86 B. B. Boltwood — Ultimate Disintegration Products 



ined. The densities of only the first ten minerals are known, and 

 these densities with the corresponding values for R are given 

 in the table which follows. 







Table 



V. 







No. 



Sp. gr. 



R 



No. 



Sp. gr. 



R 



]. 



962 



79 



6. 



8-93 



11 



2. 



9-35 



65 



7. 



8-89 



13 



3. 



9-14 



13 



8. 



8-32 



12 



4. 



9-08 



9 



9. 



8-29 



6 



5. 



8'96 



11 



10. 



8-07 



20* 



None of the minerals listed in Table IY contains more 

 helium than is to be expected from the assumption that helium 

 is produced by the disintegration of uranium only, and in gen- 

 eral with greater density of the mineral a greater proportion 

 of the total helium formed has been retained within it. 



Age of Minerals. 



If the quantity of the final product occurring with a known 

 amount of its radio-active parent and the rate of disintegra- 

 tion of the parent substance are known, it becomes possible to 

 calculate the length of time which would be required for the 

 production of the former. Thus, knowing the rate of disinte- 

 gration of uranium, it would be possible to calculate the time 

 required for the production of the proportions of lead found 

 in the different uranium minerals, or in other words the ages 

 of the minerals. 



The rate of disintegration of uranium has not as yet been 

 determined by direct experiment, but the rate of disintegration 

 of radium, its radio-active successor, has been calculated by 

 Rutherfordf from various data. Rutherford's calculations 

 give 2600 years as the time required for half of a given 

 quantity of radium to be transformed into final products. 

 The fraction of radium undergoing transformation per year is 

 accordingly 2*7xl0~ 4 , and preliminary experiments by the 

 writer on the rate of production of radium by actinium £ have 

 given a value which is in good agreement with this number. 

 The quantity of radium associated with one gram of uranium 

 in a radio-active mineral has also been determined! and was 

 found to be 3*8 XlO -7 gram. On the basis of the disintegra- 

 tion theory, when radium and uranium are in radio-active 

 equilibrium, an equal number of molecules of each disin- 



* It should be explained that No. 10 is really a secondary uraninite and is, 

 therefore, not directly comparable with the others. 



fPhil. Mag. (6), xii, 367, 1906. % This Journal, xxii, 537, 1906. 



§ Rutherford and Boltwood, this Journal, xxii, 1, 1906. 



