38 



PRESIDENTIAL ADDRESS SECTION A. 



growing earth would by compressing the whole cause a general 

 rise of temperature. In spite of this and of cooling by radiation 

 during the process of growth, it is clear that it is the outer and 

 not the central parts of the earth that must have been most 

 heated in the process of earth formation, and I do not doubt 

 that a somewhat similar state of things still prevails. Apart 

 from radio-activity then, I think the curve of temperature along 

 a diameter of the earth is likely to run soinewhat as in the 

 diagram (Fig. 2). 



The other source of heat is the presence of uranium and its 

 product radium in the surface rocks. In most of these, radium 

 has been shown by Strutt and Joly to be present to the exces- 

 sively small amount on an average of abo-ut five parts by weight 

 in 1,000,000,000,000 parts. But the heating effect of the radium 

 emission is so enormous that if radium were present in that 

 quantity even to a depth of only 45 miles, it would provide all 



Fig. 2. Probable distribution of tenijxivature along a diameter 



of the earth. 



the 'heat that is at present being lost by the earth's surface. 

 As we cannot admit that none of the original heat of aggrega- 

 tion is now being lost or that the earth's geological age is likely to 

 be much, if at all, less than 100,000,000 years, we must agree 

 with Strutt in his contention that the distribution of radium and 

 its parent, uranium, is almost wholly superficial, confined proba- 

 bly for tbe most part to the outer 30 miles or thereabouts. This 

 contention is confirmed by the observation that meteoric iron 

 which is not unlikely tO' be similar to a large part of the earth,: 

 is found to contain little or no radium. 



Joly has pointed out that when sedimentary materials are 

 eroded and deposited they carry their uranium and radium with 

 them, and begin to rise in temperature in their new position. The 

 rise is slow and its total extent increases rapidly with the thick- 



