204 J. A. POLLOCK. 



THE ORIGIN OF THE SMALL BUBBLES OF FROTH. 



By J. A. Pollock, d. sc, 



Professor of Physics in the University of Sydney. 



With Plates VIII, IX, X. 



[Read brfore the Royal Society of N. S. Wales, August 2, 2911.] 



I. Introductory.— When a glass jar containing air and 

 uncontaminated water is vigorously shaken, it will be found, 

 immediately after the agitation, that air-bubbles have been 

 formed in the liquid of comparatively large dimensions; 

 these bubbles rise rapidly to the surface, but burst so 

 quickly that there is only a momentary appearance of froth. 

 With slightly concentrated solutions of many organic sub- 

 stances, or with water contaminated with a drop or two of 

 insoluble oil, the result is strikingly different. In these 

 cases small bubbles of various sizes, which are entirely 

 absent when an uncontaminated liquid is used, are produced 

 in great numbers. Some of the bubbles are so minute that 

 they remain temporarily suspended in the liquid, while the 

 larger ones, which appear almost immediately on the sur- 

 face, constitute the most lasting part of the froth which 

 is here such a characteristic outcome of the agitation. 

 So numerous are the minute bubbles in many cases that 

 they give a milky appearance to the liquid, thus forming 

 with it a mixture which may conveniently be called an air, 

 or gas, emulsion. 



In referring to the question of the durability of liquid 

 films, Lord Rayleigh 1 mentions Marangoni 2 as the first, in 

 1871, to state in this connection the necessary condition 



' Lord Rayleigh, Proc. Eoy. Soc, Vol. xlvii, 1890; Scientific Papers, 

 Vol. in, p. 341. 



2 Marangoni, Nuovo Cimento, Vols, v, vi, 1871-2. 



