390 Prof. E. Eutherford and Mr. F. Sodcl 



V on 



fact almost identical with the ordinary curve (fig. 2) that 

 has been produced back to cut the vertical axis, and there is- 

 thus no doubt that there is a residual activity of thorium 

 unconnected apparently with ThX, and constituting about 

 one fourth of the whole. 



The decay-curves of several of the fractions of ThX 

 separated in this experiment after varying intervals of time 

 were taken for the first few days. All of them showed the 

 initial rise of about 15 per cent, at the end of 18 hours, and 

 then a normal decay to zero. The position is thus proved 

 that the initial irregularities are caused by the secondary 

 radiation excited by ThX upon the surrounding matter. By 

 suitably choosing the conditions the recovery-curve can be 

 made to rise normally from a constant minimum, and the 

 decay-curve be shown to consist of two curves, the first the 

 rate of production of excited radioactivity, and the second 

 the rate of decay of the activity as a whole. 



So far nothing has been stated as to whether the excited 

 radioactivity which contributes about 21 per cent, of the 

 total activity of thorium is the same or different from the 

 known type produced by the thorium emanation. All that 

 has been assumed is that it should follow the same general 

 law ; i. e. the effect will increase with the time of action of 

 the exciting cause, and decrease with time after the cause is 

 removed. If the rate of rise of the excited activity be worked 

 out from the curves given (fig. 5) it will be found to agree 

 with that of the ordinary excited activity, i. e. it rises to half 

 value in about 12 hours. Curve 1 is the observed decay- 

 curve for ThX ; curve 2 is the theoretical curve, assuming 

 that it decreases geometrically with time and falls to half 

 value in four days. Curve 3 is obtained by plotting the 

 difference between these tw T o, and therefore constitutes the 

 curve of excited activity. Curve 4 is the experimental curve 

 obtained for the rise of the excited radioactivity from the 

 thorium emanation when the exciting cause is constant. But 

 the exciting cause (ThX) in the present case is not constant, 

 but is itself falling to half value in 4 days, and hence the 

 difference curve, at first almost on the other, drops away 

 from it as time goes on, and finally decays to zero. There 

 is thus no reason to doubt that the effect is the same as that 

 produced by the thorium emanation, which is itself a 

 secondary effect of ThX. Curve 3 (fig. 2) represents a similar 

 difference curve for the decay of excited activity, plotted from 

 the recovery curve of thorium. 



Since this effect of excited activity is caused by the emana- 

 tion, it seemed reasonable to suppose that it will be greater, the 



