282 



Lord Rayleigh. On the Tension of 



[Mar. 6, 



means of variations in this coating, Marangoni explains the indisput- 

 able fact that in vertical soap films the effective tension is different 

 at various levels. "Were the tension rigorously constant, as it is 

 sometimes inadvertently stated to be, gravity would inevitably assert 

 itself, and the central parts would fall 16 feet in the first second of 

 time. By a self-acting adjustment the coating will everywhere 

 assume such thickness as to afford the necessary tension, and thus 

 any part of the film, considered without distinction of its various 

 layers, is in equilibrium. There is nothing, however, to prevent the 

 interior layers of a moderately thick film from draining down. But 

 this motion, taking place as it were between two fixed walls, is com- 

 paratively slow, being much impeded by ordinary fluid viscosity. 



In the case of soap, the formation of the pellicle is attributed by 

 Marangoni to the action of atmospheric carbonic acid, liberating the 

 fatty acid from its combination with alkali. On the other hand, 

 Sondhauss* found that the properties of the liquid, and the films 

 themselves, are better conserved when the atmosphere is excluded by 

 hydrogen ; and I have myself observed a rapid deterioration of very 

 dilute solutions of oleate of soda when exposed to the air. In this 

 case a remedy may be found in the addition of caustic potash. It is 

 to be observed, moreover, that, as has long been known, the capillary 

 forces are themselves quite capable of overcoming weak chemical 

 affinities, and will operate in the direction required. 



A strong argument in favour of Marangoni's theory is afforded by 

 his observation,f that within very wide limits the superficial tension 

 of soap solutions, as determined by capillary tubes, is almost 

 independent of the strength. My purpose in this note is to put 

 forward some new facts tending strongly to the same conclusion. 



It occurred to me that, if the low tension of soap solutions as com- 

 pared with pure water was due to a coating, the formation of this 

 coating would be a matter of time, and that a test might be found in 

 the examination of the properties of the liquid surface immediately 

 after its formation. The experimental problem here suggested may 

 seem difficult or impossible; but it was, in fact, solved some years 

 ago in the course of researches upon the Capillary Phenomena of Jets.J 

 A jet of liquid issuing under moderate pressure from an elongated, 

 e.g., elliptical, aperture perforated in a thin plate, assumes a chain-like 

 appearance, the complete period, X,, corresponding to two links of 

 the chain, being the distance travelled over by a given part of the 

 liquid in the time occupied by a complete transverse vibration of the 

 column about its cylindrical configuration of equilibrium. Since the 



* ' Poggendorff, Annalen,' Erganzungsband 8, 1878, p. 266. 

 t ' Poggendorff, Annalen,' vol. 143, 1871, p. 342. The original pamphlet dates 

 from 1865. 



% * Eoy. Soc. Proc.,' May 15, 1879. 



