700 Prof. 0. W. Richardson on the 



I think the fact that the phosphate with the large emissivity 

 gave off' ions for which m/H does not correspond to any of 

 the chemical constituents of the substance rules this view out 

 of court. 



Effect of an Electric Field on tJie Decay of the Emission. 



In 1906 * the writer showed that the rate of decay of the 

 positive ionization from a new platinum wire is greater when 

 the wire is positively charged than when it is charged 

 negatively, indicating that the ionization is not due to the 

 platinum alone, but to some foreign matter present in it, 

 which is emitted in the form of positive ions and is thus 

 unable to escape when the wire is negatively charged. This 

 effect has recently been confirmed by Dr. W. Wilson f, who 

 has adopted the explanation of it given by the writer. 



This property, which in the case of pure platinum is con- 

 fined to new wires, that is to say, to wires which have not 

 been heated for a long period of time with a positive charge 

 on them, occurs at low as well as high pressures. I have 

 frequently found it to be well developed in a good vacuum. 

 It also occurs in the case of heated salts, and I think it may 

 be made use of to obtain valuable information about the 

 mechanism of the emission in the case of these bodies. A 

 number of experiments have already been made with alu- 

 minium phosphate which exhibit a very different behaviour 

 in this respect under different conditions. In some cases the 

 emission appears to be due to a vapour which is strongly 

 ionized, whilst in other cases the ionization would seem to be 

 very feeble. These experiments will be considered more 

 fully in a later paper. 



General Discussion. 



While the results of the experiments which have been 

 described are quite bewildering in their complexity when 

 considered in detail, I think certain facts and principles are 

 established by the evidence brought forward in this paper, 

 which will prove to be of fundamental importance in the 

 further development of this subject. In the first place all 

 the direct evidence which appears to be trustworthy points 

 to the view that the positive ions emitted by heated salts are 

 metallic atoms. These are not necessarily atoms of the salt 

 which appears to be under examination, but may be atoms 

 of some other metal whose salts are present as an impurity. 



* Phil. Trans. A. vol. ccvii. p. 30 (1906). 

 t Phil. Mag. [6] vol. xxi. p. 636 (1911). 



