August 20, 1891] 



NATURE 



377 



exception of the chief nebular line. The associalion of white 

 stars with nebular matter in Orion, in the Pleiades, in the region 

 of the Milky Way, and in other parts of the heavens, may be 

 regarded 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 narrower than in 

 the solar spectrum. In these stars the density-gradient is 

 probably still more rapid, the depth of hydrogen may be less, 

 and possibly the hydrogen molecules may be affected by a larger 

 number of encounters with dissimilar 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 has 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 temperature 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 Rayet, 

 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 Dj, 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 nebulae, Pickering suggests that these stars 

 should be placed in one class with them, but the brightest 

 nebular line is absent from these stars. The simplest concep- 

 tion of their nature would be that each star is surrounded by a 

 nebula, the bright groups being due to the gaseous matter out- 

 side the star. Mr. Roberts, however, has not been able to 

 bring out any indication of nebulosity by prolonged exposure. 

 The remarkable star i\ Argiis 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 nebulae 

 gave place to that which regarded them as external galaxies, 

 cosmical "sand-heaps," too remote to be resolved into separate 

 stars ; though indeed, in 1858, Mr. Herbert Spencer showed that 

 the observations of nebulae up to that time were really in favour 

 of an evolutional progress. 



In 1864, 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 sidereal life. 



At that early time our knowledge of stellar spectra was small. 

 For this reason partly, and probably also under the undue in- 

 fluence of theological opinions then widely prevalent, I unwisely 

 wrote in my original paper in 1864, "that in these objects we 

 no longer have to do with a special modification of our own type 

 of sun, but find ourselves in presence of objects possessing a 

 distinct and peculiar plan of structure." Two years later, how- 

 ever, in a lecture before this Association, I took a truer posi- 

 tion. " Our views of the universe," I said, "are undergoing 

 important changes ; let us wait for more facts, with minds un- 

 fettered by any dogmatic theory, and therefore free to receive 

 the teaching, whatever it may be, of new observations." 



Let us turn aside for a moment from the nebulae in the sky to 

 the conclusions to which philosophers had been irresistibly led 

 by a consideration of the features of the solar system. We have 

 before us in the 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 formulated the nebular hypo- 

 thesis, resting it on gravitation alone, for at that time the science 

 of the conservation of enei^y 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 miss, 

 the formation of the sun and planets could be reasonably ac- 

 counted 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, at the rate of 

 about 220 feet a year. Whether at the present time the sun is 



NO. 1 138, VOL. 44] 



getting hotter or colder we do not certainly know. We can 

 reason back to the time when the sun was sufficiently expanded 

 to fill the whole space occupied by the solar system, and was 

 reduced to a great glowing nebula. Though man's life, the life 

 of the race perhaps, is too short to give us direct evidence of any 

 distinct stages of so august a process, still the probability is 

 great that the nebular hypothesis, especially in the more precise 

 form given to it by Roche, does represent broadly, notwithstand- 

 ing some difficulties, the succession of events through which the 

 sun and planets have passed. 



The nebular hypothesis of Laplace requires a rotating mass of 

 fluid which at successive epochs became unstable from exces; of 

 motion, and left behind rings or more probably perhaps lumps, 

 of matter from the equatorial regions. 



The difficulties to ^which I have referred have suggested to 

 some thinkers a different view of things, according to which it 

 is not necessary to suppose that one part of the system gravita- 

 tionally supports another. The whole m xy consist of a congeries 

 of discrete bodies even if these bodies be the ultimate molecules 

 of matter. The planets may have been formed by the gradual 

 accretion of such discrete bodies; On the view that the material 

 of the condensing solar system consisted of separate particles or 

 masses, we have no longer the fluid pressure which is an essential 

 part of Laplace's theory. Faye, in his theory of evolution froii 

 meteorites, has to throw over this fundamental idea of the nebular 

 hypothesis, and he formulates instead a different succession of 

 events, in which the outer planets were formed last ; a theory 

 which has difficulties of its own. 



Prof. George Darwin has recently shown, from an investiga- 

 tion of the mechanical conditions of a swarm of meteorites, that 

 on certain assumptions a meteoric swarm might behave as a 

 coarse gas, and in this way bring back the fluid pressure exercised 

 by one part of the system on the other, which is req lireJ by 

 Laplace's theo-y. One chief assumption consists in supposing 

 that such inelastic bodies as meteoric stones might attain the 

 effective elasticity of a high order which is necessary to the 

 theory through the sudden volalilizaion of a part of their mass 

 at an encounter, by which what is virtually a violent explosive is 

 introduced between the two colliding stones. Prof. Darwin is 

 careful to point out that it must necessarily be obscure as to how 

 a small mass of solid matter can take up a very large amount of 

 energy in a small fraction of a second. 



Any direct indications from the heavens themselves, however 

 slight, are of so great value, that 'I should perhap; in this con- 

 nection call attention to a recent remarkable photograph, by Mr. 

 Roberts, of the great nebula in Andromeda. On this plate we 

 seem to have presented to us some stage of cosmical evolution 

 on a gigantic scale. The photograph shows a sort of whirlpool 

 disturbance of the luminous matter which is distributed in a plane 

 inclined to the line of sight, in which a series of rings of bright 

 matter separated by dark spaces, greatly foreshortened by per- 

 spective, surround a large undefined central mass. We are 

 ignorant of the parallax of this nebula, but there can be little 

 doubt that we are looking upon a system very remote, and there- 

 fore of a magnitude great beyond our power of adequate com- 

 prehension. The matter of this nebula, in whatever state it 

 may be, appears to be distributed, as in so many other nebulae, 

 in rings or spiral streams, and to suggest a stage in a succession 

 of evolutional events not inconsistent with that which the nebular 

 hypothesis requires. To liken this object more directly to any 

 particular stage in the formation of the solar system would be 

 "to compare things great with small," and might be indeed to 

 introduce a false analogy ; but, on the other hand, we should 

 err through an excess of caution if we did not accept the remark- 

 able features brought to light by this photograph as a presump- 

 tive indication of a progress of events in cosmical history following 

 broadly upon the lines of Laplace's theory. 



The old view of the original matter of the nebulae, that it con- 

 sisted of a "fiery mist," 



"a tumultuous cloud 

 Instinct with fire and nitre," 



fell at once with the rise of the science of thermodynamics. In 

 1854, Helmholtz showed that the supposition of an original fiery 

 condition of the nebulous stuff was unnecessary, since in the 

 mutual gravitation of widely separated matter we have a store of 

 potential energy sufficient to generate the high temperature of 

 the sun and stars. We can scarcely go wrong in attributing the 

 light of the nebulae to the conversion of the gravitational energy 

 of shrinkage into molecular motion. 



The idea that the light of comets and of nebulae may be due 



