378 Prof. E. Rutherford and Mr. F. Soddv 



on 



In another experiment, a small quantity of thoria was shaken 

 many times with large quantities of water. In this case, the 

 radioactivity of the residue was examined and found to be 

 about 20 per cent, less radioactive than the original sample. 



The influence of Time on the activity of Thorium and ThX. — 

 The preparations employed in our previous experiments were 

 allowed to stand over during the Christmas vacation. On 

 examining them about three weeks later it was found 

 that the thorium hydroxide, which had originally possessed 

 only about 36 per cent, of its normal activity, had almost 

 completely recovered the usual value. The active residues,, 

 on the other hand, prepared by both methods, had almost 

 completely lost their original activity. The chemical separa- 

 tion effected was thus not permanent in character. At this 

 time M. Becquerel's paper (Compter Rendus, cxxxiii. p. 977, 

 Dec. 9th, 1901) came to hand, in which he shows that the 

 same phenomena of recovery and decay are presented by 

 uranium after it has been partially separated from its active 

 constituent by chemical treatment. 



A long series of observations was at once started to 

 determine — 



(1) The rate of recovery of the activity of thorium 



rendered less active by removal of ThX ; 



(2) The rate of decay of the activity of the separated 

 ThX; 



in order to see how the two processes were connected. The 

 results led to the view that may at once be stated. The 

 radioactivity of thorium at any time is the resultant of two 

 opposing processes — 



(1) The production of fresh radioactive material at a 



constant rate by the thorium compound ; 



(2) The decay of the radiating power of the active 



material with time. 

 The normal or constant radioactivity possessed by thorium 

 is an equilibrium value, where the rate of increase of radio- 

 activity due to the production of fresh active material is 

 balanced by the rate of decay of radioactivity of that already 

 formed. It is the purpose of the present paper to substantiate 

 and develope this hypothesis. 



IV. The Rates of Recovery and Decay of Thorium 

 Radioactivity. 



A quantity of the pure thorium nitrate was separated from 

 ThX in the manner described by several precipitations with 

 ammonia. The radioactivity of the hydroxide so obtained 



