554 Prof. Richardson and Mr. Hulbirt on the 



One of these might well be the same substance as that which 

 gives rise to the emission of the positive ions by the other 

 metals investigated, and the high values of ejm obtained 

 would be due to the presence of some other substance which 

 gave rise to ions of smaller mass. The fact that when fhe 

 highest values of ejm were obtained a sooty deposit was 

 observed, would lead one to suspect that this substance was 

 carbon, which was dissolved in or chemically combined with 

 the iron. On this view, it would be necessary to suppose 

 that the dissolved or combined carbon was given off in the 

 form of positive ions. This view is supported by the fact 

 that the value of mjH corresponding to the cases which gave 

 rise to the largest values of ejm is almost identical with the 

 atomic weight of carbon. Against this we have to set tfye 

 fact that in the former paper experiments were made on the 

 positive ions emitted by carbon itself, and these were found ijo 

 possess the same value of ejm as those emitted by platinum. 

 This is not, however, of necessity conclusive. It may be 

 necessary for the carbon to be dissolved in a metal before it 

 can be given off in the ionized form. In fact there is now a 

 good deal of evidence to support the general statement that 

 when a substance is expelled by heat from a metal in which 

 it has been dissolved, some of it is in the form of positive 

 ions. One of the authors* has adduced a considerable 

 amount of evidence in favour of the view that the per- 

 manent positive ionization produced by hot platinum in 

 oxygen and hydrogen is caused by the emission of those 

 gases from the metal, in which they have been either dissolved 

 or absorbed. 



The discussion of the nature of the substance which gives 

 rise to the heavier ions from iron may be conveniently post- 

 poned until the various substances under investigation can 

 be discussed together. 



Tantalum. 



Tantalum was found to resemble gold in so far as it gave 

 small values of ejm when first heated and larger ones after- 

 wards. It was not found possible to heat tantalum con- 

 tinuously for any length of time as it burnt away rapidly, 

 even when the pressure was kept below *001 mm. The 

 numbers which were obtained are given in the following 

 table : — 



* Phil. Trans. A. ccvii. p. 1 (1906). 



