602 Mr. A. M. Tyndall on the 



It was found, however, that the contractions on the average 

 were the same in both cases. It seems, therefore, that in the 

 purest possible hydrogen, with point and plate no longer 

 "fresh/' emissions from the cathode are only corpuscular in 

 nature. 



Again it may be argued that this contraction was due to 

 residual impurity in the gas. The reasons for not accepting 

 this view are set forth in the above paper. 



There is no doubt that, in negative discharge in pure 

 hydrogen, a little back discharge occurs. If the ions in this 

 are generated in the gas at the surface of the plate, the 

 above conclusions will not be affected. But if they are 

 emitted from the metal itself, this is not the cuse. However, 

 it is shown above that it is very probable that the amount of 

 back discharge was quite small, so that its effect on the 

 result may be neglected. 



The theory of Franck will also explnin the amount of 

 combination which is produced in negative discharge between 

 hydrogen and oxygen, when the latter is present in small 

 quantities. In the above paper it was shown that when 

 oxygen is present in hydrogen, combination occurs between 

 them, and as far as one could tell between them only, even 

 when nitrogen is present in large excess. 



The results receive a simple explanation on the view that 

 to make oxygen and hydrogen combine, they must be suitably 

 presented to one another together with an electric charge. 

 The efficiency of negative discharge is greater because the 

 affinity of negative ions for oxygen is great. Since their 

 affinity for nitrogen is, according to Franck, even less than 

 that for hydrogen, the tendency to unite nitrogen and 

 hydrogen will be small, at any rate with small currents. 

 One may assume that having caused a certain amount of 

 combination, a negative ion may either be retained by the 

 products or in some way may be rendered no longer efficient 

 as a combining agent. Other corpuscles will attract oxygen, 

 but may not be suitably presented to hydrogen before the 

 time of entry into a molecular life. 



Now if, as suggested, some of this oxygen in negative dis- 

 charge is concentrated as a film on the point, then, as the 

 percentage of oxygen increases, the density of this film 

 increases. At first the chances of free corpuscles producing 

 combination in the neighbourhood of the point will greatly 

 increase with this increasing density, and the combination 

 per coulomb will rapidly increase. With higher percentages 

 of oxygen, however, oxygen will predominate over hydrogen 

 in the surface film, and the free corpuscles will be decreased in 



