﻿676 



Messrs. A. Holmes and R. W. Lawson on 



There is as good an agreement between this mean and that 

 found by the previous method, as was to be expected from 

 the value of the lead-uranium ratio there found. 



(c) The value of m can also be calculated from the four 

 analyses already used, and without any assumption as to the- 

 distribution of lead. The unknowns in the expression 

 *Pbt=i~Pb + k."U t -+-m.Thi are clearly Pb , k, and m. By 

 insertion of the results of three of the chosen analyses, three 

 equations are obtained which can be solved lor the unknown 

 terms. From the four sets of three equations obtained by 

 use of the analyses 3, 6, 7, 12, the values of m given in the 

 following table were obtained. The mean value of m = 7.10~ & 

 here obtained is practically the mean of those obtained by 

 the two preceding methods, and corresponds to a half-period ? 

 for thorium E of 7 . lO" 5 x 1*5 . 10- 10 = 1-05 . 10 6 years. It 

 is interesting to note that the present method is quite inde- 

 pendent of the stability or instability of thorium E. More- 

 over, since the lead producing power of thorium is only 0*4 

 that of uranium, it follows that, were thorium E a stable 

 isotope of lead the value of m would be 0*4 x 0042 = 0'017,. 

 instead of 7 . 10 -5 as found (O042 being the uncorrected 

 lead-uranium ratio for minerals of Devonian age). The 

 wide difference between these two values of m leaves little 

 doubt that thorium E is relatively unstable. With the object 

 of fixing the value of the half-period of thorium E more 

 definitely, it is the intention of the authors to examine other 

 suitable minerals of different ages, and to apply to these the 

 same method for the evaluation of the halt-period. That 

 the results for the unknowns given in Table VI. c differ from, 



Table VI. c. 



Combination 

 of Analyses. 



Value of Pb . 



Value of k. 



Value of m. 



Nos. 3, 6, 7 

 „ 3, 7, 12 

 „ 3, 6, 12 

 „ 6, 7, 12 



o-ooio 



0000 

 00001 

 0-0026 



0-044 

 0-042 

 0-042 

 0-042 



2.10-5 



11-4.10-5 



10.10-5 



5-10-5 



Mean=7-1 . 10~ 5 



the mean more than in the other cases is only to be expected,, 

 since any slight error in the values of the substitutes will 

 most probably be magnified during the process of solution 



