20 president's address — SECTION A. 



Durack (Sydney), Gray (Melbourne), Lusby (Sydney), Glasson 

 (Adelaide), Florance (New Zealand) are all contributing to physics 

 from various English laboratories. 



It is to be hoped that in the future an increasing amount of 

 such investigation will be carried out in Australasian laboratories, 

 so that these laboratories will come to be generally regarded not 

 merely as places where existing knowledge is taught, but also 

 where there flourishes an enlightening spirit of investigation. 

 When our laboratories come to be generally regarded in this light 

 it can but increase their reputation in all directions, and make the 

 community have that confidence in science which is so typical 

 of the German people, and which many believe is intimately 

 connected with their unprecedented industrial progress. 



The History of the Earth. — The doctrine of uniformity in 

 geology stated by Hutton in the words, " we find no vestige of a 

 beginning and no prospect of an end," was accepted by many till 

 Lord Kelvin surprised this school of geologists in 1868 by drawing 

 a very decided hmit to the possible age of the earth.* 



Lord Kelvin assumed that in the remote past the earth was 

 molten, that it cooled down as a whole uniformly until the crust 

 just solidified. Then the earth's interior was at a definite tem- 

 perature, which we can now roughly estimate from the known 

 melting points of the rocks of the crust, while the surface had much 

 the same temperature as now. The rate of cooling was determined 

 by the thermal conductivity of the crust, i.e., by the rate at which 

 the interior heat could escape. 



For such a body it becomes possible to calculate what the 

 temperature gradient near the surface will be at any subsequent 

 time ; or conversely, if we know the temperature gradient, to 

 calculate what time has elapsed since the crust solidified. 



Lord Kelvin showed by applying Fourier's theorem that the 

 temperature gradient at a depth x and time t is equal to 



where 6^ is the initial surface temperature, *.- the conductivity 

 of the solid. 



In applying this to the earth we notice that x is small and t 

 large, so that 



doidx=zOoi \':i;^ 



All of the quantities in this relation other than i are known. 



Lord Kelvin's estimates of the antiquity of the earth varied 

 a good deal, but 40 million years was the maximum he would admit 

 latterly. 



• By the age or antiquity of the earth I understand Lord Kelvin means the time 

 that has elapsed since the crust soliditied. The " geological age " would be less than this. 

 The antiquity of a rock (or mineral) would only in the case of the oldest rocks be the same 

 as the geological age. Thus the age of a mineral is a minimum estimate of the earth's age. 



