tlie Method of its Transmission. 97 



appreciably influenced by temperature or by the most drastic 

 chemical treatment. 



From the rate o£ diffusion of these emanations, it appears 

 that they must possess a considerable molecular weight. An 

 investigation o£ the rate of diffusion of the radium emanation 

 into air, a preliminary account of which appeared in ' Nature/ 

 1901, p. 157, and Proc. Roy. Soc. Canada 1901, showed that 

 its molecular weight probably lay between 40 and 100. On 

 account of the rapid loss of activity of the thorium emanation it 

 has not so far been found possible to determine with certainty 

 its rate of diffusion into air or other gases. 



The emanations from thorium and radium possess very 

 similar properties. They both readily diffuse through gases 

 and porous substances ; they both possess the power of 

 ionizing the gas in their neighbourhood and producing excited 

 radioactivity on bodies. 



The differences between them can be readily accounted for 

 by supposing them to be radioactive gases or vapours of 

 different molecular weights. According to the results which 

 have been given in previous papers, radioactivity is an 

 accompaniment of chemical change. Taking this view, the 

 difference in the rates of decay of the radioactivity of the 

 emanations from thorium and radium merely indicates a 

 difference in their rate of chemical change, and does not imply 

 any fundamental difference in nature. 



Unlike the radiations from the emanations, the excited 

 radiation due to thorium decays much more slowly than that 

 due to radium (see Rutherford and Miss Brooks, Phil. Mag. 

 July 1902, p. 18). In this case, the chemical change pro- 

 ceeds more rapidly in the material responsible for the excited 

 radioactivity from radium than in that from thorium. 



Excited radioactivity is always produced on bodies when 

 the radioactive emanations from thorium and radium are 

 present. In order to show the very close connexion existing 

 between the presence of these emanations and excited radio- 

 activity, the following experimental facts may be mentioned : 



(1) Only the radioactive substances which emit emanations, 

 viz. thorium and radium, have the power of exciting radio- 

 activity. Uranium and polonium, not giving off any ema- 

 nation, do not possess the power of exciting radioactivity. 



(2) The amount of excited radiation obtained from thorium 

 and radium compounds is directly proportional xo the 

 amount of emanation present. For example, thoria gives 

 out far more emanation and produces far more excited 

 activity than thorium nitrate in the solid state. Thoria 

 and radium chloride, partly deemanated by strong heating, 

 lose their power of exciting activity in like ratio. 



Phil. Maa. S. 6. Vol. 5. No. 25. Jan. 1903. H 



