Soap Bubbles. The outside of a liquid acts just as though it were an elastic skin stretched into a particular shape. If, therefore, we could get rid of the effect of the weight of a drop of liquid, we should see only the effect of this skin. When a very small drop is taken, we are approaching this state of things, and we notice that the drop is very nearly spherical, since the elastic skin pulls it till its surface is the smallest possible for the given quantity of liquid, and the sphere gives this minimum surface. A soap bubble shows this in a beautiful manner, for in this case we have practically isolated the elastic skin itself. That the skin is exerting pressure upon the air inside it can be shown in the following way:- Blow a bubble at the end of a tube or pipe, and then remove the pipe from the mouth; the bubble immediately begins to force the air back again along the stem of the pipe, and out through the open end, while it subsides into a flat film. It is easily shown also that a small bubble exerts a greater pressure on the contained air than a larger one. If the interiors of two bubbles of different sizes be connected by a tube, the small one will grow smaller, while the larger one will increase, owing to the greater pressure of the smaller skin. The pressure, therefore, depends on the curvature of the bubble, and this is true whether the bubble be spherical or of any other shape, only that the value of the curvature is more readily realised in the case of a sphere than in that of a cylinder or other curved surface.
Two bubbles can be made to push each other about, but yet the actual films do not touch; there is a thin layer of air between the two, and this thin layer is present when one bubble is blown inside another, so that the two bubbles do not (if carefully blown) coalesce or burst. When they are merely externally resting against each other, the presence of an article electrified to the slightest degree will cause their union. Hence such a pair of bubbles form a delicate test for small quantities of electricity. The blowing of bubbles inside others, although apparently a very simple matter, is really a difficult feat to accomplish. Newton devoted much thought to the study of soap bubbles. He observed that, as the liquid thins away from the top of the, bubble, coloured rings grow in regular order, spreading outwards till they attain their greatest diameter: then they close in gradually on the under side and vanish at the bottom. The colours pass through the most beautiful tints, eventually becoming dark red; then they increase in lightness to a dirty-white, which again darkens, till at last a black spot appears at the top, and this reaches a diameter of 1/2 or 3/4 of an inch, and then the bubble bursts. As a bubble is blown larger and larger its skin resists stretching, and Lord Kelvin has shown, in his lecture on the size of atoms, that the film could not keep up its tensile strength to the point when its thickness is as little as 1/100000000 of a centimetre; but he says it is scarcely conceivable that there can be any falling off in this tensile strength as long as the film is several molecules in thickness; hence, when the tensile strength fails, i.e. when the bubble bursts, we can assume the film is only a single molecule thick. The thickness of the black spot has been shown by Professors Reinold and Rucker to be only slightly more than 1/1000000 centimetre, and as the film breaks soon after this spot appears, it is probably then something like 1/10000000 centimetre thick. This, therefore, is the order of size of a molecule of water.