190 Prof. J. A. Pollock on the 



with surface forces, the knowledge which affords a basis of 

 fact to explanations advanced from theoretical considerations. 



So far as I am aware, no explanation has yet been given 

 why, with vigorous agitation, the bubbles are so much 

 smaller and more numerous with a slightly concentrated 

 solution than when a pure solvent is used, and it was to the 

 discovery of the origin of these small bubbles that my own 

 observations have been directed. The phenomenon is a 

 striking one, and I feel that an explanation may have 

 occurred to others, but no reference to its publication can be 

 found. 



Recently the matter of the frothing of solutions has become 

 of importance in the mining industry in connexion with a 

 floatation process for the concentration of ores ; and 1 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 concentration, when 

 added in small quantity to w ater. 



The list, which is, however, not put forward as exhaustive, 

 includes organic acids, alcohols, ethereal salts, ketones, alde- 

 hydes, aromatic hydroxy compounds and essential oils, the 

 substances being used, in the practical application, up to an 

 amount which, in proportion to the water, is generally but a 

 very small fraction of one per cent. 



II. Origin of the Small Bubbles. 



The froth is readily produced when gas under pressure 

 is forced into the solution from the orifice of a pipe fixed 

 vertically in the liquid, and for purposes of investigation 

 this method of production is perhaps the most convenient 

 one. With this arrangement, using a glass vessel to 

 contain the solution, it is seen that the small bubbles are 

 produced at the base of the main bubbles which are con- 

 tinuously being formed on the mouth of the pipe by the 

 incoming gas. The inference is that the smaller bubbles 

 are created from the newly formed surface separating the 

 liquid from the mass of gas entering the solution, and 

 instantaneous shadow photographs * of the bubbles, some of 

 which are reproduced, in natural size, in Plates VI., VII., 

 VIII., confirm this view. 



Two classes of bubbles have, however, to be clearly 

 distinguished : the one comprises those formed even when 

 pure water is used, while the other class refers to the much 



* The photographs have been taken by the method described by Lord 

 Rayleigh, Proc Eoy. Inst., vol. xiii.p. 261, i v eb. 1891 ; Scientific Papers, 

 vol. iii. p. 442. 



