LIGHT AND ELECTRIFICATION. 425 



that is the discharge of positive electricity — i.e., the recep- 

 tion of negative by certain substances. It is a phenomenon 

 which undoubtedly occurs as an experimental fact, but if 

 we proceed to look into the cause of it we find its detailed 

 character by no means so obvious Certainly it depends a 

 great deal on the surroundings, and there is reason to be- 

 lieve that if a positively charged body were surrounded by 

 a surface incapable of giving off negative electricity, then 

 the apparent discharge of positive might not occur. 



The question is complicated by the simple facts (a) that 

 we cannot have a charged body without an equal opposite 

 charge on surfaces opposed to it, and (6) that every sur- 

 face reflects and scatters some of the incident light which 

 therefore partly falls upon the oppositely electrified sur- 

 face. Hence when a positively charged body loses its 

 charge, it may be not through a direct action of the light 

 upon itself, but by reason of the action of the reflected 

 and scattered light on the negatively electrified surfaces in 

 its neighbourhood. 



On this hypothesis a surface which appears to lose 

 positive more quickly that negative is one which of itself 

 hardly loses any electricity at all ; it loses negative slowly 

 but it is exposed to surfaces which can emit it more quickly, 

 and hence when it is positively electrified and they are 

 inductively negative, it receives from them a negative 

 charge more rapidly than it was able to give one out. 



A large number of exoeriments have been made in the 



O J. 



writer's laboratory to test this point, mostly by means of 

 regular reflectors so as to avoid scattered light as far as 

 possible ; the details are somewhat technical and trouble- 

 some, and the verydust of the air is apt to scatter a good 

 deal of active light ; but the result is, on the whole, to 

 substantiate the above-mentioned idea, which also possesses 

 the powerful support of Messrs. Elster and Geitel, that the 

 loss of positive electricity under the action of light is an 

 indirect and secondary phenomenon. 



It appears, however, that under X rays both points of 

 electricity are discharged equally, and if these X rays are, 

 as everything now indicates them to be, an extension into 



