SYMPOSIUM OX RADIOACTIVITY. 63 



of the earth are subject to special agencies that lead in part to 

 greater concentration and in part to wider distribution, and that 

 these act co-ordinately with the general dispersing agencies that 

 give radioactivity' to the derivative rocks, to the waters, and to 

 the air. 



If it were permissible to reason from what is known of surface 

 phenomena, particularly from the broad fact that radioactivity 

 increases as we go from air to water, from water to sediment, 

 and from sediment to igneous rock, it might be inferred very 

 plausibly that radioactivity would be found to reach its maximum 

 concentration in the heart of the earth, and certainly that the 

 deeper parts would be as rich as the superficial ones. This pre- 

 sumption might ver}- justly be felt to be strengthened by the fact 

 that the atoms of uranium, radium, and thorium are among the 

 heaviest known and that if the earth were ever gaseous or liquid, 

 these heavy atoms might naturally be expected to be concentrated 

 toward its center unless the viscosity of the fluid mass were too 

 great to permit this, in which case the distribution should be 

 either equable or indilterent to depth. 



But Strutt® early called attention to the fact that if such an 

 increasing abundance exists toward the center of the earth, or 

 if there were an equable distribution in depth, the heat gradient 

 as the earth is penetrated would be higher than observation shows 

 it to be. By computations on the data then available he concluded 

 that a distribution of radioactive substance equal to that of the 

 surface rocks for a depth of only 45 miles would give the rise of 

 heat actually observed in wells, mines and other deep excavations. 

 Later data and closer scrutiny seem to confirm the general sound- 

 ness of Strutt's inference, and to make the limitations even more 

 narrow. Joly, approaching the problem from the geological as 

 well as the physical point of view, and with the advantage of later 

 data, reached the conclusion that radioactivity of the amount 

 observed at the surface, if continued to a depth ranging from 

 27 to 37 kilometers (17.2 to 23.5 miles) would give rise to heat 

 equal to that impHed by the loss at the surface.^ According to 

 Joly, however, a complete concentration of radioactivity in a 

 shell of this depth does not meet the apparent requirements of 

 igneous phenomena if this be assigned to radioactivity. A deeper 

 distribution of a part of the radioactive matter and a less concen- 

 tration in the lower part of the crust is felt by Joly to be required 



•Proc. Rov. Soc. 77A, 1906, p. 472, and 78A, p. 150. 

 ^Radioactivity and Geology, 1909, p. 175. 



