320 Mr Henry, Experiments on Effect of Ultra-violet [May 24, 



as 210° C, which was the temperature inside the tube during the 

 experiments. The iodine was introduced at any time when re- 

 quired in the end of a small glass tube and soon surrounded the 

 disc with vapour, which gradually escaped at the open end of the 

 large tube, but allowed sufficient time, — three or four minutes — 

 for the necessary observations. 



Although a positively charged body is not discharged directly 

 by ultra-violet light, yet I found that the disc, although positively 

 charged, was discharged by the arc-light when the disc was in the 

 tube as shown above, and also that the rate of discharge depended 

 on the distance " x " between the disc and quartz plate. 



Table I. gives the results of some experiments on this dis- 

 charge, which show that it is due to the discharge of the negative 

 electricity induced on the quartz plate by the positive charge on 

 the disc, this induced charge becoming very small as " x" in- 



creases. 



Column I. gives the distance " x " in cms. 



„ II. gives the leak per minute in scale divisions for the 



positive charge. 

 „ III. gives the leak per minute in scale divisions for 



the negative charge. 

 „ IV. gives the total charge in scale divisions. 



Table I. 



In trying the effect of the arc-light on the conducting pro- 

 perties of iodine vapour it was necessary to have the distance 

 '" x " small, otherwise all the effective light would have been 

 absorbed by the vapour before reaching the disc, as was shown 

 by experiments on the leak of the negative charge with iodine 

 vapour present, so that it was not possible to get rid of the 

 positive leak by increasing the distance of the disc from the end 

 of the tube. 



