88 Lord Bayleigh [Marcli 28, 



a drop of water, no effect ensues ; but if I take a little oil, or better 

 still a drop of saponine, or of soap-water, and allow that to be 

 deposited upon the middle of the surface, we shall see a great 

 difference. The surface suddenly becomes dark, the whole of the 

 dust being swept away to the boundary. That is the result of the 

 spread of the film, due to the presence of the oil. 



How then is it possible that we should get a lens-shaped mass of 

 oil, as we often do, floating upon the surface of water ? Seeing that 

 the general tendency of oil is to spread over the surface of water, 

 why does it not do so in this case? The answer is that it has 

 already sj^read, and that this surface is not really a pure water surface 

 at all, but one contaminated with oil. It is in fact only after such 

 contamination that an equilibrium of this kind is possible. The 

 volume of oil necessary to contaminate the surface of the water is 

 very small, as we shall see presently ; but I want to emphasise the 

 point that, so far as we know, the equilibrium of the three surfaces 

 in contact with one another is not possible under any other conditions. 

 That is a fact not generally recognised. In many books you will 

 find descriptions of three bodies in contact, and a statement of the law 

 of the angles at which they meet ; that the sides of a triangle, 

 drawn parallel to the three intersecting surfaces must be in proportion 

 to the three tensions. No such equilibrium, and no such triangle, is 

 possible if the materials are pure ; when it occurs, it can only be duo 

 to the contamination of one of the surfaces. These very thin films, 

 which spread on water, and, with less freedom, on solids also, are of 

 extreme tenuity; and their existence alongside of the lens, proves 

 that the water prefers the thin film of oil to one of greater thickness. 

 If the oil were spread out thickly, it would tend to gather itself back 

 into drops, leaviug over the surface of the water a film of less thickness 

 than the molecular range. 



One experiment by which we may illustrate some of these effects 

 I owe to my colleague, Professor Dewar. It shows the variation in 

 the surface tension of water, due to the presence on it of small quan- 

 tities of ether. I hold in my hand masses of charcoal, which can be 

 impregnated with ether. The greater part of the surface of the char- 

 coal is covered with paraffin wax, and, in consequence, the ether which 

 has already penetrated the charcoal can only escape from it again on 

 one side. The result is that the water in the rear of this boat of char- 

 coal will be more impregnated with ether than the part in front, so the 

 mass of charcoal will enter into motion, and the motion will extend 

 over a considerable interval of time. As long as the ether remains 

 in sufficient quantity to contaminate the water in the rear, so long is 

 there a tendency to movement of the mass. The water covered with 

 the film of ether has less tension than the pure water in front, and 

 the balance of tensions being upset, the mass is put in motion. If 

 the nature of the case is such that the whole surface surrounding the 

 solid body is contaminated, then there is no tendency to movement, 

 the same balance in fact obtaining as if the water were pure. 



