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stores from -which * mechanical efTects may be drawn by man.' It is possible that 

 in the ingenious radium clock which we owe to Mr. Strutt we have a source of 

 mechanical energy unsuspected in 1881, but, at all events, regarded from a com- 

 mercial standpoint, it can hardly be considered as ' available by man.' Neverthe- 

 less, there is a sense in which it may be said that we are profiting by atomic 

 energy, for we are no longer bound to limit our estimate of the energy due to the 

 radiant heat of the sun and the internal heat of the earth by previously known 

 dj'namical considerations, and, in consequence, our opinions with regard to the 

 limit of the ages which the physicist could allot to the evolutionist have undergone 

 profound modification. 



I here wish to draw attention to some of the conclusions to which we are led 

 by the work of Mr. Strutt. 



Assuming the earth to be in thermal equilibrium, then, even if the whole of 

 this interior heat be due to radium alone, the mean quantity per cubic centimetre 

 cannot much exceed 1-75 x lO-is gramme. The conclusions of Rutherford, based 

 on somewhat different values for the constants involved, give an equivalent of 

 1*52 X 10"^^. Now Strutt has found that tlie poorest igneous rock examined by 

 him, namely, Greenland basalt, contains more than ten times this quantity, and an 

 average rock fifty or sixty times the amount. The assumption that the earth is 

 cooling only aggravates the difficulty, and facts appear to tell against the theory that 

 it is getting hotter. Also, we must take into consideration the heat due to the 

 existence of uranium, thorium, &c. 



We appear, therefore, to be driven to one of two assumptions : either (a) that 

 the rate of heat production by radium diminishes as we approach the centre of 

 the earth; or (6) that the interior of the earth differs markedly in constitution 

 from the exterior crust. 



It is true that Mr. Makower has shown that there is a slight change of activity 

 in one of the radium products about the temperature of 1200° C, and it is very 

 desirable that this inquiry should be pushed to much higher limits. At the same 

 time, it appears evident that but a very slight change in activity takes place at tem- 

 peratures below 1500° C. 



Now Mr. Strutt has shown, arguing from known data, that the maximum 

 temperature at the bottom of a crust of about forty-five miles in thickness, must 

 be in the neighbourhood of 1530° C, although some amount of uncertainty is 

 necessarily induced by our want of knowledge of the conductivity of rock at Iiigh 

 temperatures. Anyhow, it is probable that at the depth indicated the temperature 

 does not exceed the melting-point of platinum. Such a crust would contain 

 about one-thirtieth of the earth's volume, and if throughout it the radium heat 

 energy were of the average of that exhibited by many samples examined by Strutt, 

 the temperature of the earth could be maintained until our stores of uranium 

 suffered sensible depletion. Such an assumption would lead to the conclusion that 

 the whole of the central portion of the earth consists of non-radio-active sub- 

 stances at an approximate uniform temperature somewhat below the melting-point 

 of platinum. A brief summary of the evidence previously at our disposal may not 

 be out of place. 



In the first edition (1867) of Thomson and Tait's 'Natural Philosophy' we 

 find the tidal evidence summarised as follows : ' It seems certain, therefore, that 

 the tidal efiective rigidity of the earth must be greater than that of glass.' 



In the 1883 edition of the same work a discussion of the question by Professor 

 George Darwin is given. He states: 'On the whole we may fairly conclude, 

 whilst there is some evidence of a tidal yielding of the earth's mass, that yielding 

 is certainly small, and that the effective rigidity is at least as great as steel.' 



In a later paper 1 Darwin pointed out that this conclusion was based on the 

 assumption that oceanic tides would have their equilibrium value, and that the 

 validity of this assumption was open to doubt. Nevertheless, the evidence clearly 

 indicated a high degree of effective rigidity. 



Hough- discussed the variation of latitude, and, after correcting a small 

 mistake of Newcomb's (who was the first to suggest the explanation), found 



•^ Ptoo. Soy. Soc, 1885. •' » Phil. Trans,, A, 1895, 1896. 



