370 Mr. G. W. White on the 



The equation would then take the form 



(H + /(I))(}-f>K'T. 



Written thus it recalls the equation of van der Waals for 

 fluids, field strength and magnetic intensity here playing 

 the part of pressure and density. 



(12) In conclusion, it is of interest to compare the 

 equation obtained in paragraph (5) and the curves giving 

 the relation of I and H to the strength of the alternating- 

 current running through the material with the equation and 

 curves giving the relation of I and H to temperature dis- 

 cussed in paragraphs (9) and (10). Above a critical value 

 of the alternating current the intensity of magnetization 

 tends to become inversely proportional to the strength of the 

 current, subject to a limit to the intensity. Below the 

 critical value of the current the curves of intensity have a 

 part less inclined to the horizontal, which suggests that there 

 are other magnetic forces in action within the material 

 besides that immediately due to the external field. 



More generally since the effects of temperature on 

 magnetism are like those due to an alternating current 

 setting up a magnetic agitation, the probability that the 

 former are kinetic effects of thermal agitation is considerably 

 enhanced. 



I wish to thank Mr. R. P. Shaw for much gratuitous 

 help in preliminary work and in repeating some of the 

 experimental results in this paper. 



XXXVIII. The Properties of Selenium Blocks. 

 By G. W. White, B.Sc. (University of Bristol) *. 



IT is well known that the substance selenium in a certain 

 crystalline form possesses the peculiar property of 

 changing its electrical resistance on exposure to light. Any 

 device consisting of two electrodes bridged over by the 

 selenium is commonly, but not appropriately, called a 

 " Selenium Cell." Since in this investigation the resistance 

 change of selenium was observed, and not the production of 

 an E.M.F. on illumination, the term " Selenium Bridge " is 

 preferred and will be used throughout. 



* Communicated by Dr. A. M. Tvndall. 



