Origin oj the Small Bubbles of Froth. 191 



smaller bubbles which are only created with a contamin- 

 ated liquid, and constitute the characteristic feature of the 

 lasting froth which is produced, in this case, by vigorous 

 agitation. The necessity for the distinction may be re- 

 cognized by comparing the companion photographs in Plates 

 VI. and VII. Figure 2 of Plate VI. shows the breaking 

 up of a large bubble of carbonic acid forced into uncon- 

 taminated water, while figure 3 exhibits the effect when a 

 0*1 per cent, aqueous solution of acetic acid is used, the 

 conditions being, otherwise, the same in the two cases. 

 Figures 4 and 5, Plate VII., are a similar pair of photo- 

 graphs with the pressure of the gas rather greater than in 

 the former instance. 



The photographs show that the small bubbles of the froth 

 of the acetic acid solution are the results of the disruption 

 of the lower surface of the main bubble formed by the in- 

 rushing gas, the disintegration of the surface being extra- 

 ordinarily great compared with the slight breaking up of the 

 bubble, under similar conditions, in water. 



It is interesting to find, from Whatmough's * measures 

 of the surface tension of acetic acid solutions, that such a 

 profound change, in the effect of the agitation of the water, 

 is brought about by an alteration which only diminishes the 

 surface tension, under isothermal conditions, by about 1*5 

 per cent. This effect is so small that, in any attempt to 

 frame an explanation of the phenomenon, other factors, 

 besides the mere change in the surface tension, must be 

 taken into consideration. 



For a solution to give a lasting froth the surface concen- 

 tration must be greater than that of the mass of the liquid . 

 From Lord Rayleigh's work it is known that on the production 

 of a new surface in a solution capable of frothing, such as the 

 surface of the main bubble in the present instance, the excess 

 of concentration in the surface stratum is not instantly 

 established, but requires time for its development. The extra 

 concentration is produced as the result of molecular move- 

 ment, and it is not to be expected that, while in the stage of 

 growth, it will have, at any moment, the same value at all 

 points of the newly formed surface. It must actually appear 

 in the surface layer, in the first instance, in spots or patches ; 

 and wherever such a patch of extra concentration is formed 

 the surface tension becomes, as a necessary consequence, less 

 than that of the surrounding parts. The conditions here are 

 not isothermal ones, and the lowering of the tension may 

 be greater than that mentioned above. The newly formed 

 * Whatmough, Zeit.fiir Phys. Chem. vol. xxxix. p. 166 (1902). 



