410 Experiments on Slow Cathode Rays [March 26, 



AVith each of these methods the following results were obtained. 

 The energy in the radiation from the target is very small until the 

 cathode rays have an energy corresponding to about 15 volts. The 

 energy then rises quickly with the potential ; then, when the energy 

 is measured by the second method, reaches a maximum, and then 

 decreases very slowly. This refers to the energy in the Rontgen 

 radiation emitted by the target. It is known, however, that when a 

 target is struck by cathode rays, negative electrons moving at a much 

 slower speed than the primary cathode rays are also given out, and 

 the variation in the number of these negative electrons with the 

 voltage of the incident cathode rays has been investigated l)y Leitz, 

 Campbell and others. This variation is very similar indeed to the 

 variation in the intensity of the Rontgen radiation, for it begins 

 between 10 and 15 volts, reaches a maximum between 200 and 

 300 volts, and then slowly decreases. The number of electrons- 

 does not depend on the nature of the target (carbon targets are 

 an exception). As far as our experiments go at present a similar 

 statement is true for the Rontgen radiation produced by the cathode 

 rays ; this does not change much when one metal replaces another 

 for the target. 



The close correspondence between the emission of negative elec- 

 trons and of the soft Rontgen radiation supports a view of the origin 

 of this radiation which I expressed some years ago ; it is that the 

 radiation originates in the return of an electron to an atom or a 

 molecule which has been ionized, the kind of vibration depending on 

 the nature of the electron which has been ejected when the atom was 

 ionized. If this was one of the electrons on the surface, the radiation 

 would be in the region of ordinary light ; if it was from the innermost 

 ring of electrons in the atom it would correspond to the K radiation ; 

 if from a ring not so close to the centre the L radiation ; and so on. 

 As each unit of radiation emitted corresponds to the return of a free 

 electron, it is evident that there would be a close connection between 

 the number of free electrons produced by the cathode rays and the 

 energy in the Rontgen radiation due to those rays ; as we have just 

 seen such a correspondence does exist. T\^e may hope to gain im- 

 portant information about the atom from the study of these soft 

 radiations, for they are given out by the electrons not far from the 

 surface of the atom, and it is just these electrons which determine 

 the chemical properties of the atom. 



[J.J.T.] 



