Positive Ionization from Hot Salts. 701 



Until new direct evidence to the contrary is presented, I think 

 it is necessary to accept as a fact the statement that the positive 

 ions emitted by heated salts are charged metallic atoms. 



Another proposition which it is necessary to admit the 

 truth of is that the ionization emitted by heated salts is not 

 always due to the salt which is believed to be under exami- 

 nation, but may arise from minute quantities of other sub- 

 stances which are present as impurities. The evidence in 

 this direction in the case of aluminium phosphate appears to 

 be overwhelming. 



The points which remain to be considered are, I think, 

 of a more speculative character. As far as the relative 

 efficiency of the different salts in emitting positive ions is 

 concerned it seems to the writer that what is required for a 

 good emitter is a combination of volatility in the possible 

 compounds formed together with high electro-positiveness of 

 the metallic constituent. Thus barium sulphide, which is more 

 volatile, is apparently more efficient than barium sulphate, 

 and this in turn is more effective than barium oxide which 

 gives a large negative but no positive emission when pure. 

 In the series of the alkali metals the most electropositive 

 are the most efficient emitters, and ihe same is true in the 

 alkaline earth group. The zinc haloids are very efficient and 

 they combine the qualities postulated. In discussing a 

 question of this character it is of course necessary to rule out 

 of court substances like aluminium phosphate which are not 

 self emitting. 



A large number of facts which are otherwise very difficult 

 to explain can be reconciled by the view that the ionization, 

 in certain cases at least, is not emitted directly from the 

 heated salt but by the action on the platinum of vapours 

 which the salt evolves. In the first place this would explain 

 the very small ionization at very low pressures which is 

 observed by the strip method. The vapours diffuse away 

 from the strip before they have time to get ionized. The 

 effect of increasing the gas pressure is to prevent the vapour 

 from diffusing away from the strip, and so the ionization 

 increases. In the tube method the vapours are unable to 

 diffuse away from the hot platinum, and so there is very 

 little change of the emission with changing pressure at low 

 pressures. It is very difficult to account for this great 

 variation with the pressure at low pressures, obtained by the 

 strip method, on any other view except by falling back on 

 chemical changes. These would have to act in the same way 

 in such different gases as air, oxygen, hydrogen, carbon 

 monoxide, and carbon dioxide, for such different salts as 

 sodium phosphate and sodium sulphate and the impurities 

 Phil. Mag. S. 6. Vol. 22. No. 131. Kov, 1911. 3 A 



