1897.] Mr Henry, Experiments on Ultra-violet Light, etc. 319 



(4) Experiments on the Effect of Ultra-violet Light on the 

 Conductivity of Iodine Vapour. By J. Henry, Trinity College, 

 Cambridge (1851 Exhibition Science Scholar, Queen's College, 

 Galway). 



It is well known that ultra-violet light causes negatively 

 charged bodies on which it falls to lose their charges, but it has 

 not been tried so far as I am aware whether ultra-violet light, like 

 Rontgen rays, enables a gas through which it passes to conduct 

 electricity ; the following paper is an account of some experiments 

 made to test this point, the gases used being the vapours of Iodine 

 and Amyl Nitrite. 



These vapours were used because it seemed probable that they 

 would show this effect of light very readily if it existed, since they 

 are sensitive to ultra-violet light in other ways*. 



The apparatus used in the experiments on iodine vapour con- 

 sisted of an insulated copper disc, connected to a pair of quadrants 

 of an electrometer and supported on a stiff copper wire inside a 

 glass tube, which was closed at one end by a quartz plate, the 

 other end being open. This tube was fixed in a sand bath so that 

 it could be heated to a high temperature, the closed end being at 

 the temperature of the bath ; the diameter of the tube was 3'8 cms., 

 and the length about 20 cms. The light was given by a carbon arc 

 and brought in a converging beam on the copper disc by a quartz 

 lens (see fig. 1). 



d 



to electrometer 



•a- 



fg 



-£m^ 





-<-— 



.-■' A 



Fig. 1. 



a is a sand bath. 



b, a screen to protect insulated wire from hot gases. 



c, a brass cap holding quartz plate. 



d, a screen to cut off the scattered light. 



The copper disc had to be supported on a wire insulated out- 

 side the tube at e, as the iodine always destroyed the insulation of 

 any support inside, apparently by condensing on the insulating 

 substance (discs of mica), even when its temperature was as high 



* Contributions to Molecular Physics, Memoir X. Tyndall. 

 VOL. IX. PT. VI. 26 



