the Cause and Nature of Radioactivity. 385 



VII. Influence of Conditions on the Changes occurring 

 in Thorium. 



It has been shown that in thorium compounds the decay 

 of radioactivity with time is balanced by a continuous pro- 

 duction of fresh active material. The change which produces 

 this material must be chemical in nature, for the products of 

 the action are different in chemical properties from the 

 thorium from which they are produced. The first step in 

 the study of the nature of this change is to examine the 

 effects of conditions upon its rate. 



Effect of conditions on the rate of decay. — Since the activity 

 of the products affords the means of measuring the amount 

 of change, the influence of conditions on the rate of decay 

 must be first found. It was observed that, like all other types 

 of temporary radioactivity, the rate of decay is unaltered by 

 any known agency. It is unaffected by ignition and chemical 

 treatment, and the material responsible for it can be dissolved 

 in acids and re-obtained by the evaporation of the solution, 

 without affecting the activity. The following experiment 

 shows that the activity decays at the same rate in solutions 

 as in the solid state. The remainder of the solution that had 

 been used to determine the decay curve of ThX (fig. 2) was 

 allowed to stand, and at the end of 12 days a second quarter 

 was evaporated to dryness and ignited, and its activity com- 

 pared with that of the first which had been left since 

 evaporation upon its original platinum dish. The activities 

 of the two specimens so compared with each other were the 

 same, showing that in spite of the very different conditions 

 the two fractions had decayed at equal rates. After 19 days 

 a third quarter was evaporated, and the activity, now very 

 small, was indistinguishable from that of the fraction first 

 evaporated. Re-solution of the residues after the activity 

 had decayed does not at all regenerate it. The activity of 

 ThX thus decays at a rate independent of the chemical and 

 physical condition of the molecule. 



Thus the rate of recovery of activity under different con- 

 ditions in thorium compounds affords a direct measure of the 

 rate of production of ThX under these conditions. The 

 following experiments were performed : — 



One part of thorium hydroxide newly separated from ThX 

 was sealed up in a vacuum obtained by a good Topler pump,, 

 and the other part exposed to air. On comparing the samples 

 12 days later no difference could be detected between them 

 either in their radioactivity or emanating power. 



In the next experiment a quantity of hydroxide freed from 



