1868.] 



Constitution of the Sun and &tars. 



47 



tance of the orbit of meteors as assigns to them a position into which either 

 Jupiter or Saturn could have brought them. 



78. If these solar meteorites exist, they would seem to have had time to 

 extend themselves round the greater part of their orbit, and leave the vacant 

 space smaller at present than that which is occupied, so as to render the 

 phenomenon depending upon their absence, viz. the increase in the 

 number and size of spots, that which develops itself in the most marked 

 manner. We may, then, perhaps presume that the epoch at which they 

 came into the solar system was long before the year 126, though, cosmi- 

 cally speaking, a recent occurrence. 



79. This appears the proper place to observe that the heat which is so 

 lavishly dispersed by the sun cannot be kept up, as has sometimes been 

 supposed, by the continual falling in of meteorites moving in orbits round 

 him ; since if that were so, the outer parts of the solar atmosphere would 

 be kept intensely heated, which is contradicted by all the phenomena. In 

 the next part of this memoir, which will treat of other stars, I will offer 

 what appears to me a possible account of the proximate source of solar 

 heat. 



Part II. Of other Stars. 

 Section I. — Of Solitary Stars. 



80. Observations with the spectroscope having apprised us of the pre- 

 sence in the sun and other stars of several of the elementary bodies with 

 which we are familiar on the earth, we are bound to assume provisionally 

 and until something offers to warrant a different belief, that those which 

 are abundant on the earth and in the sun are abundant elsewhere also. 

 Let us then consider how such differences as we must presume to exist be- 

 tween star and star would affect a body like the sun. 



81. Star manifestly differs from star in mass ; they probably also differ 

 in temperature. Let us therefore inquire how a great change in the sun's 

 mass or in his average temperature would operate. Strange to say, an in- 

 crease of his temperature would produce many of the same effects as a dimi- 

 nution of his mass. This is because the dilatation of the sun's bulk, and 

 the consequent removal of the outer parts of his atmosphere to a greater 

 distance from the centre would lessen the force of gravity upon them. In 

 either case, therefore, the effect upon the atmosphere would be the same 

 as if the gases constituting it became specifically lighter. They would all 

 be able to maintain their footing with feebler molecular motions. In other 

 words, each gas would rise in the atmosphere until the distance between 

 its outer layer exposed by radiation to the intense cold of the sky, and the 

 inner layer heated by the photosphere, interposes a space of such thickness 

 as will, in obedience to the laws of conduction, reduce the temperature 

 on the outside to the lower minimum which the gas can now endure. 

 Accordingly, the spectral lines of a star, either hotter or less massive than our 



