218 Royal Society : — 



cing it was found to be little more than half what it was in the 



darkness. 



The following were the general results of the experiments : — 

 Results. — (1) That the resistance of selenium is largely affected 



by exposure to light. 



(2) That this effect is not produced by the actinic rays, but is 

 at a maximum at, or just outside the red rays, at a place nearly 

 coincident with the locus of the maximum of the heat-rays. 



(3) That the effect of varying resistances is certainly not due 

 to any change of temperature in the bar of selenium. 



(4) That the effect produced on exposure to light is sensibly in- 

 stantaneous, but that, on cutting off the light, the return to the 

 normal resistance is not so rapid. 



It would seem that there exists a power in rays nearly coincident 

 with the heat-rays of high intensity, of altering instantaneously 

 and without change of temperature the molecular condition of this 

 particular element. 



May 15. — William Spottiswoode, M.A., Treasurer and Vice- 

 President, in the Chair. 



The following communications were read : — 



"Determination of the Number of Electrostatic Units in the 

 Electromagnetic Unit made in the Physical Laboratory of Glasgow 

 University." By Dugald M'Kichan, M.A. 



The object of this paper is to describe experiments made at 

 intervals from 1870 to 1872 in the Physical Laboratory of Glasgow 

 University to determine the relation between the fundamental units 

 in the two systems of absolute electrical measurement, the electro- 

 magnetic and the electrostatic. A summary is also given of the 

 results of similar observations made by W. E. King in 1867 and 

 1868. 



The two systems of electrical measurement, or the units which 

 they employ, are founded on the fundamental units of time, mass, 

 and space applied to the observed effects of electricity at rest and 

 electricity in motion. The dimensions of quantity in the two sys- 

 tems are such that the ratio of the electromagnetic and the elec- 

 trostatic unit of quantity is expressible as a velocity. 



This velocity, usually known as v, is not only of great im- 

 portance in all combinations of electromagnetic and electrostatic 

 action, but it is also of great scientific importance in the theory 

 of the propagation of electromagnetic disturbances through a di- 

 electric medium. It occupies a very important place in the deve- 

 lopment of the electromagnetic theory of light by Professor Clerk 

 Maxwell, according to whose theory this velocity v is the same as 

 the velocity of light. 



The first experimental determination of v was made by "Weber 

 from a common electrostatic and electromagnetic measure of capa- 

 city. As the result of Weber's experiments, v was found to be 

 310*74 x 10 8 centims. per second. 



Another determination was made by Prof. Clerk Maxwell in 



