64 Mr. E. M. Wellisch on the Motion of 



be fairly elastic, this elasticity being especially marked with 

 light gases, such as hydrogen or helium, which have a simple 

 structure and a small coefficient of polarization. 



In the case of the positive ion the charge is either more 

 centrally situated than is the electron in the negative ion or y 

 what is effectively the same, the positive charge is not discrete 

 but distributed : the ionic charge will thus act so as to produce 

 a collision of small elasticity; the ion will probably penetrate 

 an appreciable distance into the molecule, and the mobility 

 will in consequence be diminished. 



For the heavier gases and vapours we would expect the 

 negative charge in the ion to be situated more centrally than 

 for the light gases ; in any case the discrete nature of the 

 electronic charge would not be so readily manifested with 

 these complex molecules which would approximate more 

 closely to metallic conductors. The forces due to the 

 approach of a positive or negative ion would be more nearly 

 equal and, in consequence, there would be no great dif- 

 ference in the values of the two mobilities. 



In pure hydrogen at atmospheric pressure the negative 

 carriers consist practically entirely of electrons ; a trace of 

 an impurity such as oxygen is sufficient to convert the carriers 

 into ions. An interesting question arises as to the nature 

 of the negative ion in slightly impure hydrogen : is it con- 

 stituted by the hydrogen or by the molecules of the impurity ? 

 Haines * has recently made an investigation with regard to 

 the negative carriers in hydrogen, commencing with the gas 

 in a very pure state and allowing impurities to accumulate. 

 In this manner he has brought into evidence three distinct 

 types of negative ions, the normal ion being the slowest of 

 the three. His conclusion is that these ions are composed 

 of clusters of hydrogen molecules, each type of ion com- 

 prising a definite number of molecules. The possibility that 

 these ions are composed of the molecules of the impurity 

 present is not discussed in the paper, nor indeed does the 

 part played by the impurity receive consideration. No 

 evidence was obtained in the present experiments of the 

 intermediate types of ions described by Haines : this was 

 possibly due to an excess of impurity in the hydrogen 

 employed by the writer, although it should be observed 

 that it was sufficiently pure to yield a copious supply of 

 electrons at atmospheric pressure, whereas in some of the 

 curves given by Haines the intermediate ions are in evidence 

 when free electrons are practically absent, With regard to 

 the question as to the nature of the negative ion in impure 

 * Haines, loc. cit. 



