2.^2 



SCIENCE 



[Vol. XVllT. No. 455 



stance, would be favorable foi" great brilliancy. Though the 

 stars are built up of matter essentially similar to that of the 

 sun, it does not follow that the proportion of the different 

 elements is everywhere the same. It may be that the sub- 

 stances condensed in the photospheres of different stars may 

 differ in their emissive powers, but probably not to a great 

 extent. 



All the heavenly bodies are seen by us through the tinted 

 medium of our atmosphere. Aocordina: to Langley, the so- 

 lar stage of stars is not really yellow, but, even as gauged 

 by our imperfect eyes, would appear bluish white if we could 

 free ourselves from the deceptive influence of our surround- 

 ings. 



From these considerations it follows that we can scarcely 

 infer the evolutional stages of the stars from a simple com- 

 parison of their eye-magnitudes. We should expect the 

 white stars to be, as a class, less dense than the stars in the 

 solar stage. As great mass might bring in the solar type of 

 spectrum at a relatively earlier time, some of the brightest 

 of these stars may be very massive, and brighter than the 

 sun — for example, the brilliant star Arcturus. For these 

 reasons the solar stars should not only be denser than the 

 white stars, but perhaps, as a class, surpass them in mass 

 and eye-brightness. 



It has been shown by Lane that, so long as a condensing 

 gaseous mass remains subject to the laws of a purely gaseous 

 body, its temperature will continue to rise. 



The greater or less breadth of the lines of absorption of 

 hydrogen in the white stars may be due to variations of the 

 depth of the hydrogen in the line of sight, arising from the 

 causes which have been discussed. At the sides of the lines 

 the absorption and emission are feebler than in the middle, 

 and would come out more strongly with a greater thickness 

 of gas. 



The diversities among the white stars are nearly as nu- 

 merous as the individuals of the class. Time does not per- 

 mit me to do more than record that, in addition to the three 

 sub classes into which they have been divided by Vogel, 

 Scheiner has recently investigated minor differences as sug- 

 gested by the character of the third line of hydrogen near 

 G. He has pointed out, too, that so far as his observations 

 go the white stars in the constellation of Orion stand alone, 

 with the exception of Algol, in possessing a dark line in the 

 blue which has apparently the same position as a bright line 

 in the great nebula of the same constellation; and Pickering 

 finds in his photographs of the spectra of these stars dark 

 lines corresponding to the principal lines of the bright-line 

 stars, and the planetary nebulae with the exception of the 

 chief nebular Hues. The association of white stars with 

 nebular matter in Orion, in the Pleiades, in the region of the 

 Milky Way, and in other parts of tlae heavens, may be re- 

 garded as falling in with the view that I have taken. 



In the stars possibly further removed from the white class 

 than our sun, belonging to the first division of Vogel's third 

 class, which are distinguished by absorption bands with their 

 stronger edge towards the blue, the hydrogen lines are nar- 

 rower than in the solar spectrum. In these stars the den- 

 sity-gradient is probably still more rapid, the depths of hy- 

 drogen may be less, and possibly the hydrogen molecules 

 may be affected by a larger number of encounters with dis- 

 similar molecules. In some red stars with dark hydrocarbon 

 bands, the hydrogen lines have not been certainly observed; 

 if they are really absent, it may be because the temperature 

 lias fallen below the point at which hydrogen can exert its 

 characteristic absorption ; besides, some hydrogen will have 



united with the carbon. The coming in of the hydrocarbon 

 bands may indicate a later evolutional stage, but the tem- 

 perature may still be high, as acetylene can exist in the 

 electric arc. 



A number of small stars more or less similar to those 

 which are known by the names of their discoverers, Wolf 

 and Eayet, have been found by Pickering in his photographs. 

 These are remarkable for several brilliant groups of bright 

 lines, including frequently the hydrogen lines and the line 

 D3, upon a continuous spectrum strong in blue and violet 

 rays, in which are also dark lines of absorption. As some 

 of the bright groups appear in his photographs to agree in 

 position with corresponding bright lines in the planetary 

 nebulffi, Pickering suggests that these stars should be placed 

 in one class with them, but the brightest nebular line is ab- 

 sent from these stars. The simplest conception of their na- 

 ture would be that each star is surrounded by a nebula, the 

 bright groups being due to the gaseous matter outside the 

 star. Mr. Roberts, however, has not been able to bring out 

 any indication of nebulosity by prolonged exposure. The 

 remarkable star 77 Argus may belong to this class of the 

 heavenly bodies. 



In the nebulae, the elder Herschel saw portions of the fiery 

 mist or " shining fluid " out of which the heavens and the 

 earth had been slowly fashioned. For a time this view of 

 the nebulaa gave place to that which regarded them as exter- 

 nal galaxies, cosmical '' sand-heaps,'' too remote to be re- 

 solved into separate stars; though indeed, in 1858, Mr. Her- 

 bert Spencer showed that the observations of nebulas up to 

 that time were really in favor of an evolutional progress. 



In 1884 I brought the spectroscope to bear upon them: the 

 bright lines which flashed upon the eye showed the source 

 of the light to be glowing gas, and so restored these bodies 

 to what is probably their true place, as an early stage of si- 

 dereal life. 



At that early time our knowledge of stellar spectra was 

 small. For this reason partly, and probably also under the 

 undue influence of theological opinions then widely preva- 

 lent, I unwisely wrote in my original papar in 1864, " that 

 in these objects we no longer have to do with a special mod- 

 iflcatiou of our own type of sun, but find ourselves in pres- 

 ence of objects possessing a distinct and peculiar plan of 

 structure." Two years later, however, in a lecture before 

 this association, I took a truer position. "Our views of the 

 universe,'' I said, "are undergoing important changes: let 

 us wait for more facts, with minds unfettered by any dog- 

 matic theory, and therefore free to receive their teaching, 

 whatever it may be, of new observations." 



Let us turn aside for a moment from the nebulse in the sky 

 to the conclusions to which philosophers bad been irresisti- 

 bly led by a consideration of the features of the solar system. 

 We have before us in tVie sun and planets obviously not a 

 haphazard aggregation of bodies, but a system resting upon 

 a multitude of relations pointing to a common physical 

 cause. From these considerations Kant and Laplace formu- 

 lated the nebular hypothesis, resting it on gravitation alone, 

 for at that time the science of the conservation of energy 

 was practically unknown. These philosophers showed how, 

 on the supposition that the space now occupied by the solar 

 system was once filled by a vaporous mass, the formation of 

 the sun and planets could be reasonably accounted for. 



By a totally different method of reasoning, modern science 

 traces the solar system backward step by step to a similar 

 state of things at the beginning. According to Helmholtz, 

 the sun's heat is maintained by the contraction of his mass. 



