Part 5

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.

*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.

*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.

*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.

*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.

*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