POPULAR SCIENTIFIC LECTUHES. 



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oistribnted by the spectroscope over a large 

 surface, and are therefore greatly enfeebled 

 or even extinguished, while the undecompos- 

 able light of bright gas lines remains am 

 decomposed, and hence can still be seen. In 

 any case, the decomposition of tho light of 

 the nebulae shows that by far tho greater part 

 of their luminous surface is due to ignited 

 gases, of which hydrogen forms i\ prominent 

 constituent. In the planetary UiiiKsori, tho 

 spherical or discoidal, it might be supposed 

 that the gaseous mass had attained u con- 

 dition of equilibrium ; but most other neb- 

 ulas exhibit highly irregular forms, which 

 by no means correspond to such a condition. 

 AS, however, their shape has either not at 

 all altered, or not appreciably, since they 

 have been known and observed, they must 

 cither have very little mass, or they must be 

 of colossal size and distance. The former 

 does not appear very probable, because small 

 masses very soon give out their heat, and 

 hence we are left to the second alternative, 

 that they are ot huge dimensions and dis- 

 tances. The same conclusion had been orig- 

 inally drawn by Sir W. Herschel, on the 

 assumption that tho nebulre were heaps of 

 stars. 



With those nebulae which, besides tho lines 

 of gases, also show the continuous spectrum 

 of ignited denser bodies, are connected spots 

 which art partly irresolvable and partly re- 

 solvable into heaps of stars, which only show 

 the light of the latter kind. 



The countless luminous stars of the heav- 

 enly firmament, whose number increases 

 with each newer and more perfect telescope, 

 associate themselves with this primitive con- 

 dition of tho worlds as they are formed. 

 They are like our sun in magnitude, in lumi- 

 nosity, and on the whole also in the chemi- 

 cal condition of their surface, although there 

 may bo differences in the quantity of indi- 

 vidual elements. 



But we find also in space a third stadium, 

 that of extinct suns ; and for this also there 

 are actual evidences. In the first place, 

 there are, in the course of history, pretty fre- 

 quent examples of the appearance of new 

 stars. In 1572 Tycho Brahc observed euch a 

 one, which, though gradually burning paler, 



Kio. 14. 



was visible for two years, stood still like a 

 fixed star, and finally reverted to tho dark- 

 ness from which it had so suddenly emerged. 

 Tho largest of them all seems to have been 

 that observed by Kepler in the year 1604, 

 which was brighter than a star of the first 

 magnitude, and was observed from Septem- 

 ber 27th, 1604, until March, 1G06. The 

 reason of its luminosity was probably the 

 collision with a smaller world. In a more 

 recent case, in which on May 12th, 186f>, a 

 email star of tho tenth magnitude in the 

 Corona suddenly burst out to ono of tho 

 second magnitude, spectrum analysis showed 

 that it was an outburst of ignited hydrogen 

 which produced tho light. This was only 

 luminous for twelve days. 



In other cases obscure heavenly bodies 

 have discovered themselves by their attrac- 

 tion on adjacent bright stars, and the mo- 

 tions of the latter thereby produced. Such 

 an influence is observed in Sirius and Procy- 

 on. By means of a new refracting telescope 

 Messrs. Alvan Clarke and Pond, of Cam- 

 bridge, U. S., have discovered in the case of 

 Sirius a scarcely visible star, which has but 

 little luminosity, but is almost seven times 

 as heavy as the sun, has about half the mass 

 of Sirius, and whoso distance from Sirius is 

 about equal to that of Neptune from the sun. 

 The satellite of Procyon has not yet been 

 seen ; it appears to be quite dark. 



Thus there are extinct suns. The fact 

 that there arc such lends now weight to the 

 reasons which permit us to conclude that otur 

 sun also is a body which slowly gives out its 

 store of heat, and thus will some time becom* 

 extinct. 



Tho term of 17,000,000 years which I have 

 given may perhaps become considerably pro- 

 longed by tho gradual abatement of radia- 

 tion, by the new accretion of falling meteors, 

 and by still greater condensation than that 

 which I have assumed in that calculation. 

 But wo know of no natural process which 

 could spare our sun the fate which has man- 

 ifestly fallen upon other suns. This is * 

 thought which we only reluctantly admit ; it 

 seems to us an insult to the beneficent Crea- 

 tive Power which we otherwise find at work 

 in organisms and especially in living ones. 

 But wo must reconcile ourselves to th 



