12 Sir William Thomson [Jan. 21, 



the ordinary gaseous law of density, in simple proportion to pressure 

 for the same temperatures. We know this law to hold with somewhat 

 close accuracy for common air, and for each of its two chief constitu- 

 ents, oxygen and nitrogen, separately, and for hydrogen, to densities 

 of about two hundred times their densities at our ordinary atmo- 

 spheric pressure. But when the compressing force is sufficiently 

 increased, they all show greater resistance to condensation than 

 according to the law of simple proportion, and it seems most probable 

 that there is for every gas a limit beyond which the density cannot 

 be increased by any j)ressure however great. Lane remarks that 

 the density at the centre of the sun would be "nearly one-third 

 greater than that of the metal platinum," if the gaseous law held 

 up to so great a decree of condensation for the ingredients of the 

 sun's mass ; but he^ does not suggest this supposition as probable, 

 and he no doubt agrees with the general opinion that in all pro- 

 bability the ingredients of the sun's mass, at the actual temperatures 

 corresponding to their positions in his interior, obey the simple 

 gaseous law through but a comparatively small space inwards from 

 the surface ; and that in the central regions they are much less con- 

 densed than according to that law. According to the simple gaseous 

 law, the sun's central density would be thirty-one times that of 

 water ; we may assume that it is in all probability much less than 

 this, though considerably greater than the mean density, 1-4. 

 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 tempera- 

 ture, 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 exceed- 

 ingly 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 atmosj)here, 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 

 depths inwards from the bounding surface the temperature is be- 

 coming higher and higher. This would certainly be the case if the 

 gaseous law of condensation held throughout, but even then the 

 effective radiational temperature, in virtue of which the sun sheds his 

 heat outwards, might be becoming lower, because the temperatures of 

 equi-dense portions are clearly becoming lower under all circum- 

 stances. 



Leaving now these complicated and difficult questions to the 



