On the Law of Density-Numbers. 191 



that the combination of sulphur with silver is assisted by 

 radiation. 



But it is not perhaps necessary to assume that the effective 

 action of light is confined entirely to the surface of one of the 

 electrodes. If, as is commonly believed, electrolytic conduc- 

 tion involves a series of decompositions and recompositions 

 throughout the electrolyte, any cause which assists either the 

 separation or recomposition (or both) of the components of 

 the electrolyte might be expected to increase its conduc- 

 tivity ; and it seems reasonable to suppose that the same 

 influence which would assist the union of two substances when 

 they have a tendency to unite would also be favourable to 

 their separation when they have a tendency to separate. It 

 is not impossible, therefore, that radiation, acting upon the 

 surface of a thin layer of sulphide of silver through which an 

 electric current is passing, might, by facilitating the mole- 

 cular rearrangement of the atoms of sulphur and silver, exert 

 a material influence upon the conductivity of the sulphide*. 



So far as regards the explanation of the effect of light upon 

 the resistance of selenium, I am aware that this paper contains 

 little more than speculative suggestions, which are at present 

 almost entirely unsupported by experimental evidence f. It 

 is, however, noteworthy that these speculations led to the con- 

 struction of a cell which, without containing a particle of 

 selenium, behaved almost exactly as if it were composed of 

 that substance. How far this may be considered to prove 

 anything with regard to selenium I do not know ; but in any 

 case the discovery of another substance possessing the same 

 remarkable property seems in itself to be a matter of some 

 interest. 



XXIII. On a New Law, analogous to those known under the 

 names Law of Avogadro and Law of Dulong and Petit. By 

 J. A. GeoshansJ. 



[Concluded from p. 30.] 



IT may be said that, as yet, the study of the causes which 

 influence the value of x has to be commenced, and that 

 all that pertains to this constant is still uncertain. Still I 



* There are some experimental reasons, into which I am not at present 

 prepared to enter, for believing that the admixture with the sulphide of a 

 certain amount of free sulphur is necessary for the development of sensi- 

 tiveness to radiation. 



t It is especially desirable to ascertain experimentally whether the com- 

 bination of selenium with the metals used as electrodes in selenium cells 

 is assisted by light. 



% Communicated bv the Author. Translated bv W. W. J. Nicol, M.A., 

 D.Sc. 



