ORIGIN OF THE SMALL BUBBLES OF FROTH. 215 



on the nature of the solute, and it may, perhaps, vary with 

 the concentration. 1 



Where the concentration in the surface stratum is greater 

 than in the body of the solution, variations in the value of 

 the surface concentration serve, in connection with solution 

 films, the same purpose as is effected, in the case of bubbles 

 of oil-contaminated liquids, by changes in the thickness of 

 the layer of insoluble oil. Referring again to the question 

 of durability, there is in a newly formed vertical film an 

 initial movement of the solution from above downwards, 

 under the action of gravity, which makes the film wedge- 

 shaped, a fact readily deduced from the arrangement of 

 colours, in a soap film; by this movement the surface 

 concentration becomes, at least temporarily, less at the 

 upper levels than below, and there is thus established that 

 diminishing gradation of values of the surface tension, from 

 above downwards, which is required for equilibrium. 



On the other hand, with films of those solutions, in which 

 the concentration in the surface layer is less than in the 

 interior part of the liquid, the initial movement would 

 cause similar temporary differences in the actual surface 

 concentration, but with the introduction of an increasing 

 gradation of values of the surface tension; films of such 

 solutions must, therefore, be considered as even more 

 unrealizable than those of pure liquids. 



A durable froth can, thus, only occur with those solutions 

 in which the concentration in the surface stratum is greater 

 than that in the body of the liquid, which, in turn, only 

 happens, at least with dilute solutions, if increased con- 

 centration means a diminished surface tension. 



III. Origin of the small bubbles. -The facts which have 

 been stated, in connection with the foregoing presentation 



1 See Milner, loc. cit. 



