216 J. A. POLLOCK. 



of the explanation of the durability of liquid films, further 

 serve to explain the mode of formation of the small bubbles 

 which are created when a solution, capable of frothing, is 

 vigorously agitated. 



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 1 of the bubbles, 

 some of which are reproduced, in natural size, in Plates 

 VIII, IX, X, 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 

 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 

 recognised by comparing the companion photographs in 

 Plates VIII and IX. Figure 2 of Plate VIII shows the 

 breaking up of a large bubble of carbonic acid forced into 

 uncontaminated 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 



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

 Eayleigh, Proc. Koy. Inst., Vol. xm, p. 261, Feb, 1891 ; Scientific Papers, 

 Vol. in, p. 442. 



