1868.] 



Constitution of the Sun and Stars. 



31 



46. But of whatever kind these or other vehicles for the conveyance of 

 heat beyond the atmospheres of calcium and iron may be, it is certain that 

 no sodium or magnesium rays can carry heat beyond the limits of the so- 

 dium atmosphere. It is also certain that the heat borne outwards is un- 

 able to maintain beyond the iron atmosphere a temperature as high as 

 that of aBunsen's burner, and that, after passing a situation but little out- 

 side the iron, the temperature falls off from this maximum. It must 

 have sunk very low where the next considerable escape of heat takes place 

 — at the boundaries of the atmospheres of magnesium and sodium. Ac- 

 cordingly, we must regard the hydrogen in that still higher dreary waste 

 which is tenanted by hydrogen alone, as a feebly conducting body, of im- 

 mense depth, warmed but moderately beneath, and exposed on the outside 

 to a chilling radiation towards the open sky. Its outer strata must be in- 

 tensely cold. 



47. The case of a comet consisting of a gas * not found in the solar at- 

 mosphere is altogether different. As it approaches the sun it is exposed to 

 the full unveiled glare of the photosphere, and absorbs the heat of those 

 wave-lengths which correspond to the lines of its spectrum. However small 

 a part of the incident heat this may be, it may make the comet nearly as hot 

 as an opake body would become ; since the comet can lose by radiation no 

 heat except through these same spectral rays. 



48. Having now examined in detail the lines of hydrogen, sodium, 

 magnesium, calcium, and iron, we may treat in a more cursory manner the 

 other elements that have been observed in the sun's atmosphere. Chro- 

 mium, nickel, cobalt, copper, and zinc enter in small quantities into the 



* If, indeed, a comet consist of gas, which, perhaps, we ought to deem ^highly im- 

 probable. The molecules of a gas pass most of their time beyond the reach of one 

 another's molecular action, and, unless further confined by a sufficient force of gravity, 

 would each pursue an independent orbit of its own. They would therefore tend gra- 

 dually to extend like a stream of meteors along their common path ; for the orbits 

 being slightly different would have slightly different periodic times, which in the lapse 

 of ages would operate in this way. It does not appear likely that the gravity of a body 

 so large, and with so small a mass as a comet, could successfully withstand this ten- 

 dency. But if the comet were kept together by a molecular cohesion, somewhat like a 

 solid, there would be no such difficulty. Nor is it necessary to suppose that this solid, 

 if such we are to call it, would retain this constitution when subjected to an intense 

 gravity like the earth's : the hardest Archangel pitch flattens down under its own 

 weight, and in time adapts itself to its containing vessel. The matter of comets may 

 on our earth be gas. 



And, again, it seems improbable that a comet can have been raised to the temperature 

 of ignition at the distance from the sun that the earth is ; yet this was the distance of 

 Tempel's comet when its nucleus was seen by Mr. Huggins to emit a spectral ray. The 

 only bodies we know to have the property of glowing at low temperatures are phospho- 

 rescent bodies; and we know from Becquerel's observations that the spectra of phospho- 

 rescent solids consist of bands, in some cases narrow. 



The coma? of comets cannot be transparent gas, since transparent gas would not be 

 conspicuous by reflected light. The phenomena of tails, too, suggest some entirely 

 peculiar constitution. 



