720 Dr. P. B. Perkins : Determination of Periods of 



We have, therefore, three distinct classes of foaming 

 solutions, having the following characteristics : — 



(1) Surface concentration thermodynamically reversible : 

 special superficial viscosity or rigidity absent. 



(2) Surface concentration thermodynamically irreversible: 

 special superficial viscosity or rigidity absent. 



(3) Surface concentration thermodynamically irreversible : 

 the surface layers specially viscous or rigid. 



Classes (2) and (3) might each be subdivided on the basis 

 of the chemical reversibility of the process of surface 

 concentration. At present, however little seems to be 

 known on this point*. 



This classification is evidently of fundamental importance, 

 not merely in the study of foaming solutions, but also in relation 

 to the phenomenon of surface concentration in general. Thus 

 Lewis f has measured the " surface excess ' ; at the interface 

 between various solutions and a hydrocarbon oil, and found 

 in many cases that the values obtained were of a different 

 order of mao-nitude from those calculated from Gribbs's 

 thermodynamical theory J. This discrepancy is explained 

 by the fact that the solutions used did not belong to class (1). 

 to which alone Gibbs's theory applies. 



The University, Leeds, 

 Sept, 18, 1913. 



LXXXIII. A Determination of the Periods of Transformation 



of Thorium and Actinium Emanation. By P. B. Perkins,. 



PhD., Honorary Research Fellow, University of 'Manchester '§.. 



T) ECEXT investigations to determine the molecular weight 

 JLIj of the thorium and actinium emanations presuppose 

 an accurate knowledge of the half-value periods of these 

 substances. 



Owing to their rapid decay, especially that of the latter, 

 ordinary electroscope methods of determining the decay 

 constants cannot be used. 



Debierne || first measured the period of decay of actinium 



* See Robertson, " The Proteins/' University of California Publi- 

 cations in Physiologv, vol. iii. No. 16 (1909). 

 + Phil. Mag-, xvii.'p. 466 (1909). 

 % Scientific Papers, vol. i. p. 235. 

 § Communicated bv Sir Ernest Rutherford, F.R.S. 

 || Debierne, C. R. cxxxvi. p. 446 (1903). 



