Ionization produced by Heated Salts. 389 



exceed that in A, and the currents change in a different way 

 when the direction of the applied field is reversed. The fact 

 that the ionization in the tube B can be greater than that in 

 the salted tube A seems to indicate that a great deal of the 

 ionization arises from a substance which takes some time to 

 form. Thus there may be more of this substance in the 

 vapour by the time it has diffused into the tube B than there 

 was when it was first given off by the salt. 



(4) At temperatures below 402° C, the melting-point of 

 Cdl 2 , the positive ionization effects in both tubes decay 

 asymptotically with the time from an initial maximum 

 value ; when the temperature is above this point the positive 

 ionization increases with time to a maximum followed by 

 decay with time. 



(5) Iodine alone gives quite large currents. A com- 

 parative study of cadmium iodide and iodine shows that 

 some, at least, of the negative ionization effects are pro- 

 bably due to the subsequent ionization of the iodine 

 vapour which is liberated by chemical dissociation in visible 

 quantities. 



(6) The distilled salt is less active than the undistilled. 

 The successive distillates exhibit alternations in their power 

 of negative and positive emission. Those distillates which 

 show a comparatively large negative emission exhibit a small 

 positive ionization and vice versa. In the case of the dis- 

 tillates in which the negative emission is small the positive 

 emission exceeds that of the original salt. 



(7) Continued distillation diminishes the percentage of 

 iodine in the salt, but does not produce much, if any, change 

 in its appearance. 



Several points suggested by these results are being tested 

 by further experiments. 



In closing this paper the writer expresses his thanks to 

 Professor 0. W. Richardson, who suggested this line of 

 research, and to whom he is indebted for many suggestions 

 during its progress. He also wishes to thank Mr. C. Bol 

 for the construction of the various pieces of glass apparatus 

 used. 



Palmer Physical Laboratory, 

 Princeton University. 



