1868.] 



Constitution of the San and Stars. 



11 



hot regions beneath. The dust in the ascending currents would be the 

 warmest, and therefore the brightest, and if the currents of convection 

 were on a sufficiently extensive scale, we might expect as a result such a 

 granulated appearance as the sun presents. But it would be one which 

 would be incompatible with the gradation of brightness which extends from 

 the centre to the margin of the sun's disk. The stratum in which these 

 convection-currents exist could affect the light coming from beyond merely 

 as a partial screen, since there would be no marked* difference in point of 

 transparency between the ascending and the descending currents, so that 

 the peculiar action which the honey-combed structure of the stratum would 

 otherwise produce is not developed. There would accordingly be scarcely 

 any diminution of brightness till quite close to the edge of the disk ; and 

 there it should fall off very rapidly. As these are not at all the appear- 

 ances which present, themselves, we must give up the hypothesis of a cloud 

 of fixed solid or liquid matter. The hypothesis of clouds precipitated from 

 vapour is therefore the only one not excluded ; and we have found that it 

 appears consistent with all the phenomena that have been yet discussed. 



Section II. — Collateral Inquiries. 



21. The only class of bodies about the molecular constitution of which 

 we have any satisfactory f information is gases. These appear to consist 



* [The increase of transparency of the heated portions would be due to the separation 

 of the particles of dust caused by the expansion of the intermingled air. Now at these 

 high temperatures an addition to the temperature produces an immense alteration in 

 the quantity of heat and light radiated (see § 68). Hence the elevation of temperature 

 cannot be great ; and accordingly the volume of the air, which varies as the temperature 

 measured from the absolute zero, is but little increased. Such a change would deter- 

 mine great currents of convection, but would not materially separate interspersed parti- 

 cles of dust. — September 1868.] 



t The dynamical theory of the molecular constitution of gases, which, if I mistake not, 

 may be ranked in point both of importance and probability along with the wave theory 

 of light, does not appear to have yet met with that general attention and acceptance 

 which it seems to deserve. It may not be out of place, therefore, to add to the num- 

 berless proofs which have been drawn from its interpreting the phenomena of gases, by 

 many writers, but especially by Clausius, the following negative proof, which demon- 

 strates that no statical theory, whether on the hypothesis of a continuous substance or 

 of distinct particles, is possible. 



A gas is susceptible of enormous dilatation and compression without an abrupt 

 change in the laws upon which its pressure depends ; hence, if it consist of particles at 

 rest, the force which acts in any direction on any one, must be the result of forces ema- 

 nating from many others, no one contributing more than a share which may be regarded 

 as infinitesimal. Hence it is easy to see that if the density be changed, the pressure 

 will vary as the square of the density; for the force in any direction on any one particle 

 will increase as the number of the particles on that or the opposite side (according as 

 the elementary forces are attractive or repulsive) near enough to act on it, i. e. will in- 

 crease as the density ; and the number of particles subjected to this augmented force 

 which are foimd within each element of volume will also have increased in the same 

 proportion. Hence the pressure per square millimetre across any surface within the gas 

 will increase as the square of the density : and as this is a law which does not exist in 



