346 Prof. McCoy and Dr. Viol : Chemical Properties and 



at the various times, in terms of the activity, calculated 

 for time zero, as unity. The third column gives the calcu- 

 lated values of the activity when the mean value found for 

 x is used. The last column gives the values of x calculated 

 from the observed activities of this film. 



The average of fifteen values of x calculated from data on 

 one film was 0*429 ; from seventeen values on another, 

 0*424 ; from eighteen values on another, 0*423 ; and from 

 six values on a fourth, 0"431. The mean of these four 

 is 0*427. 



As will be seen from the table, the maximum activity, 

 which is reached in about 18 hours, is 12*7 per cent, greater 

 than the initial. In our preliminary experiments maximum 

 activities between 9 per cent, and 19 per cent, above the 

 initial were occasionally fouud. This variation was due to 

 several causes : (1) our first films, in which ferric oxide was 

 used, were several times as emanating as those made of 

 barium sulphate ; (2) ThX prepared directly from thorium 

 salts always contains some Msj ; (3) precipitation of thorium 

 by ammonia does not completely separate B and C from ThX. 

 Two possibilities then arise : if some B is left in the ThX 

 film the initial activity is not affected, but the maximum 

 will be too great ; while if C is originally present in the 

 film the initial activity will be too great, and in consequence 

 the maximum apparently too small. Rutherford and Soddy * 

 observed a maximum activity 1 7 per cent, above the initial, 

 while the whole experiment led them to a value of 0*44 

 for x, which does not differ greatly from our value of 0*427. 

 However, they took as periods of ThX and B 4 days and 

 11 hours respectively, instead of 3*64 days and 10*6 hours. 

 Of course, any uncertainty regarding the periods of ThX, 

 B, and C affects the value of x. The most probable values 

 of these decay constants are considered in Section IV. of this 

 paper. The values we have used in the calculation of o; are 

 those which seemed the most accurate. 



III. The Ratio of the Activity of the Products of Radio- 

 thorium to that of Radiothorium Alone. 



Radiothorium free from active impurities was prepared 

 from a barium chloride mesothorium-one preparation in the 

 following manner. After sufficient radiothorium had been 

 produced in the solution, the aluminium from 2 c.c. of a 

 five per cent, solution of aluminium chloride was precipi- 

 tated in the Ms 1? BaCL solution by means of pure ammonia. 



* Phil. Mag. [6] iv. p. 370 (1902). 



