ON THE SUN^S HEAT. 401 



This is a wide range of uncertainty, but it would 

 be unwise at present to narrow it, ignorant as we 

 are of the main ingredients of the sun's whole 

 mass, and of the laws of pressure, density, and 

 temperature, even for known kinds of matter, at 

 very great pressures and very high temperatures. 



The question, Is the sun becoming colder or 

 hotter ? is an exceedingly complicated one, and, 

 in fact, either to put it or to answer it is a 

 paradox, unless we define exactly where the 

 temperature is to be reckoned. If we ask, How 

 does the temperature of equi-dense portions of the 

 sun vary from age to age ? the answer certainly is 

 that the matter of the sun of which the density 

 has any stated value, for example, the ordinary 

 density of our atmosphere, becomes always less 

 and less hot, whatever be its place in the fluid, 

 and whatever be the law of compression of the 

 fluid, whether the simple gaseous law or anything 

 from that to absolute incompressibility. But the 

 distance inwards from the surface at which a 

 constant density is to be found diminishes with 

 shrinkage, and thus it may be that at constant 



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