12 Mr. E. A. Houstoun on the Effect of a 



which, in going through, forms the osmotic current *. On 

 this point the work of Nernst t and Flusin J may he referred 

 to. During the last year the writer has been working on 

 the osmotic pressure of ethyl-alcohol solutions with different 

 membranes. One experiment may be mentioned here (it is 

 not put forward as original). Pure alcohol and pure water 

 were separated by a gutta-percha tissue, the water being 

 within the cell. A large pressure was set up, showing an 

 inflow from the alcohol to the water. With a copper- 

 ferrocyanide membrane the direction is reversed. The 

 cause of the current is the same in each case, namely, the 

 mutual potential energy of solution of the liquids ; the 

 direction of the current is conditioned by the membrane. 



The experimental work of this paper was carried out at 

 the Cavendish Laboratory. I wish to take this opportunity 

 of thanking Prof. J. J. Thomson for his kindness during the 

 progress of the work. 



II- The Effect of a Surface-Film in Total Reflexion. By 

 Kobert A. Houstoun, M.A., B.Sc, Glasgow University 

 1851 Exhibition Scholar §||. 



IT is well known that Fresnel's Laws for determining the 

 phase and amplitude of a reflected or refracted light- wave 

 from those of the incident wave have not been found to give 

 the exact experimental result, when investigated with the 

 most accurate experimental means. There is a difference 

 between the calculated and observed results, greater than the 

 experimental error. It is assumed in deriving Fresnel's laws, 

 that at the surface where reflexion takes place the index of 

 refraction changes discontinuously in going from the one 

 medium to the other. The deviation from Fresnel's laws has 

 been satisfactorily explained on the assumption that there is 

 a " surface-film" between the two media. This surface-film 

 can be regarded as a transition-layer, in which the index of 

 refraction changes continuously from the value it has in the 



* Since this paper was written, a similar view has been arrived at by 

 Prof. Kahlenburg' (Phil. Mag. Feb. 1905, p. 228) on similar lines of 

 work. 



t Zeit. Phys. Chem. vi. p. 37 (1890). 



% Comptes Xenitus, exxxi. p. 1308 (Dec. 31, 1900). 



§ Communicated by Professor A. Gray, F.E.S. 



|| " Wirkuuo' einer Oberflachenschicht bei Total-reflexion,'' E. A. 

 Houstoun, Gottinger Nachrichten, 1903, p. 352. li Ueber die Wirkimg 

 einer Oberflachenschicht bei Total-reflexion." E. A. Houstoun, Phys. 

 Zeitschrift, vi. p. 208. 



