Ashman — Radio- Activity of Thorium. 71 



From these figures the specific activity of thorium alone can 

 be calculated. The average value of the specific activity of 

 thorium dioxide plus radiothorium and its products given by 

 samples " C " and " D " is 939. That of thorium plus radio- 

 thorium obtained from the same samples is 289. Therefore 

 that of the products alone is 650. From samples "A" and " B " 

 the value of the activity of Tha? (Rt + Pr) is found to be 390. 

 That of Th + x (Rt) is 107 : therefore xYy is 223 and x = 

 0*3446. Since Th + Et = 289 it follows that Kt = 185, and 

 therefore the specific activity of thorium dioxide is 104, or that 

 of thorium is nearly 119. 



Taxing the period of mesothorium as 5*5 years and that of 

 radiothorium as 737 days, McCoy and Ross* have shown that it 

 is possible to calculate the activity of a sample of thorium 

 dioxide at any time, t, after the separation of all the mesothorium, 

 thorium- X and its products. The maximum activity of sample 

 "A," Oct. 1, 1907, was 400. On Feb. 27, 1908, 149 days later, 

 it was 346. In this interval of time the radiothorium present 

 on the former date would decay to e~ 14Q7i * = 0*869, where \ 

 is the disintegration constant of radiothorium, and the prod- 

 ucts of radiothorium would disappear in the same proportion. 

 The specific activity of thorium dioxide itself is 104. If no 

 radiothorium had been formed from the new mesothorium 

 produced from thorium during the interval of 182 days between 

 the date of preparation of the oxide, Aug. 28, 1907, and Feb. 

 27, 1908, the activity would have been 0*869 (400-104) + 

 104 = 361. The activity of the equilibrium amounts of radio- 

 thorium and all its products is 835. Now some radiothorium 

 must have been produced from the mesothorium formed from 

 thorium during the interval 182 days ; the activity thus gained, 

 ""according to the formula of McCoy and Ross, would be 



836 | 1-1"" ^ 2 (e~ A ^ - e~ x A "1 - e~ M 1=4. 



The calculated specific activity on Feb. 27, 1908, is therefore 

 361 + 4 = 365. The observed activity was 346. Table II 

 gives the activities observed and calculated of samples "A," 

 " B " and " D " on various dates since their preparation. 



Table II. 

 Date of 

 Sam- prepar- Date and max. 

 pie ation activity Date and activity Date and activity 



1907 1907 1908 Obs. Gal. 1908 Obs. Gal. 



"A" Aug. 28 Oct. 1 400 Feb. 27 346 3G5 Aug. 26 308 336 



"B" Oct. 19 Nov. 2 1 380 Feb. 27 346 359 Aug. 29 308 331 



"D" Oct. 23 Nov. 23 948 Apr. 841 854 Aug. 28 767 765 



* Loc. cit. 



