306 Mr Vincent, On the Action of 



Lenard. The effect was stopped by placing even the thinnest 

 sheet of mica or gelatine between the spark and the small quartz 

 window, and also by a layer of coal-gas issuing from a fish-tail 

 burner in the same position. 



Thus ionisation is produced in dry dust-free air by ultra-violet 

 light, and the production of the cloud may be due to some other 

 effect in the presence of moisture. 



I next tried to prove the presence of hydrogen peroxide in 

 air which had been subjected to the influence of ultra-violet 

 light. In a previous paper 1 it was shown that hydrogen per- 

 oxide in the presence of ozone would produce a developable picture 

 on an ordinary photographic dry plate. Air was saturated with 

 moisture by passing it over water in a long horizontal glass 

 tube. This method was used so as to avoid any complications 

 that might be introduced by using air that had been bubbled 

 through water. The air was then led into an airtight glass 

 vessel provided with a thin quartz window, close to which the 

 aluminium spark was placed. The air then passed into a brass 

 box which contained the photographic plate. The plate was 

 completely screened from light and a part of its surface was 

 covered with a flat plate of quartz. On development any effect 

 such as was sought for would have been evident by the appearance 

 of a white picture of this bit of quartz on a black ground. No 

 such effect was produced. The same experiment was tried under 

 various conditions as to moisture of the air and length of exposure 

 but without any result. It thus appears that if hydrogen per- 

 oxide is formed in moist air by ultra-violet light it must be 

 in very small quantities. The same air which failed to give 

 this test for hydrogen peroxide turned starch and potassium 

 iodide blue in a few minutes; it also rendered india-rubber 

 photographically active 2 , thus confirming Lenard's observation 

 of the formation of ozone. 



In order to find out if the separate droplets in the cloud 

 formed by ultra-violet light were charged the following form 

 of apparatus was employed after the trial and rejection of some 

 others. 



Two stout circular discs D of brass about 14? cm. in diameter 

 formed the top and bottom of the cloud chamber. These discs 

 were fastened to a ring of glass G 2 cm. high by sealing- 

 wax. This glass had a portion about 3 cm. long cut out of 

 it, which was replaced by a sheet of quartz Q, which served to 

 introduce the ultra-violet light into the chamber. The ultra- 

 violet light was obtained from an aluminium spark S, three or 

 four millimetres long, which was worked by a large induction 



1 Vincent, Chem. Neios, July, 1898. 



2 Vincent, loo. cit. 



