the Cause and Nature of Radioactivity. 383 



precipitant. This reagent even in strongly acid solution 

 precipitates almost all of the thorium. When the filtrate is 

 rendered alkaline by ammonia, filtered, evaporated, and 

 ignited, the residue obtained is inactive. 



In the case where sodium phosphate is used as the precipi- 

 tant m ordinary acid solution, the part that comes down is 

 more or less free from ThX. On making the solution 

 alkaline with ammonia, the remainder of the thorium is 

 precipitated as phosphate, and carries with it the whole of 

 the active constituent, so that the residue from the filtrate is 

 again inactive. 



In fact ammonia is the only reagent of those tried capable 

 of separating ThX from thorium. 



The result of Sir William Crookes with uranium, which we 

 have confirmed working with the electrical method, may be 

 here mentioned. UrX is completely precipitated by am- 

 monia together with uranium, and the residue obtained by 

 the evaporation of the filtrate is quite inactive. 



There can thus be no question that both ThX and UrX are 

 distinct types of matter with definite chemical properties. 

 Any hypothesis that attempts to account for the recovery of 

 activity of thorium and uranium with time must of necessity 

 start from this primary conception. 



YI. The Continuous Production of ThX. 



If the recovery of the activity of thorium with time is due 

 to the production of ThX, it should be possible to obtain 

 experimental evidence of the process. The first point to be 

 ascertained is how far the removal of ThX by the method 

 given reduces the total radioactivity of thorium. A pre- 

 liminarv trial showed that the most favourable conditions for 

 the separation are by precipitating in hot dilute solutions by 

 dilute ammonia. A quantity of 5 grams of thorium nitrate, 

 as obtained from the maker, was so precipitated by ammonia, 

 the precipitate being redissolved iu nitric acid and re- 

 precipitated under the same conditions successively without 

 lapse of time. 



The removal of ThX was followed by measuring the 

 activity of the residues obtained from the successive 

 filtrates. The activity of the ThX from the first nitrate was 

 equivalent to 1-25 grams of thoria, from the second to 0*33 

 gram, and from the third to 0*07 gram. It will be seen that 

 by two precipitations practically the whole of the ThX is 

 removed. The radioactivity of the separated hydroxide was 

 18 per cent, of that of the standard de-emanated sample of 

 thoria. 



