41S SCIENCE PROGRESS. 



Now a silver plate 14 centimetres square, under certain 

 conditions of arc illumination, was found in the writer's 

 laboratory to lose negative electricity at the rate of 30 

 electrostatic units per minute when kept electrified to 80 

 volts in fairly free space. Hence the time that would 

 elapse before the above plate would lose a tenth of a 

 milligramme of its substance is ten million minutes or nearly 

 a century. 



Such a ratio of loss as that could not be detected by a 

 balance, even in the case of silver, which is the substance 

 most suitable for detecting a small electrolytic loss by 

 weight. But now suppose that the discharge is not of so 

 atomic a character, but that little flakes or pieces of the 

 metal are driven off under the electric stress, so that the 

 charge per gramme lost is very much less. 



In that case the disintegration of surface might be 

 perceived, but there are many difficulties in the way of 

 supposing such an action. 



The electric tension even when on the verge of dis- 

 ruption, when the surface is charged to many thousand 

 volts, is by no means comparable to the forces of cohesion. 



And the action of light occurs at so low a tension that 

 it is impossible that its action is a mere bringing down of 

 the limit at which disruptive discharge begins. 



The action of light is much more like a quiet atomic or 

 molecular process than it is like a disruptive discharge from 

 the substance in bulk. 



It may, however, be worth noticing that the electric re- 

 pulsive force experienced by an atom when 011 a surface 

 charged to the disruptive limit is not incomparably less than 

 the average force of cohesion acting on such an atom. The 

 tenacity of a metal may be taken as io 9 cgs. units, or about 

 icr 7 degree, per superficial molecule. The electric force 

 acting on an atom in a potential gradient of 30,000 volts per 

 centimetre, which is the disruptive limit under ordinary 

 atmospheric conditions, is about io -9 degrees — one-hun- 

 dredth of the average cohesive force ; so it would not be 

 unduly speculative to conceive it possible that circumstances 

 connected with heat and other motors should occasionally 



