LIGHT AND ELECTRIFICATION. 421 



belono- to something in the gas, and not to the illuminated 

 body. 



It may be asked whether dust in the air has any part in 

 the action, but, so far as I can find, it has none at low 

 tensions. The discharge rate from the silver surfaces, for 

 instance, was just about as rapid in a dust-free atmosphere 

 as when dust was present. 



The proof that the discharge is effected by molecules of 

 some kind, or at least by something which travels along the 

 lines of electrostatic force was given by Righi. He elec- 

 trified a small metallic cylinder of which only one generating 

 line was free from varnish, and therefore clean enough to dis- 

 charge electricity. This cylinder being negatively electrified 

 in front of an earthed plate, an exploring terminal of an 

 electroscope could ascertain which part of the plate was 

 receiving a charge, as the cylinder was rotated on its axis, 

 a movable slit being arranged in the plate for this purpose ; 

 and it was found to be always near one extremity of a circular 

 arc of which the discharging line constituted the other 

 extremity. He further found that if the illuminated body 

 were free to move it receded like an electric windmill, 

 proving that it had imparted its charge to something 

 possessing appreciable inertia. 



The inertia of the gaseous particles would indeed cause 

 some divergence from the above circular orbits in which 

 the electrical force is urging them, but the force is so great 

 and the mass is so small that the deviation is not noticeable. 

 Moreover, the charged atom has to make its way among a 

 crowd of others by a process very similar to what occurs in 

 electrolysis, so that the path of the electric charge follows 

 almost accurately the line of electric force. In that sense it 

 may be said to represent the motion of an electron or free 

 electric charge, without committing the speaker to the 

 hypothesis that such charges divorced from their usual 

 boundary conditions on matter can really exist. 



If a gaseous atom can receive a charge from an elec- 

 trified surface there is no difficulty in understanding what 

 it does with it, nor how, by such a process, the electrified 

 body gets discharged, but the difficulty is to realise how an 



