ORIGIN OF THE SMALL BUBBLES OF FROTH. 217 



cases. Figures 4 and 5, Plate IX, are a similar pair of 

 photographs 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 

 inrushing gas, the disintegration of the surface being extra- 

 ordinarily great compared to the slight breaking up of the 

 bubble, under similar conditions, in water. 



It is interesting to find, from Whatmough's 1 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. 



It has just been shown that if a solution is to give a 

 lasting froth the surface concentration must be greater 

 than that of the mass of the liquid. Prom Lord Rayleigh's 

 work, previously mentioned, it is known that on the pro- 

 duction 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 develop- 

 ment. The extra concentration is produced as the result 

 of molecular movement, 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 



1 Whatmough, Zeit. fur Phya. Chem., Vol. xxxix, p. 166, 1902. 



