484 il/r Hughes, On the Mobilities of the Ions 



lost negative electricity in accordance with the ordinary photo- 

 electric effect at the surfaces of solids or liquids. This view was 

 confirmed by the experiments of Bloch*, who repeated Lenard's 

 experiments in such a way that the air could be rendered free 

 from dust. With ordiuary air he obtained Lenard's results, with 

 dust-free air he was unable to detect any conductivity in the air 

 due to the action of the ultra-violet light. 



Professor Sir J. J. Thomson f has carried out some experiments 

 in which he found that the ultra-violet light from the glow in the 

 lime cathode discharge produced a small conductivity in a stream 

 of air, increasing the current to about six times the natural leak. 



In a letter to Nature, Palmer| stated that he found that air 

 was rendered conducting by ultra-violet light of wave length 

 shorter than about \ 18.50 (Angstrom units). No details are 

 given. 



Stark § has investigated the effect of ultra-violet light on the 

 conductivity of gases and has obtained positive results with 

 certain complex organic vapours — anthracene, diphenylmethane, 

 diphenylamine and a naphthylamine. Stark concludes from his 

 experiments on air that Lenard's results do not indicate a true 

 ionisation of the air by ultra-violet light. He points out in dis- 

 cussing researches on the ionisation of air by ultra-violet light, 

 that, before one can say that a conductivity in air is really due 

 to ionisation of air molecules, the absence of dust particles and 

 droplets must be proved and that positive as well as negative ions 

 must be shown to exist in the air. 



I was unable to obtain any definite indication of ionisation of 

 air due to ultra-violet light which had passed through thin quartz 

 plates. The sources of light used were the mercury arc, the glow 

 in a lime cathode discharge and the discharge in hydrogen. 



There was reason to believe that, if the ionisation of air 

 depended upon the wave length of the light employed, the 

 shorter the wave length, ihe more ionisation one would expect. 



Professor Lyman has carried out several important researches 

 upon ultra-violet light especially in the region of very short wave 

 lengths. He found that the ultra-violet spectrum of hydrogen 

 was full of closely packed lines extending down to X;1030||. 

 Another research IT of his was carried out to find whether there 

 was any substance more transparent to ultra-violet light than 

 quartz. Many substances were tried, but only one — fiuorite — 

 was found to transmit ultra-violet light of shorter wave length 



* Bloch, Le Radium, p. 240, 1908. 



t Sir J. J. Thomson, Proc. Camb. Phil. Soc. xiv. p. 417, 1907. 

 J Palmer, Nature, 77, p. 582, 1908. 

 . § Stark, Phijs. Zeits., Sept. 15, 1909. 

 II Lyman, Astrophysical Journal, xxiii. p. 181, 1906. 

 ir Ibid., XXV. p. 45, 1907. 



