ORIGIN OF THE SMALL BUBBLES OF FROTH. 209 



As has been mentioned, the surface tension, in the case 

 of a vertical iilm, must, for stability, continuously diminish 

 from above downwards, the rate of variation depending on 

 the weight of liquid to be supported. The difference, 

 therefore, between the surface tension at the top and at 

 the base of the film, necessary to be established for its 

 equilibrium, will depend, other things being equal, on the 

 height of the film. Taking the relation shown in figure 1 

 as an illustration, it is possible, in the case of a large bubble, 

 that the layer of oil might not be sufficient to allow the 

 necessary difference in the extreme values of the surface 

 tension to be attained, while quite enough to ensure the 

 stability of smaller bubbles. Such a consideration may 

 explain why the larger bubbles of froth are often evanescent 

 while the smaller ones are quite persistent. 



Frothing, however, is not confined to oil-contaminated 

 liquids, but, on the other hand, is strikingly shown in the 

 case of dilute aqueous solutions, for instance, of a great 

 many organic substances. This is well known, the effect 

 being apparent with some solutes even when present in 

 the minute proportion to the water of only one part in a 

 million. 1 



Recently the matter of the frothing of solutions has 

 become of importance in the mining industry in connection 

 with a floatation process for the concentration of ores, and 

 I am indebted to Mr. H. Howard Greenway for a long list 

 of organic substances which have been found to give a 

 suitable froth, for the purpose of this method of concentra- 

 tion, when added in small quantity to water. 



The list, which is, however, not put forward as exhaus- 

 tive, includes organic acids, alcohols, ethereal salts, 



1 See Lord Rayleigh, Lecture on Foam, Proc. Roy. Inst., Vol. xiii, 1890; 

 Scientific Papers, Vol. in, p. 351 ; 

 and Ramsden, Proc. Roy. Soc, Vol. lxxii, p. 156, 1903. 



N— Sept. 6, 1911. 



