188 Prof. E. M. Wellisch on 



generally, the electrons are not sensitive to oxygen as an 

 impurity. To consider only the case of hydrogen, as I did not 

 experiment with the inert gases : I fomidthat as the pressure 

 was reduced the sensitivity to oxygen was markedly decreased ; 

 e. #., to take actual figures, although a small amount of air 

 (less than 1 mm. pressure) would rob hydrogen at 1 atmosphere 

 of its free electrons, the electrons existed in considerable 

 numbers in a mixture of hvdrooen at 821 mm. and air 

 at 2-J mm. At still lower pressures it was found possible to 

 increase the number of electrons by adding small amounts 

 of air ; this arose from the extra number of electrons supplied 

 by the air more than compensating for the diminution in the 

 number supplied by the hydrogen. 



For the explanation of these results, and indeed of many 

 diverse results that have arisen during the course of the 

 experiments, I have been constantly guided by the following- 

 idea, which has proved of the greatest assistance even should 

 it subsequently be found not to correspond with reality: 

 viz., that an electron cannot effect a permanent union with an 

 uncharged molecule to form a negative ion unless the relative 

 velocity at collision exceed a critical value characteristic of 

 the molecule concerned ; in other words, there is a definite 

 potential of ion formation just as there is a definite ionization 

 potential, the former of course being smaller than the latter, 

 probably of the order of one half. 



To revert to the case of hydrogen with oxygen as the 

 impurity. If an electron be expelled from a hydrogen 

 molecule, we can imagine a sphere A drawn round the 

 parent molecule of such a radius that the electron will be 

 effective in forming a negative ion for any collision with a 

 hydrogen molecule within this sphere ; also a sphere B of 

 larger radius corresponding to the formation of a negative 

 oxygen ion. Now in pure hydrogen the free electrons that 

 are observed are those which pass the boundary of the 

 sphere A ; if a trace of oxygen be present, an electron may 

 form a negative oxygen ion in the region between A and B. 

 In hydrogen the electrons must have at collision a fairly 

 high degree of elasticity, so that we may regard the velocity 

 of the electron between A and B as being approximately a 

 function of its distance from the parent molecule. Now the 

 chance of meeting an oxygen molecule between A and B is 

 much greater if the hydrogen be at a high than at a low 

 pressure, because in the former case the time taken for the 

 electron to cross the region is considerably greater owing 

 to the larger number of collisions. The sensitivity of the 



