372 Prof. C. Sheard on the 



effects from heated salts be examined, using apparatus which 

 does not involve the heating of* the salt in contact with 

 metal. 



The nature of the ionization process or processes has not 

 been adequately investigated. Richardson * examined the 

 positive ionization from several sulphates and phosphates by 

 the " strip " and " tube " methods. The tube method gave 

 no such diminution in the ionization at low pressures as was 

 observed when the salts were heated on the platinum strip. 

 These results are explicable if the ionization arises in part or 

 in whole from the decomposition, not of the salt itself, but 

 of an intermediate product. The results presented in this 

 article will show that there is both an ionization of the 

 salt vapour and an emission of ions from the salt. 



Determinations of the specific charge! of the emissions 

 from heated salts have shown positive ions characteristic of 

 the base of the salts and electrons only. In this paper it 

 will be shown that with fresh samples of cadmium iodide 

 there is a large emission of negative ions when it is first 

 heated. In general, the current-E.M.F. curves for the 

 negative ionization are very similar to those for the positive. 

 This indicates that the negative ions are comparable in mass 

 with the positive ions. This point of view* is confirmed by 

 measurements, carried out about the same time by Professor 

 Richardson J, of the value of e/rn for the negative ions given 

 out by the iodides of the alkaline earth-metals. The values 

 found agreed with the view that the ions were atoms of 

 iodine carrying a single electronic charge. I have not yet 

 had an opportunity to investigate the effect of a magnetic 

 field on the initial negative emission from cadmium iodide. 



Apparatus and Methods of Experimentation, 



Fig. 1 represents a form of apparatus used in the 

 earlier experiments. It consisted of hard glass tubes, each 

 30 cm. long and 2*5 cm. internal diameter, joined by a 

 cross-connexion of glass 2 cm. long and 1*5 cm. diameter. 

 Each tube was closed with a ground-glass stopper, into 

 which were sealed the electrodes. The outer electrodes were 

 cylinders of platinum gauze each 6 cm. in length. When 

 in place they fitted closely against the surfaces of the con- 

 taining tubes. This ensured the heated tube and salt being 

 at the potential of the gauze, and also prevented the passage 



* Phil. Mag. [6] xxii. p. 669 (1911). 



t Richardson (Phil. Mag. xx. p. 981, 1910). Davisson (Phil. Mag. 

 xxiii. p. 121, 1912). 



X Phys. Rev. xxxiv. p. 386 (1912). 



