1034 



where « represents the number of ionising collisions per cm. of 

 path described in the direction of the field for a negative ion,/? the 

 same quantity for a positive ion. S will thus be large, if « is large 

 jj^jj(j if /J — which is always small as compared to a — does not 

 become relatively large. Argon belongs to the so-called "elastic" 

 gases in which the negative ions in collisions, with molecules which do not 

 give rise to ionisation, do not lose any energy. For these gases 



K. T. CoMPTON 'j has recently calculated - {p = gas-pressure) as a 



P 

 function of the ionisation-potential V^, the mean free path /and the 



X a 



intensity of the field per unit pressure — . He finds - much larger 



for the elastic gases than for the non-elastic gases. Substituting in 

 the table given by him the values of V„ and / for argon, neon and 

 helium (the value of / being taken proportional to the value calculated 

 according to the kinetic theory) « is found largest for argon. As 

 there is no obvious reason for assuming that ,i would be much larger 

 for the elastic gases than for the non-elastic ones, the large value 

 of « can make us foresee that the sparking-potential of argon should 

 be particularly small. 



As regards the sparking-potential of the mixtures we may again refer 

 to a calculation of Compton's '). This calculation was not carried to 

 the end, but it indicates, that for a mixture of an elastic and a 

 non-elastic gas « should have about the character of « for non- 

 elastic gases. Now nitrogen is not altogether un-elastic K. T. Compton 

 and J. M. Bknadk ') have proved, that the loss of energy in a 

 collision of an electron with a nitrogen molecule depends on the 

 smallest distance of their respective paths, more particularly, that it 

 is inversely proportional to the 10^^ power of this distance. In this 

 way the problem certainly becomes very complicated. 



The other limiting case of a mixture of two completely elastic gases 

 has been discussed by Franck and Hertz '), although in a somewhat 

 different connection. In that case the gas with the smaller ionisation- 

 potential practically completely assumes the conduction. For nitrogen 

 the ionisation-potential is considerably smaller than for argon. The 

 part played by nitrogen will therefore be in the first place a 

 reduction of « by its un-elastic nature, whereas on the other hand 



1) Compton, Phys. Rev. (7;. 517, 1916 



2) loc. cit. p. 516. ( 

 3; Phys. Rev. (8) 449. 1916. 



1) Franck and Hertz, Ber. D. phys. Ges. (18) 213. 1916. 



