*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.