1897.] Light on the Conductivity of Iodine Vapour. 



321 



Table II. gives the rates of discharge of the disc for different 

 values of the distance " cc," the smallest being T cm. in experiment 

 4 ; the potential of the charged disc was in each case 12 volts. 



Column I. gives the sign of the charge on the disc. 



„ II. gives the total charge on the disc in scale divisions. 

 „ III. gives the leak per minute due to light when there 



is no iodine present. 

 „ IV. gives the leak per minute due to light when there 

 is iodine present. 

 By placing a plate of clear glass in the path of the light the 

 leak whether of the positive or of the negative charge was at once 

 destroyed. 



We see by these figures that the leak when iodine vapour is 

 present is always much less than when it is absent, and also, as 

 was to be expected, that the negative leak diminished more rapidly 

 with the increase of the distance " x " when the vapour was present 

 than the positive leak did, since in the latter case the light fell 

 on the quartz plate before being sifted of its ultra-violet rays by 

 the vapour ; the diminution of the positive leak when the iodine 

 is present seems to show that the vapour not only does not 

 conduct, but even tends to prevent the escape of the negative 



Table II. 



electricity from the quartz plate, this effect may however be due 

 to the absorption in the iodine vapour of the light reflected from 

 the copper disc, back again to the quartz plate, this light in the 

 absence of iodine increasing the discharge from the quartz. 



Owing to the leak of the positive charge, whether iodine 



