Radiation of Gas Molecules by Collision with Electrons. 2±7 



X 



the value of 7 for small — is not constant, or for small 



X P 



— an inelastic shock does not take place at every collision 



of the electron, possessing the critical amount of energy, 

 with the gas molecule, but depends upon their relative 

 position at the moment of the shock. 



On the strength of the present experimental results it 

 is impossible to explain the shape of the curve for small 



values of — . Whatever cause determines the form of the 



p X 



curve, its influence disappears with the increase of — ' 



For the determination of values of 7 we obviously must 

 take the straight part of the curve, prolong it as far as the 

 intersection with the axis of abscissa}, and take this point for 

 the new origin. In this way we can determine the value of 

 a for a given X, and further, using for V the data found by 

 Franck and Hertz, we then can calculate the numerical value 

 of 7 by moans of the equation (3). 



The described method has as yet been applied to argon 

 and helium only, and the following results were obtained. 



Argon. The point of intersection with the axis of abscissas 

 of the prolonged straight part of the curve corresponds to 



—=30. The ionizing potential of argon, according to 



Franck and Hertz, is equal to 12 volts. Thus 7= about 0*4- 

 Helium. In the case of helium the experimental results 

 are not sufficiently definite. Gill and Pidduck in a careful 

 investigation* have given a characteristic curve for the gas. 

 containing 2 per cent, of impurities. The point of intersection 



for this curve is — =8. The ionizing potential of helium is 



20*5 volts. The value of 7, calculated by means of these data, 

 is about 0*9. So far as pure helium is concerned, the results 

 are not so certain, and the curve is not characteristic enough 

 to permit the applieation of this method. There is no doubt, 

 however, that the ionization increases with the purity of the 

 gas, and the value 7 for the pure gas must consequently be 

 greater than 0*9. 



From the difference of values of 7 for the two gases it is to 

 be expected that under the impact of electrons helium will 

 bo more readily ionized than argon, while argon will be more 

 capable of emitting light. The following observations, made 

 by Collie and Ramsayf, may be considered as a confirmation 



* Phil. Mag. xvi. p. 280 (1908). 



f Proc. Roy. Soc. lix. pp. 257-270 (1896;. 



