382 Prof. O. Sheard on the 



the maximum currents obtained in both tubes under negative 

 potentials exceeded the maximum positive currents in the 

 ratio o£ 3 to 1. This is in agreement with results published 

 by A. Campetti *, in which he showed that the electric 

 dispersion in iodine with platinum electrodes and for tem- 

 peratures up to 500° C. was greater for negative than for 

 positive potentials. 



The positive ionization with iodine in A rose to a maximum 

 value in twenty minutes and then decayed with time. There 

 is no similarity between the positive time-current relations 

 in A, using iodine and cadmium iodide under similar con- 

 ditions of pressure and temperature. The negative current 

 curves are, however, comparable in form. The positive and 

 negative current relations in tube B are similar in the two 

 cases. The following ratios were found in the case of iodine 

 (fig. 4) and cadmium iodide (Table IV.). 



(i) 



(2) 

 (3) 



Ratio. 

 Max. Negative in B _ 

 Max. Negative in A 



Max. Negative in B _ 

 Max. Positive in B 



Max. Positive in B 

 Max. Positive in A 



I. 



Cdl 2 . 



13 



17 



3 



6 



20 



25 



Considering the experimental difficulties involved, especially 

 in getting the initial, and hence the maximum, current 

 readings, it is fairly definitely proven that there is a true 

 negative ionization from cadmium iodide due to the iodine 

 constituent. 



Emission of Ions from the Salt. — The Existence of 

 Negative Ions. 



The existence of a current when the salt-covered plate, 

 usually of platinum, is negatively charged, has been taken to 

 indicate the emission of negative ions from the salt or volume 

 ionization of the vapours given off from the salt. However, 

 it might equally well arise from the vapour or the sublimed 

 salt at the opposite electrode giving an emission of positive 

 ions which would then saturate to the negatively charged 

 electrode. For this reason the results with a hot detecting 

 electrode, such as was used in the foregoing experiments and 

 also by Schmidt in his work, may be equivocal. Furthermore, 



* Scl Torino, Atti, 40, 1, pp. 55-64 (1904-5). 



