402 Mr Rutherford, The Discharge of [Feb. 21, 



the action of a magnetic field on the discharge at low pressures. 

 Stole tow 1 investigated in detail the relation between the current 

 and electromotive force for the discharge at low voltages and at 

 different pressures. 



Most of these papers have dealt with the general character of 

 the discharge, but the subject of the nature of the conduction 

 and of the carrier that discharges the electrification has not been 

 specially attacked. In a very interesting paper Lenard and Wolff 2 

 investigated the effect of a surface, on which ultra-violet light fell, 

 on the condensation of a steam jet in the neighbourhood, and 

 their results led them to the conclusion that many bodies were 

 disintegrated under the action of ultra-violet light and that the 

 particles torn off became nuclei for the condensation of the steam 

 jet. In the light of more recent experiments these results are 

 however capable of other interpretations. 



R. v. Helmholtz 3 has shown that a steam jet is acted on when 

 chemical action is going on in its neighbourhood. Richarz 4 has 

 shown that Rontgen rays produce condensation in a steam jet, 

 and Wilson 5 has recently observed that ions produced under the 

 action of Uranium and Rontgen radiation become under certain 

 conditions nuclei for the condensation of water around them. He 

 has also demonstrated the important fact, which appears to have 

 been overlooked, that ultra-violet light produces clouds in ordinary 

 moist air quite independent of any solid body in its neighbourhood. 

 The presence of this effect must have complicated the effects of 

 Lenard and Wolff, and the more general results on the properties 

 of ions in producing condensation seem to show that possibly their 

 results may be ascribed to the presence of free gaseous ions rather 

 than disintegrated particles of metal. 



It is the object of this communication to give results of 

 investigations on the nature of the carriers of the negative 

 electrification produced under the action of ultra-violet light, and 

 to show that probably the greater part of the electrification is 

 carried by gaseous ions produced at the surface of the negatively 

 electrified plate. 



In order to obtain a discharge with ultra-violet light, the 

 light must fall on a negatively electrified surface. There is no 

 discharge produced by allowing the light to fall between two 

 plates without impinging on either. In this respect the action 

 of ultra-violet light is very different to Rontgen and Uranium 

 radiation, which produce a volume ionisation of the gas through 



1 Journal de Buss. Phys. xxi. 1889. Journal de Physique, ix. p. 468. 1890. 



2 Wied. Annal. xxxvu. p. 443. 1888. 



3 Wied. Annal. xxxn. p. 1. 1887. xl. p. 161. 1890. 



4 Wied. Annal. lix. p. 592. 1896. 



5 Gamb. Philos. Soc. Vol. ix. Pt. vn. 1897. 



