SOME UNSOLVED SCIENTIFIC PROBLEMS 



planets. So another source of the sun's energy had to 

 be sought. It was found forthwith by that other great 

 German, Helmholtz, who pointed out that the falling 

 matter through which heat may be generated might just 

 as well be within the substance of the sun as without ; 

 in other words, that contraction of the sun's heated body 

 is quite sufficient to account for a long-sustained heat- 

 supply which the mere burning of any Known substance 

 could not approach. Moreover, the amount of matter 

 thus falling towards the sun's centre being enormous 

 namely, the total substance of the sun a relatively small 

 amount of contraction would be theoretically sufficient 

 to keep the sun's furnace at par, so to speak. 



At first sight this explanation seemed a little puzzling 

 to many laymen and some experts, for it seemed to im- 

 ply, as Lord Kelvin pointed out, that the sun contracts 

 because it is getting cooler, and gains heat because it 

 contracts. But this feat is not really as paradoxical as 

 it seems, for it is not implied that there is any real gain 

 of heat in the sun's mass as a whole, but quite the reverse. 

 All that is sought is an explanation of a maintenance of 

 heat-giving capacity relatively unchanged for a long, but 

 not an interminable, period. Indeed, exactly here comes 

 in the novel and startling feature of LTelmholtz's calcu- 

 lation. According to Mayer's meteoric hypothesis, there 

 were no data at hand for any estimate whatever as to the 

 sun's permanency, since no one could surmise what might 

 be the limits of the meteoric supply. But Helmholtz's 

 estimate implied an incandescent body cooling keeping 

 up a somewhat equable temperature through contraction 

 for a time, but for a limited time only ; destined ulti- 

 matety to become liquid, solid ; to cool below the tem- 

 perature of incandescence to die. Not only so, but it 



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