LIGHT AND ELECTRIFICATION. 419 



render individual atoms detachable under stresses approach- 

 ing so near to the average limit, and this would be one way 

 of representing disruptive discharge. 



Against this, however, must be set the fact that the dis- 

 ruptive limit depends greatly on the atmospheric condi- 

 tions, on the pressure and nature of the gas in contact with 

 the metal ; therefore it would appear that even for disrup- 

 tive discharge we must look to an interaction between the 

 molecule of the metal and that of the medium in contact 

 with it, rather than to a simple disruption of the metal 

 alone. 



What is certain is that the charge is carried away by 

 particles (atoms or otherwise) which travel along the lines 

 of force to the oppositely electrified surfaces. It may con- 

 ceivably be that the conveyers of charge are the electrons 

 themselves ; in other words, that the negative ends of the 

 lines of force are detached from the charged body under 

 the action of light, and that the line therefore promptly 

 shuts up. It is more probable, however, there are no such 

 detached electrons or atomic charges divorced from matter, 

 but that the negative charge is conveyed by material atoms, 

 whether they be the atoms of the metal or of the sur- 

 rounding gas. To examine the question whether the con- 

 veying atoms belonged to the metal or to the gas, a number 

 of experiments have been made in my laboratory with the 

 object of testing the presence of metallic particles or vapour 

 near an electrified metal rapidly discharging under the 

 action of light. 



The metals most easy to detect in small quantities are 

 in general perhaps silver, iron and sodium. Silver, by its 

 reflecting power when deposited upon glass ; iron, by its 

 magnetic properties ; and sodium, by the light it causes a 

 non-luminous flame to emit. Silver plates, with their clean 

 edges opposed to the surface of plate glass, were oppositely 

 electrified so that any charge given off from the silver edge 

 should be deposited upon the glass as upon the dielectric of 

 a Leyden jar, and were kept thus strongly illuminated by 

 an arc light for hours ; the glass was then examined for 

 transparency. A decided deposit was found near the illu- 



