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, 
10.7X10* cubic centimeters of helium (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 helium 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 (ThO,) is equivalent, in its rate of production of 
helium, to 0.203 gram of U,O;. 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 helium 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 helium produced. 
The latter, calculated from the volume, is found to be 1.881074 
gram. The associated lead will be 1.22107 gram. That is, the 
presence of one gram of uranium involves the production of 1.22107” 
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 
1 Proe. Roy. Soc., October, 1910. 
