192 Radioactivity produced by Thorium Compounds. 



expect to obtain the same total amount of radioactivity spread 

 over a vessel due to diffusion of the particles, as can be 

 obtained bv concentration of all the radio-active particles on 

 the — electrode ; and the amount should be independent of 

 the pressure and nature of the gas, provided it does not act 

 on the thorium. Some experiments seem to point to the 

 conclusion that the radio-active particles are not charged till 

 they diffuse out into the gas, but that they gain a + charge 

 in the course of time. A possible explanation is that the + 

 charge is obtained by the diffusion of the ions to the surface 

 of the particles. Since there is reason to believe that the 

 — ions in most cases move faster than the + ions in an electric 

 field, there is always an excess of 4- ions in the gas, and the 

 particles in the gas thus tend to become positively charged. 

 On this supposition, the diminution of the amount of radio- 

 activity on the — electrode at low pressures is due to the 

 fact that there is not a sufficient number o£ ions in the gas 

 to charge the particles, which thus diffuse to the sides of the 

 vessel. 



As far as experiments have gone, the power of exciting 

 radioactivity appears to be confined to thorium compounds. 

 Xeither uranium nor radium nor polonium has so far shown any 

 trace of action ; but the specimens * of radium and polonium 

 used were not very radio-active and contained considerable 

 amounts of impurity. A plate made radio-active is not able 

 to excite any appreciable radioactivity in another plate 

 near it. I have tested the + and — electrodes after the 

 passage for several hours of a strong current between them 

 due to Rontgeu rays, flames, and discharge from points, but 

 no trace of radioactivity on them has been observed. 



Macdonald Physics Building, 

 McGill University. Montreal. 

 Nov. 22nd, 1899. 



* As this paper was passing: through the press the Compter Readus of 

 Xov. 6th was received, which contains a paper by Curie and a note by 

 Becquerel on the radiation excited in bodies by radium and polonium. 

 Curie has used specimens of these substauces 10,000 to 50,000 times 

 more radio-active than uranium and the phenomena observed are, in 

 some respects, similar to those exhibited by thorium compounds ; but 

 there are not sufficient data on which to base any comparison. Xo mention 

 is made of the effect of an electric field, or whether there is an " emanation " 

 from radium and polonium, as there is from thorium compounds. Curie 

 concludes that the results obtained are due to a kind of phosphorescence 

 excited by the radiation : while in the case of thorium the author has 

 shown tflat such a theory is inadmissible. Further experiments on the 

 comparison of the radioactivity produced by thorium with that produced 

 by ^radium and polonium will be of interest. 



