128 Lord Rayleigh. [June 5, 



mainly the result of the deformation of the meniscuses developed at 

 the contacts on the two sides with the liquid surface. This view 

 does not appear to me to be sound ; for a deformation of a meniscus 

 due to inertia would not involve any dissipation of energy, nor per- 

 manent resistance to the movement. But the second suggestion of 

 Marangoni is of great importance. 



On various grounds the Italian physicist concludes that "many 

 liquids, and especially those of Plateau's third category, are covered 

 with a superficial pellicle ; and that it is to this pellicle that they owe 

 their great superficial viscosity." After the observations of Dupre* 

 and inyself,t supported as they are by the theory of Professor Willard 

 Gibbs,J the existence of the superficial pellicle cannot be doubted ; 

 and its mode of action is thus explained by Marangoni : " The 

 surface of a liquid, covered by a pellicle, possesses two superficial 

 tensions ; the first, which is the weaker and in constant action, is 

 due to the pellicle ; the second is in the latent state, and comes into 

 operation only when the pellicle is ruptured. Since the latter 

 tension exceeds the former, it follows that any force which tends to 

 rupture the superficial pellicle upon a liquid encounters a resistance 

 which increases with the difference of tensions between the liquid 

 and the pellicle." In Plateau's experiment the advancing edge of the 

 needle tends to concentrate the superficial contamination, and the 

 retreating edge to attenuate it ; the tension in front is thus inferior 

 to the tension behind, and a force is called into operation tending to 

 check the vibration. On a pure surface it is evident that nothing of 

 this sort can occur, unless it be in a very subordinate degree, as the 

 result of difference of temperature. 



There is an important distinction, discussed by Willard Gibbs, 

 according as the contamination, to which is due the lowering of 

 tension, is merely accidentally present upon the surface, or is derived 

 from the body of the liquid under the normal operation of chemical 

 and capillary forces. In the latter case, that, for example, of solu- 

 tions of soap and of camphor, the changes of tension which follow 

 an extension or contraction of the surface may be of very brief 

 duration. After a time, dependent largely upon the amount of con- 

 taminating substance present in the body of the liquid, equilibrium 

 is restored, and the normal tension is recovered. On the other 

 hand, in the case of a surface of water contaminated with a film of 



* ' Theorie Meeanique de la Chaleur,' Paris, 1869, p. 377. 



f " On the Tension of Recently Formed Liquid Surfaces," ' Roy. Soc. Proc.,' 

 vol. 47, 1890, p. 281 (supra). 



I ' Connecticut Acad. Trans.,' vol. 3, Part II, 1877-78. In my former communi- 

 cation I overlooked Prof. Gibbs's very valuable discussion on this subject. 



'Nuovo Cimento,' vol. 5-6, 1871-72, p. 260 (May, 1872). 



