376 Prof. E. Rutherford and Mr. F. Soddy on 



original plate, and comparing it with a similarly undisturbed 

 constant comparison sample. Most of the investigations 

 have been carried out by this method. 



III. The Separation' of a Radioactive Constituent from 

 Thorium Compounds. 



During an investigation of the emanating power of thorium 

 compounds, to be described later, evidence was obtained of 

 the separation of an intensely radioactive constituent by 

 chemical methods. It had been noticed that in certain cases 

 thorium hydroxide, precipitated from dilute solutions of 

 thorium nitrate by ammonia, possessed an abnormally low 

 emanating power. This led naturally to an examination 

 being made of the filtrates and washings obtained during the 

 process. It was found that the filtrates invariably possessed 

 ■emanating power, although from the nature of their pro- 

 duction they are chemically free from thorium. If the 

 nitrate is evaporated to dryness, and the ammonium salts 

 removed by ignition, the small residues obtained exhibit 

 radioactivity also, to an extent very much greater than that 

 possessed by the same weight of thorium. As a rule these 

 residues were of the order of one- thousandth part by weight 

 of the thorium salt originally taken, and were many hundred, 

 in some cases over a thousand, times more active than an 

 equal weight of thoria. The separation of an active con- 

 stituent from thorium by this method is not all dependent on 

 the purity of the salt used. By the kindness of Dr. Knofier, 

 of Berlin, who, in the friendliest manner, presented us with 

 a large specimen of his purest thorium nitrate, we were 

 enabled to test this point. This specimen, which had been 

 purified by a great many processes, did not contain any of 

 the impurities found in the commercial salt before used. But 

 its radioactivity and emanating power were at least as great, 

 and the residues from the nitrates after precipitation by 

 ammonia were no less active than those before obtained. 

 These residues are free from thorium, or at most contain 

 only the merest traces, and when redissolved in nitric acid 

 do not appear to give any characteristic reaction. 



An examination of the penetrating power of the rays from 

 the radioactive residue, showed that the radiations emitted 

 were in every respect identical with the ordinary thorium 

 radiation. In another experiment the nature of the emanation 

 from a similar intensely active thorium-free residue was 

 submitted to examination. The rate of decay was quite 

 indistinguishable from that of the ordinary thorium emanation; 

 that is, substances chemically free from thorium have been 



