NOVEMBEE 20, 1908] 



SCIENCE 



705 



and certain parts of the Deeean have given 

 results in favor of such a decrease. On the 

 other hand, as will be seen later, the granite 

 at the north end of the St. Gothard and 

 the primitive gneiss of the Simplon show 

 no diminution. According to the view I 

 have put forward above as to the origin 

 of the surface richness in radium it is I 

 think to be expected that, while the richest 

 materials would probably rise most nearly 

 to the surface, there might be considerable 

 variability in the radio-activity of the 

 deeper parts of the upper crust. 



XJBANroM AND THE INTERNAL HEAT OP THE 

 EAETH 



While forced to deny of the earth's in- 

 terior any such richness in radium as pre- 

 vails near the surface, the inference that 

 uranium exists yet in small quantities far 

 down in the materials of the globe is highly 

 probable. This view is supported by the 

 presence of radium in meteoric substances 

 and by its very probable presence in the 

 sun — that greatest of meteorites. True, 

 the radio-thermal theory can not be sup- 

 posed to account for any great part of solar 

 heat unless we are prepared to believe that 

 a very large percentage of uranium can be 

 present in the sun, and yet yield but feeble 

 spectroscopic evidence of its existence. 

 Taken all together, the case stands thus as 

 regards the earth. We are assured of 

 radium as a widely distributed surface ma- 

 terial, and to such depths as we can pene- 

 trate. By inference from the pi-esence of 

 radium in meteoric substances and its very 

 probable presence in the sun, from which 

 the whole of terrestrial stuff probably 

 originated, as well as by the inherent likeli- 

 hood that every element at the surface is in 

 some measure distributed throughout the 

 entire mass, we arrive at the conclusion 

 that radium is indeed a universal terres- 

 trial constituent. 



The dependent question then confronts 



us— Are we living on a world heated 

 throughout by radio-thermal actions ? This 

 question— one of the most interesting which 

 has originated in the discovery that in- 

 ternal atomic changes may prove a source 

 of heat— can only be answered (if it can be 

 answered) by the facts of geological sci- 

 ence. 



I will not stop to discuss the evidence for 

 and against a highly heated interior of the 

 earth. I assume this heated interior the 

 obvious and natural interpretation of a 

 large class of geological phenomena, and 

 pass on to consider certain limitations to 

 our knowledge which have to be recognized 

 before we are in a position to enter on the 

 somewhat treacherous ground of hypoth- 

 eses. 



In the first place, we appear debarred 

 from assuming that the surface and central 

 interior of the earth are in thermal con- 

 nection, for it seems certain that, since the 

 remote period when (probable) convective 

 effects became arrested by reason of in- 

 creasing viscosity, the thermal relations of 

 the surface and interior have become de- 

 pendent solely on conductivity. From this 

 it follows if the state of matter in the in- 

 terior is such as Lord Kelvin assumed— 

 that is, that the conductivity and specific 

 heat may be inferred from the qualities of 

 the surface materials— we have remained 

 in thermal isolation from the great bulk of 

 the interior for hundreds of millions of 

 years, and perhaps even for more than a 

 thousand million of years. Assuming a 

 diffusivity similar to that of surface rocks, 

 and starting with a temperature of 7,000° 

 Fahr., Kelvin found that after 1,000 mil- 

 lion years of cooling there would be no 

 sensible change at a depth from the surface 

 greater than 568 miles. In short, even if 

 this great period— far beyond our esti- 

 mates of geological time— has elapsed since 

 the consist entior status, the cooling surface 



