Conductivity produced by Heating Salts. 737 



V. Effect of the nature of the surrounding Gas upon the 

 rate of Leak. 



The air was pumped out o£ the apparatus until the pressure 

 was reduced to 20-30 mm. of mercury. The gas was then 

 introduced, and the apparatus again pumped down to 30 mm. 

 of mercury. More gas was then admitted, and the pressure 

 again reduced ; the process being repeated so as to get rid of 

 as much of the air as possible. 



The gases experimented with were hydrogen, oxygen, and 

 carbon dioxide. 



In the case of hydrogen the results obtained were somewhat 

 irregular, although in the majority of cases a decreased rate 

 of leak occurred in the presence of this gas. 



No certain change in the previous rate of leak was noted 

 in the case of either of the other gases tried. 



VI. Effect of strongly heating the Zinc Iodide previously. 



Some zinc iodide was placed in a piece of platinum-foil, 

 and was then heated in the Bunsen flame until it was just 

 melted. 



After this, it was placed in the apparatus and tested as in 

 the other cases. The rate of leak which was so produced 

 was found to be of the same order as that produced by zinc 

 iodide which had not been previously heated. 



VII. Experiments carried out with the Zinc Iodide at the 



ordinary temperature of the laboratory. 



These experiments required the use of an instrument 

 of small capacity. A Wilson* sulphur-bead gold-leaf 

 electroscope was therefore used. The gold-leaf system was 

 enclosed in a brass tube, which was itself placed inside a 

 glass vessel which could be evacuated at will. The pressure 

 could be ascertained by means of a Macleod gauge. 



(a) At ordinary atmospheric pressure an extra leak due to 

 the positive ions was noted when zinc iodide was placed 

 inside the electroscope. A small leak was also noted when 

 the leaves were charged positively. On the average, it was 

 found that with positively charged leaf the rate of leak was 

 increased 30 per cent, when zinc iodide was placed in the 

 electroscope. 



(b) At reduced pressures. Effect of a strong magnetic 

 field,. — A series of observations was taken with zinc iodide in 

 the apparatus, the leaves being positively charged and the 

 pressures ranging from 70 mm. of mercury to 15 mm. of 



* Proc. Roy. Soc. vol. lxviii. p. 152 (1901). 

 Phil Mag. S. 6. Vol. 13. No. 78. June 1907. 3 D 



