28 PRESIDENT’S ADDRESS. 
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 behaves 
in the interior of the earth as it does at the surface, rocks similar to those 
in the earth’s crust cannot extend to a depth of more than forty-five 
miles below the surface. ; 
It is remarkable that Professor Milne from the study of earthquake 
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 thirty miles, and con- 
cludes that at a depth greater than this the earth is fairly homogeneous. 
Though the discovery of radio-activity has taken away one method 
of calculating the age of the earth, it has supplied another. 
The gas helium is given out by radio-active bodies, and since, 
except in beryls, it is not found in minerals which do not con- 
tain radio-active elements, it is probable that all the helium in these 
minerals has come from these elements. In the case of a mineral con- 
taining uranium, the parent of radium in radio-active equilibrium, with 
radium and its products, helium will be produced at a definite rate. 
Helium, however, unlike the radio-active 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 exampie, shown in this way that a speci- 
men of the mineral thorianite must be more than 240 million years old. 
The physiological and medical properties of the rays emitted by 
radium is a field of research in which enough has already been done to 
justify the hope that it may lead to considerable alleviation of human 
suffering. It seems quite definitely established that for some diseases, 
notably rodent ulcer, treatment with these rays has produced remarkable 
cures ; it is imperative, lest we should be passing over a means of saving 
life and health, that the subject should be investigated in a much more 
systematic and extensive manner than there has yet been either time 
or material for. Radium is, however, so costly that few hospitals could 
afford to undertake pioneering work of this kind; fortunately, however, 
through the generosity of Sir Ernest Cassel and Lord Iveagh a Radium 
Institute, under the patronage of his Majesty the King, has been founded 
in London for the study of the medical properties of radium, and for the 
