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 [pg 67] 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 [pg 68] 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.
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 [pg 69] 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 [pg 70] 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.
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 [pg 71] 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]
Fig. 39.—Network formed from a film of tar-oil on the surface of water.