PRESIDENT S ADDRESS — SECTION A. 



23 



There is rather a wide difference between the mean of Joly's 

 large number of determinations and the mean of other observations. 

 There is clearly a discrepancy, which would probably be most 

 quickly elucidated by the chief observers, determining the radium 

 in specimens of the same rock carefully ground and mixed. If we 

 give equal weight to the mean of Joly's observations 7 x 10^^ and to 

 1.3x10— ^2 the mean of the other observers, the final average is 

 4.1xl0~^2 gm. Now the heat given out by radium^ in complete 

 radio-active equilibrium (uranium to radium F)^ is .06 calories 

 per sec. per gm., so that each gm. of the earth's crust on account 

 of the radium it contains is the source of 4.1 x 10— ^^x .06 = 2.5 x 10—^^ 

 calorie per sec of heat, a source unaffected so far as experiment has 

 shown by temperature or pressure. 



Thorium in the Earth's Crws/.— But the uranium radium series 

 of radioactive elements are not the only source of such heat ; 

 thorium is also widely distributed. Fewer rocks have been ex- 

 amined for it than for radium, but the following results have been 

 recorded : — 



Thorium in Igneous Rocks. 



The heat emitted by thorium in radio-active equilibrium is 

 5 X 10—^ calories per sec. per gm^, and that by the average amount 

 of thorium in rocks 6.5x10— i* cal. sec— ^ gm." ^ 



Heat due to Radium and Thorium in Rocks. — Thus the heat 

 emitted by the uranium, radium and thorium found in surface 

 rocks is (24.6x6.5) 10-i*=3x lO-i^ cal. sec-i gm.-i. Blanc^ 

 having found nearly four times as much thorium as our mean value, 

 concluded that thorium contributed as much heat as uranium 

 and radium. 



Distribution of Radio- Active Elements. — If the whole mass of 

 the earth (6 x 10^^ gnis.) were the source of as much radio-active 

 heat as the surface rocks, the heat emitted would be 1.8x10^^ 

 calories per sec. or about 300 times the heat flowing from the interior 

 as deduced from the conductivity and temperature gradient of the 

 surface rocks. But if the interior of the earth gains more heat than 

 it loses, then its temperature is rising, nor is the geological and other 



1. Von Schweidler and Hess " Le Radium," Feb., 1909. 



2. Boltwood, Amer. Journ. Science., 1908. 



3. Pegram and Webb, Phy. Rev., 1908. 



4. Science Abs.. No. 1057, 1909. 



