Mr Vincent, On the Action of Ultra-Violet Light etc. 305 



The Action of Ultra-Violet Light on Moist Air. By J. H. 

 Vincent, D.Sc, B.A., St John's College, Head of the Physics 

 Department, Paddington Technical Institute. . 



[Read 23 November 1903.] 



The experiments to be described in this paper were under- 

 taken in the hope that they might lead to some satisfactory 

 explanation of the genesis of the remarkable cloud formed in 

 moist air by ultra-violet light. Although this hope was not 

 gratified, some observations were made which increase the interest 

 of the phenomenon. 



C. T. R. Wilson 1 showed that when the light from a strong arc 

 was concentrated by means of a quartz lens on to air saturated 

 with moisture, a cloud made its appearance, and a similar result 

 was obtained with oxygen and carbonic acid gas. It was found 

 possible to produce such clouds even when the relative humidity 

 was less than 90 °/ . The cloud was not produced in pure steam 

 nor in hydrogen. The view taken by Wilson was that the 

 droplets owed their origin and growth to the formation of hy- 

 drogen peroxide. 



Lenard 2 subsequently showed that ultra-violet light produced 

 ozone in air or oxygen and that it produced a volume ionisation 

 in some gases. 



The conductivity which Lenard observed is not necessarily 

 connected with the presence of moisture, for I found that it 

 occurred in air which had been standing over strong sulphuric 

 acid for several weeks. For the leaking system I used a gold 

 leaf and brass plate in a metal box provided with quartz windows. 

 The box also had a plane glass window through which the gold 

 leaf could be observed with a telescope. A beam of ultra-violet 

 light from an aluminium spark was sent through the dry air in 

 the box. It was admitted through a small quartz window and 

 passed out through a large one so that no direct light fell on 

 anything but the air in the box and the face of the large quartz 

 window. The gold leaf and brass plate were well removed from 

 the direct beam of light ; the metal inside the box was not 

 cleaned ; it is thus very unlikely that any Hallwachs' effect could 

 be present. The gold leaf lost electricity whether it was charged 

 positively or negatively when the spark was on. The rate of 

 leak was much greater in the former case, as was observed by 



1 G. T. E. Wilson, Phil. Trans. A, Vol. cxcn. 



2 Lenard, Ann. d. Phys. i. 4, and in. 4, 1900. 



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