664 Messrs, Narayan and Subrahmanyam on Surface 



of matter whose inherent capillary force is less than that of 

 the mass. In the case of soap, the formation of the pellicle 

 is attributed to the action of atmospheric carbon dioxide 

 liberating the fatty acid from its combination with alkali. 

 This pellicle, Rayleigh remarks, is more akin to a solid than 

 to a liquid. 



Since the formation of a coating would be a matter of 

 time, Rayleigh confines himself to this particular aspect 

 of the question, viz., how the surface tension of a soap 

 solution (oleate of soda) changes with the freshness of the 

 surface formed. From observations upon a jet of liquid 

 issuing from an elliptical aperture, he concludes that the 

 addition of a little oleate of soda does not change the surface 

 tension if the surface be fresh (Proc. Ro} r . Soc. xlviii. pp. 127- 

 140, 1890), proving thereby beyond all doubt that the change 

 in the tension is due to external agency. The observations of 

 Dupre ( Theorie Mecanique de la Chaleur, Paris, 1869, p. 377), 

 and the theory of Prof. W. Gribbs (Connecticut Acad. Trans, 

 vol. iii. pt. 2, 1877-78) lend strong colour to Rayleigh's 

 conclusions. 



A strong argument in favour of Marangani's theory is 

 afforded by his observations (Pogg. Ann. vol. cxliii. p. 342, 

 1871) that within very wide limits the superficial tension of 

 soap solutions as determined by capillary tubes is almost 

 independent of strength. In a recent issue of the Journal 

 of Physical Chemistry, vol. xxiv. pp. 617-6*29 (1920)> in the 

 course of a long article on the surface tension of soap 

 solutions, and their emulsifying power, Messrs. M. G. White 

 and J. W. Marden described a series of experiments with the 

 capillary tube, with the result that the surface tension of 

 soap solutions decreases with increase of concentrations, 

 from 49 to 20 and 40 to 21 dynes per centimetre, and that 

 the emulsifying power is closely connected with the surface 

 tension. 



As th^re is wide discrepancy between the results obtained 

 by Lord Rayleigh and others on the one hand and the above 

 authors on the other, the present investigation is taken up 

 with a view to test the accuracy of the results, in which a 

 long series of determinations of the surface tension of soap 

 solutions at different concentrations is made by different 

 methods — strictest possible precautions being observed in 

 maintaining the temperature uniform, and in measuring the 

 various quantities. 



Taking advantage of the bubble-forming property of a 

 soap solution, the authors have thought it desirable U 



