*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._]