*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 10 1/2 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 deg. F. or 24 deg. 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 deg. 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 5 3/4 centimetres, or 2
1/4 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 deg. 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 deg. 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 deg. 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 deg. 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:--
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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