Ashman — Radio- Activity of Thorium. 67 



activity after one month was about four times the initial activ- 

 ity. McCoy and Ross* found the ratio of final to initial activ- 

 ity to be approximately 2-5 for various samples of thoria 

 prepared from a commercial nitrate. The corresponding ratio 

 for thoria prepared from minerals was about 3*2. The authors 

 at that time considered this to prove conclusively that thorium 

 is itself active, and estimated that the specific activity of 

 thorium dioxide was between 100 and 130. The work here 

 described was undertaken at the suggestion of Professor 

 McCoy with the object of determining the a-ray activity of 

 thorium itself with accuracy. A brief preliminary mention 

 of my results was made in the paper of McCoy and Ross. 



The activity of thorium itself is found from the experi- 

 mental data as follows : The activity of new thorium freed 

 from the easily separated products thorium-X emanation, 

 tliorium A, thorium B, and thorium C, is due to Th -f- Rt. 

 Rut in the course of a month the products (Pr) accumulate in 

 equilibrium amounts, and the activity is then due to Th + Rt + 

 Pr. The activity of old thorium freed from the products 

 is due to Th + ccRt, where x is the fraction of the equili- 

 brium amount of radiothorium in the sample ; and after an 

 interval of one month the activity is due to Th + x (Rt + 

 Pr ). These four relations lead to four equations by means of 

 which the four unknown quantities, activity of Th, Rt and Pr, 

 and the fraction x may be found. McCoy and Ross explained 

 that the uncertainty of the data upon which their estimate 

 was based was due to the fact that by the large number of 

 precipitations with ammonia, which were necessary to remove 

 the separable products, Th-X, etc., large quantities of silica 

 and other impurities from the glass and reagents were intro- 

 duced into the thorium. A further disadvantage of the am- 

 monia method arises from the fact that Th-A, with the com- 

 paratively long period of 11 hours, is precipitated with the 

 thorium. This makes it necessary to carry out 12 to 15 pre- 

 cipitations in the course of three or four days in order com- 

 pletely to remove the separable products. 



Schlundt and Mooref found that in the precipitation of 

 thorium by fumaric acid, according to Metzger's method for 

 the analytical separation of thorium from other rare earths, 

 Th-A as well as Th-X remains in the solution to a very large 

 extent, while Th-B is carried down with the thorium. This 

 method would therefore seem to offer an easier way to separ- 

 ate the active products of thorium than the precipitation with 

 ammonia ; but it had the disadvantage of requiring an expen- 



* J. Am. Chem. Soc, xxix, 1709, 1907. 

 f J. Phys. Chem., ix, 682, 1905. 



