AGE OF THE EARTH — JOLY. 283 



may be regarded as known with considerable accuracy. It may be 

 assumed that 1 gram of uranium in equilibrium gives rise to, closely, 

 10.7 XlO" 8 cubic centimeters of hehum (measured under standard 

 conditions) per year. Thorium and its products of change are just 

 as widespread in occurrence as uranium. The contribution of helium 

 derived from the thorium group must, therefore, in most cases 

 be also taken into account. Failing direct measurements of the rate 

 of generation of hehum by thorium, it is possible to estimate this in 

 terms of the output due to uranium by a comparison of the ionization 

 effects of the two families of substances. This comparison has been 

 made by Boltwood. Allowance has further to be made for the differ- 

 ent ionizing activity of the alpha rays from the uranium and thorium 

 series due to their differing velocity and range. The final result is 

 that 1 gram of thoria (Th0 2 ) is equivalent, in its rate of production of 

 helium, to 0.203 gram of U 3 8 . The "helium ratio" of a mineral is 

 the helium in cubic centimeters per gram of "total equivalent" 

 uranium oxide present. This is the usage adopted by Strutt. In a 

 recent paper * Strutt experimentally verifies this procedure by direct 

 measurement of the hehum evolved by minerals rich in uranium and 

 thorium. 



The use of lead as a measure of geological time involves the assump- 

 tion that Boltwood's theory is correct, i. e., that lead is the final 

 product of decay in the uranium series. There is strong evidence in 

 favor of this view. Notably the fact that the atomic weight of 

 uranium, less that of the eight alpha particles which are known to be 

 emitted during its several stages of disintegration, descends to that 

 of lead. The universal association of the two elements and the con- 

 nection of this association with geological time, constitute further 

 evidence. 



The mass of lead generated in one year per gram of uranium is 

 easily found from a knowledge of the mass of hehum produced. 

 The latter, calculated from the volume, is found to be 1.88 XlO -11 

 gram. The associated lead will be 1.22 XlO -10 gram. That is, the 

 presence of one gram of uranium involves the production of 1 .22 X 10 -10 

 gram of lead per annum. A small correction may be required for 

 the exhaustion of the uranium. 



The most obvious criticism which the radioactive method suggests 

 may be embodied in the following possibilities : 



(a) Risk of the original presence of helium or lead in the minerals 

 investigated. 



(&) Risk of loss of hehum or lead, or their gain from spurious 

 sources. 



As regards the first of these heads there is evidence that hehum 

 or lead may be originally present in the substance. In fact, we may 



» Proe. Roy. Soc, October, 1910. 



