Ionization by Charged Particles, 81& 



agreement with experiment in predicting that, for fast rays,, 

 the forms of the curves of figs. 3 and 5 or the numbers in 

 Table I. should be independent of the nature of the primary 

 rays. Different substances differ probably in the forces- 

 binding the electrons to the atoms ; since the agreement of 

 Thomson's theory with experiment in the matter of the number 

 of ions formed shows that these forces may be neglected if 

 the primary rays are fast enough, the theory is also in 

 agreement with experiment in predicting that the properties 

 of the 8 rays should be independent of the material on which 

 the primary rays fall. 



10. But the predicted proportion of 8 rays having an 

 energy greater than W does not agree in absolute magnitude 

 with experiment. If W = 11 volts, then half the rays 

 should emerge with energies greater than 11 volts, whereas 

 not more than 0*1 of the rays have so great an energy. Two 

 hypotheses may be introduced to explain the discrepancy. 



In the first place, it may be supposed that the velocities 

 with which the 8 rays emerge from a solid body are not 

 those with which they are liberated from the atoms. If the 

 rays which emerge have undergone collisions with other 

 atoms since their liberation, the average velocity of the rays 

 will be decreased ; the proportion of rays having velocities 

 greater than W will undergo the greatest diminution, for 

 these rays will produce fresh ionization. After an infinite 

 number of collisions the velocities of all the rays would be 

 reduced to zero, but there might be an intermediate stage 

 in which the proportion of rays of any velocity existing in 

 the beam is roughly inversely proportional to the power 

 of those rays to produce ionization; it may be that the distri- 

 bution of velocities which is observed among the emergent 

 8 rays is really that corresponding to this state. 



Some support for this view might be found in the absence 

 of any difference between the velocities of the emergence and 

 incidence 8 rays (see (10)) ; if the 8 rays emerged with the 

 velocities with which they were liberated from the atoms, the 

 emergence rays should certainly be faster on the average than 

 the incidence. Again the result, obtained in the experiments 

 described in § 7, that the distribution of velocities among the 

 8 rays is independent of the potential Vi (when Yjis greater 

 than 30 volts), provides some evidence for the same view. 

 For in those experiments the field between A and Z un- 

 doubtedly diminished very greatly the number of rays 

 emerging from A, and yet it diminished the number of rays 

 of all speeds in nearly the same ratio ; if the distribution 

 observed represented a state of equilibrium arising from the- 



