204 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
the heat which flows from the hot interior of the earth to the colder 
crust. Now, when the earth first solidified it only possessed a certain 
amount of capital in the form of heat, and if it is continually spend- 
ing this capital and not gaining any fresh heat it is evident that the 
process can not have been going on for more than a certain number 
of years, otherwise the earth would be colder than it is. Lord Kelvin 
in this way estimated the age of the earth to be less than 100,000,000 
years. Though the quantity of radium in the earth is an exceedingly 
small fraction of the mass of the earth, only amounting, according 
to the determinations of Professors Strutt and Joly, to about 5 grams 
in a cube whose side is 100 miles, yet the amount of heat given out by 
this small quantity of radium is so great that it is more than enough 
to replace the heat which flows from the inside to the outside of the 
earth. This, as Rutherford has pointed out, entirely vitiates the 
previous method of determining the age of the earth. The fact is that 
. the radium gives out so much heat that we do not quite know what 
to do with it, for if there was as much radium throughout the interior 
of the earth as there is in its crust, the temperature of the earth 
would increase much more rapidly than it does as we descend below 
the earth’s surface. Professor Strutt has shown that if radium be- 
haves in the interior of the earth as it does at the surface, rocks 
similar to those in the earth’s crust can not extend to a depth of more 
than 45 miles below the surface. 
It is remarkable that Professor Milne from the study of earth- 
quake phenomena had previously come to the conclusion that rocks 
similar to those at the earth’s surface only descend a short distance 
below the surface; he estimates this distance at about 30 miles, and 
concludes that at a depth greater than this the earth is fairly 
homogeneous. 
Though the discovery of radioactivity has taken away one method 
of calculating the age of the earth it has supplied another. 
The gas helium is given out by radioactive bodies, and since, except 
in Beers. it is not found in minerals which do not contain radioactive 
elements, it is probable that all the helium in these minerals has come 
from these elements. In the case of a mineral containing uranium, 
the parent of radium in radioactive equilibrium, with radium and its 
products, helium will be produced at a definite rate. Helium, how- 
ever, unlike the radioactive elements, is permanent and accumulates 
in the mineral; hence if we measure the amount of helium in a sample 
of rock and the amount produced by the sample in one year we can 
find the length of time the helium has been accumulating, and hence 
the age of the rock. This method, which is due to Professor Strutt, 
may lead to determinations not merely of the average age of the crust 
of the earth, but of the ages of particular rocks and the date at which 
the various strata were deposited; he has, for example, shown in this 
