﻿On the Virlal of a Mixture of Ions. 551 



fo drive the electrons away from the surface, is more in 

 harmony with the present experiments. According to this 

 view a larger potential difference is necessary to drive 

 electrons away from; a film for incident light than for 

 emergent light, thus showing that the direction of the light 

 has something to do with the emission of electrons. 



Approximate values of the actual thicknesses of the films 

 used can be calculated in the following way : — 



Drude's values * for the absorption coefficient and re- 

 flexion coefficient for platinum for sodium light is 45,000 mm -1 

 and -70 respectively. It is doubtful whether the same 

 values hold for thin films, but an approximation can be made 

 to the true state of affairs by assuming these values for the 

 whole spectrum and for thin films. The thickest film used 

 (55 minutes) let through about 20 per cent, of the light 

 which fell on it. This was measured photometrically and 

 also estimated from the spectra. 30 per cent, of the light 

 falling on the film enters it, and thus 10 per cent, is absorbed. 

 This means that a film of 55 minutes deposit absorbs ^ of 

 the light which enters it. The intensity of light falls off 

 according to an exponential law. On plotting the curve 

 y — e -^,^ox ft j s f oun j that an absorption of 33*3 per cent, 

 corresponds to a thickness of 10~ 6 cm. 



The thicknesses of the other films can be readily calcu- 

 lated once the thickness of one has been determined. 



I have great pleasure in recording my best thanks to 

 Prof. Hicks and Dr. S. R. Milner for the interest they have 

 taken in this work, and for much valuable advice in con- 

 nexion with it. 



University of Sheffield, 

 Dec. 6, 1911. 



XLIX. The V trial of a Mixture of Ions. By 8. R. Milner, 

 D.Sc, Lecturer in Physics, The University, Sheffield]. 



THE problem considered in this paper is the determina- 

 tion of the average virial of a mixture of N -f- 

 and N — ions contained in a volume V, on the assumption 

 that every + ion repels every other + ion, and attracts 

 every — ion, with a force ^ 2 /r 2 , where r is the distance 

 between the pair of ions considered, and q the ionic charge. 

 The following statement defines both the average virial and 

 also the strict theoretical method of calculating it : — For a 



* Wied. Ann. 1890, Bd. xxxix. p. 481. 

 f Communicated by the Author. 



