150 The Hon. Robert John Strutt [March 27, 



round the speck of zircon is practically circular, and is reminiscent of 

 a spot of grease on cloth. 



Professor Joly has pointed out that this alteration in the sur- 

 rounding materials must be due to the radio-activity of the zircon. 

 That radio-active materials are capable of producing such colora- 

 tions has been known from the early days of radium. You see, for 

 instance, projected on the screen, the image of a glass bottle, in 

 which a radium preparation has been kept. Though originally of 

 colourless glass, it has been stained a deep purple by long continued 

 action of radium. 



It may, perhaps, be thought that this idea, though plausible, is 

 no more than a guess. It is however, much more than that. 

 We know, from the investigations of Professor Bragg and Mr. 

 Kleeman, that the a particles of radium, which constitute the most 

 important feature of radio-active emission, are only able to penetrate 

 a limited and definite distance into solid materials. They then lose 

 their characteristic properties, if, indeed, they are not altogether 

 stopped. This distance has been measured experimentally, and 

 Professor Joly has shown that the distance is just the same as the 

 distance to which the alteration round the zircon crystals extends. 

 Thus we have full quantitative confirmation of the theory which 

 attributes it to radio-activity. 



I will now pass from the discussion of a very minute phenomenon 

 to the discussion of a large scale one. It will be familiar to many 

 of you that, in the opinion of some, at least there is reason for 

 changing the vicAvs which have been held for two generations con- 

 cerning the earth's internal heat. We know that there is, at any rate, 

 some radium in the earth, and that radium gives out heat. Thus it 

 cannot be disputed that some part of the earth's internal heat must 

 be due to this cause ; the only question which remains is whether 

 this part is large or small ; whether in fact the earth's internal heat 

 is chiefly to be accounted for as a small remnant of the much greater 

 internal heat which it once possessed, or whether there is enough 

 radio-active material in the earth to supply most of the annual loss 

 by conduction through the crust and radiation into space. 



As I mentioned before, I have made a large number of deter- 

 minations of the quantity of radium in the rocks of which the 

 superficial portions of the earth are constituted. These are found 

 to be so rich in radium that the difficulty is not so much to account 

 for the internal heat of the earth, as determined by underground 

 observations of temperature, but rather to understand why it is not 

 much hotter. I have suggested, as an explanation, that this general 

 distribution of radio-active material, which pervades the outer parts of 

 the earth, is in reality superficial, extending only to some moderate 

 number of miles in depth, though no doubt much deeper than the 

 deepest mines. I am not wholly satisfied, however, of the sufficiency 

 of this explanation. Radium, and the series of products of which it 



