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XIII. The Origin of the Small Bubbles of Froth. By J. A. 

 Pollock, D.Sc, Professor of Physics in the University of 

 Sydney * . 



[Plates VI.-VIIL] 



I. Introductory. 



WHEN a glass jar containing air and uncontaminated 

 water is vigorously shaken, it will be found, imme- 

 diately 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 conr= 

 centrated solutions of many organic substances, 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 uncon- 

 taminated liquid is used, are produced in great numbers. 

 Some of the bubbles are so minute that they remain tempo- 

 rarily suspended in the liquid, while the larger ones, which 

 appear almost immediately on the surface, constitute the 

 most lasting part of the froth which is here such a charac- 

 teristic outcome of the agitation. So numerous are the 

 minute bubble-, in many cases that they give a milky appear- 

 ance 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 t mentions Marangoni J as the first, in 

 1871, to state in this connexion the necessary condition for 

 stability. Subsequently, in 1878, the matter was inde- 

 pendently discussed by Willard Gibbs §, who devotes, to the 

 consideration of the general problem, a section of his famous 

 Essay on the Equilibrium of Heterogeneous Substances. 



To Lord Rayleigh we are not only indebted for an account 

 of the frothing of liquids, contained in a lecture on " Foam/' 

 given at the Royal Institution in 1890 1|, but we also owe to 

 him, as a result of his experimental researches in connexion 



* Communicated by the Author. From a paper read before the 

 Royal Society of New South Wales. 



t Lord Rayleigh, Proc. Roy. Soc. vol. xlvii., 1890 ; Scientific Papers, 

 vol. iii. p. 341. 



J Marangoni, Nuovo Cimento, vols, v., vi., 1871-2. 



§ Gibbs, Trans. Conn. Acad. vol. iii. p. 467 (1878) ; Scientific Papers, 

 vol. i. p. 300. 



|| Lord Rayleigh, Proc. Roy. Inst. vol. xiii. 1890; Scientific Papers, 

 vol. iii. p. 351. 



