356 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1908. 



ephemeral indeed, and that if importance is to be ascribed to radium 

 as a geological agent we must seek to find if the radium now perish- 

 ing off the earth is not made good by some more enduringly active 

 substance. 



That uranium is the primary source of supply can not be regarded 

 as a matter of inference only. The recent discovery of ionium by 

 Boltwood serves to link uranium and radium, and explains why it 

 was that those who sought for radium as the immediate offspring of 

 uranium found the latter apparently unproductive, the actual rela- 

 tion of uranium to radium being that of grandparent. But even 

 were we without this connected knowledge, the fact of the invariable 

 occurrence in nature of these elements, not only in association, but in 

 a quantitative relationship, can only be explained on a genetic con- 

 nection between the two. This evidence, mainh^ due to the work of 

 Boltwood, when examined in detail, becomes overwhelmingly con- 

 vincing. 



Thus it is to uranium that we look for the continuance of the sup- 

 plies of radium. In it we find an all but eternal source. The frac- 

 tion of this substance which decays each year, or, rather, is trans- 

 formed to a lower atomic weight, is measured in tens of thousands of 

 millionths; so that the uranium of the earth one hundred million 

 years ago was hardily more than 1 per cent greater in mass than it is 

 to-day. 



As radio-active investigations became more refined and extended, it 

 was discovered that radium was widely diffused over the earth. . The 

 emanation of it was obtained from the atmosphere, from the soil, 

 from caves. It was extracted from well waters. Radium was found 

 in brick-earths, and everywhere in rocks containing the least trace of 

 demonstrable uranium, and Rutherford calculated that a quantity 

 of radium so minute as 4.6X10"" grams per gram of the earth's 

 mass would compensate for all the heat now passing out through its 

 surface as determined by the average temperature gradients. In 1906 

 the Hon. R. J. Strutt, to whom geology owes so much, not only here 

 but in other lines of advance, was able to announce, from a systematic 

 examination of rocks and minerals from various parts of the world, 

 that the average quantity of radium per gram was many times in ex- 

 cess of what Rutherford estimated as adequate to account for ter- 

 restrial heat loss. The only inference possible was that the surface 

 radium was not an indication of what was distributed throughout the 

 mass of the earth, and, as you all know, Strutt suggested a world 

 deriving its internal temperature from a radium jacket some 45 

 miles in thickness, the interior being free from radium.^ 



My own experimental work, begun in 1904, was laid aside till 

 after Mr. Strutt's paper had appeared, and a valued correspondence 



aProc. Roy. Soc. Vol. LXXVII, p. 472, and Vol. LXXVIII, p. 150. 



