

produced hy the Action of Thorium Compounds. 191 



radioactivity naturally present themselves: — (a) That the 

 radioactivity is due to a kind of phosphorescence excited in 

 the substance by the radiation from thorium; (b) or to the 

 deposition of the 4- gaseous ions produced in the gas by 

 the "emanation"; (c) or to the deposition of particles of a 

 radio-active material emitted by thorium compounds. 



The hypothesis that the radiation is a kind of phospho- 

 rescence will not explain the results observed, since sub- 

 stances are made radio-active outside the incidence of the 

 radiation, and the radioactivity can be concentrated on the 



— electrode. The question as to whether the induced radio- 

 activity is due to the deposition of a foreign substance on 

 bodies, or to the action of the + ions produced in the gas, or 

 a combination of both, is difficult to decide with certainty 

 from the experimental evidence. The theory that the 4- ions 

 produced by the emanation are responsible for the radio- 

 activity, at first sight seems to explain many of the results. 

 Since the radio-active particles of the emanation are very 

 small, the intensity of the radiation must be very great near 

 them ; and in consequence of this, ions may not only be pro- 

 duced, but the charges on the ions set in violent vibration: 

 these 4- ions would be carried to the negative electrode, and 

 gradually dissipate the energy of their vibration by radiation 

 into space. On this theory, however, it is difficult to explain 

 the variation of radioactivity with pressure. At low pressures, 

 the experiments show that the total radioactivity produced is 

 much the same as at atmospheric pressure, but the — electrode 

 receives only a small proportion of the radio-active particles. 

 On the theory that the radio-active particles are -I- ions, we 

 should expect them in a strong field to be all carried to the 



— electrode. Another experiment on the variation of the 

 amount of radioactivity with distance also does not fall in 

 readily with this view. The amount of radioactivity was 

 found to be practically the same whether the distance from 

 the radio-active surface was 3 mms. or 3 cms. In the latter 

 case, the number of 4- ions produced by the emanation is 

 much greater than in the former ; but the amount of radio- 

 activity is unaffected. 



The theory that the radioactivity is due to a deposition of 

 radio-active particles from the thorium compounds affords a 

 general explanation of all the results ; but the difficultv is to 

 advance a satisfactory reason for the particles obtainingthe -+- 

 charge which they must possess in order to be moved to the 



— electrode in an electric field. If we suppose the radio-active 

 particles from thorium compounds emitted at a uniform rate 

 independent of the nature and pressure of the gas, we should 



