
344 RADIO-ACTIVE PROCESSES [CH. 
M is expressed in grams. If we adopt Langley’s value of the solar 
constant, this heat suffices to give a supply for 12 million years. 
Lord Kelvin used Pouillet’s value for that constant, but if he had 
been able to use Langley’s, his 100 million would have been 
reduced to 60 million. The discrepancy between my results of 
12 million and his of 60 million is explained by a conjectural 
augmentation of the lost energy to allow for the concentration 
of the solar mass towards its central parts.’ Now it has been 
shown (section 205) that one gram of radium emits during its 
life an amount of heat which probably hes between 2 x 10° and 
5 x 10” gram-calories. It has also been pointed out that there is 
every reason to suppose that a similar amount of energy is resident 
in the chemical atoms of the active elements. It is not impro- 
bable that, at the enormous temperature of the sun, the breaking 
up of the elements into simpler forms may be taking place at 
a more rapid rate than on the earth. If the energy resident 
in the atoms of the elements is thus available, the time durmg 
which the sun may continue to emit heat at the present rate may 
_be from 50 to 500 times longer than was computed by Lord Kelvin 
from dynamical data. 
Similar considerations apply to the question of the probable 
age of the earth. A full discussion of the probable age of the 
earth, computed from its secular cooling from a molten mass, is 
given by Lord Kelvin in Appendix D of Thomson and Tait’s Vatural 
Philosophy. He has there shown that about 100 million years 
after the earth was a molten mass, the gradual cooling due to 
radiation from its surface would account for the average tempera- 
ture gradient of 1/50° F. per foot, observed to-day near the earth’s 
surface. 
Some considerations will now be discussed which point to the 
probability that the present temperature gradient observed in the 
earth cannot be used as a guide to estimate the length of time 
that has elapsed since the earth has been at a temperature capable 
of supporting animal and vegetable life ; for it will be shown that 
probably there is sufficient radio-active matter on the earth to 
supply as much heat to the earth as is lost by radiation from its 
surface. Taking the average conductivity K of the materials of 
the earth as ‘004 (c.G.s. units) and the temperature gradient 7 near 

