49
” ” floating on hot surface, . . 57
” ” formation of, . . 24, 33, 37
” ” overheated, . . . . 55
” ” shapes of, . . 10, 29, 30, 31
E
Elastic skin of liquids, . . . . . 5
Equi-density temperatures, . . . 16, 17, 19
Ethyl benzoate, columns of, . . . . 42
F
Fogs, . . . . . . . . 52
G
Globule, forces acting on, . . . . . 61
” the “devouring”, . . . . . 74
Globules, attraction between, . . . . 73
” expanding, . . . . . . 72
” production from films, . . . . 69
” surface movements on water, . . 63, 66
Golden syrup, experiment with, . . . . 8
I
Interfacial tension, . . . . . 22, 61
Ions, condensation on, . . . . . 53
J
Jets of liquid, . . . . . . . 38
L
Liquid clouds in liquid media, . . . . 54
” columns, . . . . . . 40
” jets, . . . . . . 38
Liquids, general properties of, . . . . 2
” origin of, . . . . . . 1
” properties of surface of, . . . 3
M
Minimum thermometer, . . . . . . 6
Mists, . . . . . . . . 49
Mono-brom-benzene, . . . . . . 48
N
Network formed from film, . . . . . 70
Nitrobenzene, drops of, . . . . 29, 37
” films, . . . . . . 69
O
Orthotoluidine columns, . . . . . 42
” drops, . . . . . . 27
” equi-density temperature of, . . . 16
” globules, movements of, . . . . 66
” jets, . . . . . . . 39
” spheres, . . . . . 11, 14
P
Petroleum, boundary surface with water, . . . 6
Plateau’s spherule, . . . . . . 25
Q
Quinoline, formation of globules of, . . . 69
” rings of, . . . . . . 71
R
Raindrops, . . . . . . . 54
S
Shape of detached masses of liquid, . . . 8
Silver floating on water, . . . . . 4
Solubility, movements due to, . . . . 63
Spheres of liquids, effect of temperature on, . 15
” ” ” production of, . . 10
Spheroidal state of liquids, . . . . 59
Spreading of oil on water, . . . . . 60
Surface skin of water, properties of, . . . 3
” tension, definition of, . . . . 21
” ” phenomena, analogies to life, . 75
” ” value for various liquids, . . 22
W
Water, column of, . . . . . . 40
” surface tension of, . . . . 21
” beetle, . . . . . . 4
X
Xylidine 1-3-4, movements of globules of, . . 66
(_Pr 1266_)
————
Butler & Tanner Frome and London
Transcription note
The following minor typographical flaws have been corrected:
- Fig. 7: _missing period at the end of the caption_
- “feeler,”’ _unnecessary additional closing quote_
- *Index:* Drops of liquid, shapes of, 10, 29, 30, 31 _missing commas_
- *Index:* Mono-brom-benzene _added hyphen to conform with reference
in text_
Footnotes have been renumbered progressively throughout the book.
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