74 . A. \V. Stewart on 



the uninium-load ratio, therefore, the figures represent the 

 luaxiimun time wliicli can have lai)sed since the stratum was iaid 

 down and are possibly an over-estimate of this period. The 

 results which have been obtained by the present. Lord Rayleigh 

 and others in this way may be given here : — 



r„n., .,T,. . , . , ^ Helium ^. „ Lead 



STlvATA Apre dediu^efl from ,- : ratio From , , — • ratio 



Uranium Uranium 



Miocene 6,300,000 years 30,000,000 years 



Eocene 31,000,000 70,000,000 



Carboniferous 146,000,000* 330,000,000 



Devonian 145,000,000* 390,000,000 



Archaean 405,000,000 940,000,000 



In another field of geology, radioactivity has i")layecl its part. 



The rate of cooling of the Earth and Sun presents peculiarities 



which in the pre-radioactivity days could hardly be explained in 



a satisfactory manner. We now know, however, that the 



quantity of radium and other radioactive materials in the Earth 



is sufficient to account for the maintenance of the terrestrial 



globe's temperature for a much longer period than could be 



deduced from the ordinary physics of contraction and radiation ; 



and thus a difficulty has been removed from the path of Science 



in this region. 



In closing an account of the progress of radioactivity, some 

 mention must be made of the ramifications which the subject has 

 produced. In physics, the radioactive properties of matter have 

 led to the development of new views of the constitution of matter 

 and have helped in the advance of our ideas of the atom. 

 Chemistry has been assisted in its definition of the chemical 

 elements and has seen its conceptions of the atoms revolutionized 

 from the radioactivity side. In medicine, radioactive elements 

 are now a recognized weapon in the cure of certain diseases ; and 



*Iti).ust be remembered that specimens from diffei-ent localities are 

 likely to vary in age, wliicli perliaps accounts for the inverted order of the 

 Carboniferous and Devonian results in the case of the uranium-lielium ratio 

 of these two. Also, as ahead}' mentioned, leakage of helium may have 

 takLMi place in some of the specimens examined, 



