Ions and Electrons through Gases. 59 1 



ions, the molecules of a gas must contain atoms either of 

 oxygen or chlorine ; we may b} r analogy include other 

 electro-negative atoms such as bromine, iodine, &c. This 

 statement is to be regarded merely as a suggestion for 

 further experiments ; a study of the ionization in pure 

 ammonia might prove of interest in this connexion. 



It is known that the presence of a trace of oxygen in an 

 inert gas or in hydrogen at atmospheric pressure will reduce 

 considerably the number of free electrons. The present expe- 

 riments showed that in hydrogen the sensitivity of the free 

 electrons to traces of oxygen was greatly decreased if the 

 gas pressure was reduced so that, for instance, a considerable 

 number of free electrons was obtained in a mixture of 

 hydrogen at 82 \ mm. pressure and air at 2\ mm. In a 

 previous communication * a definite theory in explanation 

 of these results has been given : the underlying idea is that 

 an electron cannot effect a permanent union with an un- 

 charged molecule to form a negative ion unless the relative 

 velocity at collision exceed a critical value characteristic 

 of the molecule concerned. We have seen that in a large 

 number of gases the electrons persist in the free state, so 

 that it would appear that the negative ions in these gases 

 must in general \ be formed immediately after the act of 

 ionization. We may regard the electron as being expelled 

 with a certain velocity from an uncharged molecule, but 

 owing to the positive charge acquired by the molecule the 

 velocity of th^ electron will decrease as it recedes ; in 

 accordance with the above view we may imagine a sphere 

 drawn round the parent molecule of such a radius that the 

 electron will be effective in forming a negative ion only 

 for impacts within this sphere. It is probable that the 

 circumstances of an encounter as well as the relative velocity 

 will determine the effectiveness of a collision, so that only a 

 fraction of those impncts will result in the formation of ions: 

 outside the sphere, however, the electron must continue in 

 the free state. It is easy to see that on this view the relative 

 number of electrons will increase with decreasing pressure. 



The potential required for the formation of a negative ion 

 must of course be less than that required to ionize a molecule, 

 inasmuch as in the latter case a fresh pair of ions originates. 

 We should expect that for those gases which have a high 

 ionization potential the proportion of negative ions to elec- 

 trons would in general be small. This is borne out by the 



* Wellisch, Phil. Mag. xxxi. p. 186 (1916). 



+ When the applied held is sufficiently great to generate the critical 

 velocity in the electron, negative ions will again commence to be formed. 



