96 Lord Bayleigh [Marcli 28, 



action brought into play. The part of the surface of water immedi- 

 ately in contact with the ring no doubt goes round ; but the movement 

 spreads to a very little distance. The same would be observed if we 

 added soap. But if I add some saponine, we shall find a different 

 result, and that the behaviour of the dust in the interior of the ring 

 is materially altered. The saponine has stiffened the surface, so that 

 the ring turns with more difficulty ; and when it turns, it carries round 

 the whole interior with it. The surface has now got a stiffness from 

 which it was free before ; but the point upon which I wish to fix 

 your attention is that the surface of pure water does not behave in 

 the same way. If, however, we substitute for the simple hoop another 

 provided with a material diameter (Fig. 6), lying also in the surface 

 of the water, then we shall find, as was found by Plateau in his 

 experiment, that the water is carried round. In this case, it is no 

 longer possible for the surface to be left behind, as it was with the 

 simple hoop, unless it is willing to undergo local expansions and 

 contractions of area. The difierence of behaviour proves that what 

 a water surface resists is not shearing, but expansions and contractions ; 

 in fact, it behaves just as a contaminated surface should do. On this 

 supposition, it is easy to explain the efiects observed by Plateau ; but 

 the question at once arises, can we believe that all water surfaces 

 hitherto experimented upon are sensibly contaminated ? and if yes, 

 is there any means by which the contamination may be removed ? 

 I cannot in the time at my disposal discuss this question fully, but I 

 may say that I have succeeded in purifying the surface of the water 

 in Plateau's experiment, until it behaved like alcohol. It is therefore 

 certain that Plateau's superficial viscosity is due to contamination, as 

 was conjectured by Marangoni. 



I must now return to the subject of foam, from which I may seem 

 to have digressed, though I have not really done so. "Why does 

 surface contamination enable a film to exist with greater permanence 

 than it otherwise could ? Imagine a vertical soap film. Could the 

 film continue to exist if the tension were equal at all its parts ? It 

 is evident that the film could not exist for more than a moment; 

 for the interior part, like the others, is acted on by gravity, and, 

 if no other forces are acting, it will fall 16 feet in a second. If the 

 tension above be the same as below, nothing can prevent the fall. 

 But observation proves that the central parts do not fall, and thus 

 that the tension is not uniform, but greater in the upper parts than in 

 the lower. A film composed of pure liquid can have but a very brief 

 life. But if it is contaminated, there is then a possibility of a 

 different tension at the top and at the bottom, because the tension 

 depends on the degree of contamination. Supposing that at the first 

 moment the film were uniformly contaminated, then the central parts 

 would begin to drop. The first effect would be to concentrate the 

 contamination on the parts underneath and diminish it above. The 

 result of that would be an increase of tension on the upper parts. So 

 the effect would be to call a force into play tending to check the 



