ON THE AGE OF THE SUN'S HEAT. 357 



iy. If, for instance, it were the same as that of 

 solid glass, which is about 1/40,000 on bulk, or 

 1/120,000 on diameter, per i Cent, (and for most 

 terrestrial liquids, especially at high temperatures, 

 the expansibility is much more), and if the specific 

 heat were the same as that of liquid water, there 

 r ould be in 860 years a contraction of I per cent, 

 on the sun's diameter, which could scarely have 

 escaped detection by astronomical observation. 

 There is, however, a far stronger reason than this 

 for believing that no such amount of contraction 

 could have taken place, and therefore for suspect- 

 ing that the physical circumstances of the sun's 

 mass render the condition of the substances of 

 rhich it is composed, as to expansibility and 

 specific heat, very different from that of the same 

 substances when experimented on in our terrestrial 

 iboratories. Mutual gravitation between the 

 lifferent parts of the sun's contracting mass 

 mst do an amount of work, which cannot 

 calculated with certainty, only because the 

 law of the sun's interior density is not known. 

 The amount of work performed on a contrac- 



