Tension of Soap Solutions for Different Concentrations. 663 



length o£ the pendulum will not be more than 25 to 30 cm., 

 so that the whole arrangement is very compact. Further, 

 by the electromagnet arrangement, the oscillations will have 

 a constant amplitude, and the amplitude also, if necessary, 

 can be varied within certain limits. 



It is also possible to produce very small diminution of 

 intensity unlike the case of the sector photometer. It is free 

 from the complications of a motor. 



The slit-widths are adjusted so as to get equal illumination 

 in both the spectra. If I' and I" are the intensities of the 

 incident and absorbed beams, we have 



I" = Te 



where k is extinction coefficient, and d the thickness of the 

 layer, so that 



4rf^log e =log(I'/I")=log(6'/6"), 



where b' and b" are slit-widths. 



Therefore, by knowing V and b' 1 , k the extinction coefficient 

 can be determined. 



Department of Physics, 



Maharajah's College, Vizianagram, 

 10th November, 1921. 



LXXX. Surface Tension of Soap Solutions for Different 

 (Concentrations. By A. L. Narayan, M.A., and Gr. 



SuBRAHMANYAM, B.A* 



[Plate XV.] 



THE question of surface tension of soap solutions has from 

 time to time attracted the attention of several scientists, 

 of whom the late Lord Rayleigh was one, who in a series of 

 investigations drew several conclusions of abiding interest. 

 In his paper, u On the tension of recently formed liquid 

 surfaces " (Proc. Hoy. Soc. xlvii. pp. 281-287, 1890), Lord 

 Rayleigh, following Marangani, attributed the capabilit}*- of 

 extension of a soap film into large and tolerably durable 

 lamina? to the " superficial viscosity " due to the presence 

 upon the body of the liquid of a coating or pellicle composed 



* Communicated bv the Authors. 



