78 THE POPULAR SCIENCE MONTHLY. 



temperature, till finally the main absorption of light is caused by 

 hydrogen and iron. Toward the top of the curve we get hydrogen 

 enormously developed. It seems that we deal with a greater and 

 greater quantity of hydrogen as the temperature gets higher. 



Side by side with this sequence in the case of stars, a similar 

 one up to a certain point is noted in the comets. As a rule the 

 temperature of comets is, as we should expect, very much below 

 that reached by stars. There is, therefore, no overwhelming indi- 

 cation of light-absorption, and it is only in those which closely 

 approach the sun that any indication of the absorption of light 

 caused by the presence of iron vapor is to be seen. A comparison 

 of the spectra observed gives a clear indication that the nature of 

 comets and nebulae, so far as the spectroscope can seize them, is 

 very similar : the phenomena present themselves in the same 

 order ; a line common to both begins the story, and then bright 

 carbon is found among the first substances indicated, and after- 

 ward absorption phenomena, produced by manganese and lead 

 chiefly, it is supposed, are superadded. 



After this cometary parenthesis I now return to consider the 

 top of the temperature curve. I repeat that we have this sort of 

 condition. The swarms, whether single or multiple in origin, 

 have by collisions and gravity brought about the highest point 

 of temperature which they can reach in consequence of these 

 actions. Swarms of separate meteorites now give place to a 

 globular mass of gas produced by their volatilization. It may 

 be that this very high temperature may be produced, and this 

 enormous globular mass of gas formed, long before all the mete- 

 orites and meteoritic dust in the parent swarm, or in that partic- 

 ular region of space, shall be absolutely condensed to the center ; 

 so that we see it is quite possible that this high temperature con- 

 dition may last for a very long time. Hence the curve should be 

 flat-topped in all probability very flat for, so far as the spec- 

 trum analysis of stars has gone at present, more than half of 

 those which have been examined give us evidence of extremely 

 high temperature. However that may be, it is easily to be under- 

 stood that such a mass as that we are considering must be radiat- 

 ing with tremendous energy ; for a time probably the heat which 

 it receives by the collisions and condensation of the outer mem- 

 bers of the parent swarm may be as great as the heat which it 

 radiates, and under these conditions the average temperature of 

 the gas will remain constant ; but the moment the input is less 

 than the output the mass of gas must cool, so that we have next 

 to consider what will happen to a mass of gas cooling under these 

 circumstances. 



What will cool first ? The outside. We know pretty well the 

 chemical nature of the outside of the mass of gas we are dealing 



