1898.] Electrification by Ultra-violet Light. 403 



which they pass. The question whether there is any volume 

 ionisation of a gas through which ultra-violet light passes was 

 investigated by Henry 1 who tried the vapours of iodine and methyl 

 iodide both of which are very powerful conductors under Rontgen 

 rays but with a negative result. The result of Wilson that a 

 cloud is formed in moist air with strong ultra-violet light renders 

 it possible that there is a slight volume ionisation of the gas 

 through which the light passes, but the effect appears to be too 

 small to be determined by electrical means, and in all later 

 experiments it is assumed that the surface of the negatively 

 charged plate is the seat of the action of the ultra-violet light 

 discharge. 



If we allow ultra-violet light to fall on a negatively electrified 

 surface, e.g. a polished zinc plate, since the body is slowly dis- 

 charged, it seems probable that if the discharge is due to the 

 convection of the charged particles, these charged particles can 

 be blown away by directing a sufficiently rapid blast of air across 

 their path. This has been experimentally shown by Zeleny 2 , who 

 showed that a negatively charged gas can be obtained by blowing 

 past the negatively charged plate when the ultra-violet light was 

 acting. This gas has similar properties to the charged gas 3 

 obtained by the separation of ions in Rontgen ionisation, for they 

 readily give up their charge and refuse to pass through a plug of 

 cotton-wool: 



I had independently observed the same fact, and had also 

 investigated the effects of blowing currents of air by electrodes, 

 especially from the point of view of determining the velocity of 

 the carrier of electricity. 



Before entering on the general results it is necessary to draw 

 attention to the phenomena observed by Blondlot and Bichat 4 . 

 They found that if an insulated zinc plate was acted on by ultra- 

 violet light, when all the conductors in the neighbourhood were 

 connected to earth, the potential to which it could be raised was 

 increased 6 or 7 times by allowing a blast of air to impinge on the 

 plate. They found that this action was independent of dust and 

 moisture. It is easy in this way to raise the potential of a plate 

 of amalgamated zinc to 15 volts in a few minutes, although the 

 potential to which it could be raised under the action of the light 

 alone was less than 2 volts. 



I have also found that the rate of leak of a body charged 

 negatively is much more rapid when a blast of dust-free air is 

 directed against it. A plate of polished zinc, charged to — 8 volts, 



1 Proc. Camb. Phil. Soc. Vol. ix. Pt. vi. 1897. 



2 Phil. Mag. March, 1898. 



3 Eutherford. Phil. Mag. April, 1897. 



4 G. R. xvii. p. 29. 1888. 



