Age of the Earth. 371 



helium, to 0*203 gram of U 3 8 . The " helium ratio " of a 

 mineral is the helium in cub. cms. per gram of " total equi- 

 valent" uranium oxide present. This is the usage adopted 

 by Strutt. In a recent paper (Proc. R, S. Oct. 1910) Strutt 

 experimentally verifies this procedure by direct measurement 

 of the helium evolved by minerals rich in uranium and 

 thorium. 



The use of lead as a measure of geological time involves 

 the assumption 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 favour 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- 

 nexion 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 

 helium produced. The latter, calculated from the volume, 

 is found to be 1/88 X 10~ n gram. The associated lead will 

 be 1*22 X 10~ 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. 



(b) Risk of loss of helium or lead, or their gain from 

 spurious sources. 



As regards the first of these heads there is evidence that 

 helium or lead may be originally present in the substance. 

 In fact, we may in a general way consider that the same 

 causes which lead to the segregation of uranium or thorium 

 most probably led to the concentration of other substances. 

 This at least is probable where, as in the case of zircon, none 

 of the substances dealt with are essential parts of the 

 molecule of the mineral. The magma or menstruum from 

 which the parent radioactive substances are derived may be 

 very rich in helium or lead, and the amounts of these con- 

 stituents which enter into the mineral may be considerable. 

 It follows that the absolute value of the helium or lead ratio 

 involves the events attending the genesis of the mineral. It 

 is even quite probable that substances crystallized out within 

 a plutonic mass, and which, at first sight, might be thought 



