226 Mr. A. T. Waterman on the Positive 



Of these the Zn and Cd halides give a positive emission 

 consisting o£ doubly charged atoms. 



Mn and Al vary considerably in their emission, so that 

 nothing very definite can be said about them. 



Be and Fe apparently give only the K-impurity emission 

 like that from the pure metals. 



All the rest yield a positive emission consisting of singly 

 charged atoms of the metal. 



The question naturally arises as to the reason for obtaining 

 a singly charged ion in some cases and a doubly charged in 

 others. This does not appear to be connected simply with 

 the valence as might be expected. In other words, how can 

 one predict what the emission from a given salt will be ? 

 The experiments that follow were instituted with a view to 

 attempting an answer to this question. Only positive emis- 

 sion was considered and a number of salts were tried, the 

 metals of which differed in valence. 



The following were the salts employed : — 



AgCl Cu 2 Cl 2 



AgBr A1F 3 



PbCl 2 PtCl a 



PbBr 2 



OuCl 2 . 2H 2 MoS 2 (mineral form) . 



Method. 



The method employed was identical with that used by 

 Prof. Richardson in his article on " The Ions from Hot 

 Salts," published in the Phil. Mag. for September 1913, 

 pp. 452-472. Briefly, the salt is placed on a narrow strip 

 of platinum whose emission has previously been driven out 

 by continued heating to a temperature above which measure- 

 ments with the salt are to be made. The salt is then heated 

 by passing a current through the strip, and the temperature 

 roughly ascertained from the resistance of the strip. If now 

 the strip is raised to a certain potential, emission from the 

 hot salt is driven off. A magnetic field is then applied 

 which deflects the stream of ions. The amount of this 

 deflexion is taken when the magnetic field is reversed. This 

 deflexion obviously depends on 



1. The dimensions of the apparatus. 



2. The potential applied. 



3. The strength of the magnetic field. 



4. The value of e/m for the ions. 



