1908] on Radio-active Changes in the Earth. 151 



is one, are not the only radio-active materials in the earth : there is 

 another series, of which thorium is a member ; and there is good 

 reason to suppose that thorium is present in rocks in such quantity 

 as to add appreciably to the evolution of heat. Taking this into 

 account, we should probably find, if we had exact data for calcula- 

 tion, that the thickness of rock containing radio-active material 

 was so small that the material of the interior would somewhere have 

 exuded, in the course of the violent dislocations and earth move- 

 ments which geology reveals to us. No material, however, appears 

 anywhere at the earth's surface which can plausibly be regarded as 

 representative of the unknown interior, if the suggested hypothesis is 

 accepted. It cannot be denied that the subject is at present obscure. 

 Possibly an explanation may be found by supposing that the activity 

 of uranium may be arrested at high temperatures. We have at 

 present no adequate experimental evidence on the subject. It is 

 known that there is very little effect of this kind on radium. If, 

 however, the activity of uranium were arrested at a high temperature, 

 the supply of radium and all the other members of the series would 

 fall off, and thus the aggregate heat production of the whole series 

 might be greatly diminished. 



I shall now pass to another branch of the subject. The investi- 

 gations of Sir William Ramsay and Mr. Soddy have proved that 

 there is continuous evolution of helium from the radium emanation. 

 We have good reasons, into which, however, I do not propose to 

 enter, for considering that the same is true of radio-active changes 

 in general, at all events those in which there is an emission of 

 radiation. Helium is probably evolved at each stage of the trans- 

 formation of uranium, and at each stage of the transformation of 

 thorium ; and it results that the natural minerals and ores in which 

 these elements are found, contain a store of helium, which has 

 accumulated in them, and remains locked up in their poreg. 



As already mentioned, I have succeeded in determining the 

 presence of radium in granite. Thus it becomes natural to enquire 

 whether the corresponding amount of helium is to be found there 

 too. Nothing of the kind had ever come under observation before, 

 and it was, therefore, with some interest that I made the experiment. 

 You see before you a vacuum tube of helium prepared from ordinary 

 granite. The characteristic j^ellow glow will satisfy anyone acquainted 

 with the appearance of a helium discharge of the presence of the gas. 



The facility with which helium was detected in granite suggested 

 further experimental problems. The undoubtedly radio-active ele- 

 ments are at present confined to uranium and thorium, and their 

 respective families of descendants. Evidence has been produced, by 

 myself among others, which suggests that lead and some other 

 elements possess a feeble radio-activity of their own. But this 

 evidence is somewhat equivocal. It seemed highly desirable to attack 

 the question in a new way, and the idea suggested itself of looking 



