ty 
308 RADIO-ACTIVE PROCESSES [CH. 
Similar results hold in the case of radium. The emanation 
and the residual activity of radium supply about 18 and 25 per cent. 
respectively of the total activity, and the rest is supplied by the 
changes in emanation X. These results are thus also in rough 
agreement with those obtained for thorium, and indicate that each 
change which gives rise to @ rays supplies about an equal fraction 
of the total activity. This is an important result, for 1t mdicates 
that about the same number of a particles is expelled at each 
change, which gives rise to a rays. This deduction is based on the 
observed fact that the penetrating power and consequently the 
ionization produced by such a particles is not very different. It 
therefore seems probable that, when a compound of thorium or of 
radium is in radio-active equilibrium, the same number of systems 
change per second in each of the products, and that the change in 
all cases but one is accompanied by the expulsion of about the 
same number of a particles. 
196. Conservation of radio-activity. The early observa- 
tions on uranium and thorium had shown that their radio-activity 
remained constant over the period of several years durmg which 
they were examined. The possibility of separating from uranium 
and thorium the active products Ur X and Th X respectively, the 
activity of which decayed with the time, seemed at first sight to be 
contradictory to this point of view. Further observation, however, 
showed that the total radio-activity of these bodies was not altered 
by the chemical processes, for it was found that the uranium- 
and thorium from which the active products were removed, spon- 
taneously regained their radio-activity. At any time after removal 
of the active product, the sum total of the radio-activity of the 
separated product together with that of the substance from which 
it has been separated is always equal to that of the original com- 
pound before separation. In cases where the active products, like 
Ur X and the radium emanation, decay with time according to an 
exponential law, this follows at once from the experimental results. 
If 7, is the activity of the product at any time ¢ after separation, 
ok 1 
and J, the initial value, we know that 7 =e, At the same 
0 
time the activity Z,; recovered during the interval ¢t is given by 
