lONIZATION BY NEGATIVE IONS 15 



have very different physical properties in gases at low 

 pressures. It will be seen from the results obtained 

 below that the positive ions must acquire a very much 

 larger kinetic energy than the negative ions before new 

 ions can be produced by their collisions with molecules. 

 This difference between the positive and negative ions 

 shows that the condition that new ions should be gene- 

 rated by a collision is not determined by the kinetic 

 energy of the colliding particle. Thus with equal kinetic 

 energies the ionizing power of the negative ions is much 

 greater than that of the positive, and this can only be 

 attributed to their possessing a larger velocity, and, 

 consequently, also a much smaller mass. Hence, since 

 hydrogen is one of the gases which have been examined, 

 it follows that the mass of the negative ion in any gas is 

 less than that of the positive ion in hydrogen. 



A particular instance of this result was previously 

 established, in the case of negative ions set free from 

 a metal by ultra-violet light. The ratio of the mass 

 to the charge on the ion has been found, by Sir J. J. 

 Thomson, by means of experiments on the effect of a 

 magnetic field on the motion of the ion, 1 and it was 

 deduced that the mass of an ion set free by ultra-violet 

 light was of the order 1/1000 of the mass of a hydrogen 

 atom. 



7. Comparison of the masses of negative ions in liquids and 



gases ; corpuscular state. 

 In this connection it is interesting to observe that 



it will be assumed for simplicity that they have single atomic 

 charges. See papers by the author and Mr. C. E. Haselfoot, 

 Proceedings of the Royal Society, A. Vols. Ixxx., Ixxxi., 1908, and 

 Vol. Ixxxii., 1909. 



1 J. J. Thomson, Philosophical Magazine, December, 1899. 



