A. S. Eve — Relative Activity of Radium and Thorium. 479 



(2) The specific activity of the thorium in the thorianite 



requires some consideration. Of the 454 grams of 

 thorianite employed, 360 grams consist of Th0 2 , and 

 50 grams of uranium. The 7-radiation of the uranium 

 itself can be ignored, as it could not produce a suffi- 

 cient effect through the thickness of lead employed. 

 But with the uranium there will be an amount of 

 radium present in the proportion determined by Ruth- 

 erford and Boltwood. Thus the 50 grams of uranium 

 are associated with 50 X 3*8 xlO" 7 grams of radium, or 

 the equivalent of 3*3 XlO -5 grams of radium bromide. 

 But *25 mg of radium bromide had an activity of 18*5 

 as measured by the electroscope, and, therefore, the 

 radium in the thorianite would produce a deflection of 

 18-5/-25X3-3X10-' 2 or 2*47 divisions a minute. De- 

 ducting this from the total activity of the thorianite, 

 namely, from 8*34, we have an activity of 5*87 due to 

 the thorium and all its products, so that the specific 

 activity is 5*87/360 or -0163. 



(3) An analysis of the thorium nitrate showed that 47 per 



cent of Th0 2 was present, so tha/t 213 grams of ThO a 

 had an activity of 1*38, or a specific activity of "0065. 



The specific activities here given are dependent on the elec- 

 troscope used, on the thickness of the lead employed and on 

 the distance of the substances from the electroscope. But the 

 ratios of the specific activities is independent of the condi- 

 tions of the experiments, except for a small correction due to 

 the fact that the substances cannot be concentrated at a point. 



The following results are, therefore, deduced : — 



(a) The thorium and its products in the thorianite is more 



active than the thorium and its products in the thorium 

 nitrate in the ratio of *0163 to *0065, or of 2'5 to 1. 

 This is in very good agreement with Boltwood's results. 



(b) Radium bromide in radio-active equilibrium is more active 



than an equal mass of Th0 2 in radio-active equilibrium 

 in the ratio of 74,000 to '0163 or of 4*5 X 10 s to 1. 



Therefore, radium is 6*9 XlO 6 times as active as thorium 

 when both are in radio-active equilibrium, when the activity 

 is measured by the 7-rays. 



In a previous paper the writer suggested that a kilogram of 

 commercial thorium nitrate might be a convenient standard 

 for Y-ray measurement. This suggestion was not a good one, 

 because recent work has shown that radio-thorium would be 

 continually increasing in such a standard, so that the activity 

 would vary at a rate at present unknown. The only feasible 



