﻿368 Prof. E. Rutherford on the Mass and Velocity of 



radium at its minimum activity is 6'2 x 10 10 . The calculated 

 volume of the emanation is 0*8 cubic mms., while the pro- 

 duction of helium per year is 0*11 cubic cms. per gram. 

 The two hypotheses thus lead to the same rate of production 

 of helium by radium. 



Age of Radioactive Minerals. 



I have previously pointed out that the age of the radio- 

 active minerals can be calculated from the amount of helium 

 contained in them. The method is based on the assumption 

 that, in a compact mineral, the greater part of the helium is 

 mechanically imprisoned in the mineral and is unable to 

 escape. Let us consider, for example, the mineral fergu- 

 sonite. which was found by Ramsay and Travers to contain 

 1*81 c.c. of helium per gram of the mineral. The fergusonite 

 contains about 7 per cent, of uranium. The amount of helium 

 per gram of uranium is consequently 26 c.c. Now we have 

 seen that one gram of radium produces 0*11 c,c. of helium 

 per year. The content of radium per gram of uranium is 

 3*8 x 10 — 7 gram*. Supposing that uranium emits only one 

 a particle corresponding to the five emitted by radium in 

 equilibrium where the product radium F is present, the 

 production of helium per year per gram of uranium is 

 f x *11x3'8x10- 7 or 6-3 XlO -8 c.c. per year. Assuming 

 as a first approximation that the rate of production of helium 

 has been constant since the formation of the mineral, the 

 time required for a production of 26 c.c. of helium is about 

 400 million years. This is a minimum estimate, for some of 

 the helium has probably escaped from the mineral. 



As another example, consider the mineral thorianite, which 

 contains about 72 per cent, of thorium and 10 per cent, of 

 uranium. The evolution of helium per gram of the mineral 

 was found by Ramsay to be 9*5 c.c. Bragg (Phil. Mag. 

 June 1906) has shown that thorium breaks up at '26 of the 

 rate of uranium. This was based on measurements made with 

 ordinary commercial thorium. Boltwood (Amer. Journ. Sci. 

 June 1906) has, however, drawn attention to the fact that 

 ordinary commercial thorium has in many cases only about 

 one half of the activity obtained by direct preparation of the 

 thorium from the radioactive minerals. This would double 

 the rate of breaking up of thorium observed by Bragg. 

 Remembering that a thorium atom during its transformations 

 emits five a particles, and assuming that thorium breaks up 

 at half the rate of uranium, it is seen that 72 per cent, of 

 thorium in a mineral corresponds as a producer of helium to 

 * Rutherford and Boltwood, Amer. Journ. Sci. July 1906. 



