October 23, 1891.] 



SCIENCE. 



23: 



at the rate of about 220 feel a year. Whether at the present 

 time the sua is getting hotter or colder we do not certainly 

 know. We cau reason back to the time when the sun was 

 suiliciently 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 hy- 

 pothesis, es])scially in the more precise form given to it by 

 Roche, does represent broadly, notwithstanding some diffi- 

 culties, the succession of events through which the suu and 

 planets have passed. 



The nebular hypothesis of Laplace requires a rotating 

 mass of fluid which at successive epochs became unstable 

 from excess of motion, and left behind rings, or more 

 probably perhaps lumps, of matter from the equatorial 

 regions. 



Tlie 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 

 gravitationally supports another. The whole may consist of 

 a congeries of discrete bodies, even if these bodies be the 

 ultimate molecules of matter. The planets may have been 

 farmed 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 the- 

 ory. Faye, in his theory of evolution from meteorites, has 

 to throw over this fundamental idea of the nebular hypothe- 

 sis, and he formulates instead a different succession of events, 

 in which the outor planets were formed last, a theory which 

 has difficulties of its own. 



Professor George Darwin has i-ecently shown, from an in- 

 vestigation of the mechanical conditions of a swarm of me- 

 teorites, 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 required by Laplace's theory. One chief assump- 

 tion consists in supposing that such inelastic bodies as mete- 

 oric stones might attain the effective elasticity of a high or- 

 der which is necessary to the theory through the sudden 

 volatilization of a part of their mass at an encounter, by 

 which what is virtually a violent explosive is introduced be- 

 tween the two colliding stones. Professor 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, how- 

 ever slight, are of so great value, that I should perhaps in 

 this connection call attention to a recent re.markable photo- 

 graph, 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 mat- 

 ter which is distributed in a plane inclined to the line of 

 sight, in which a series of rings of bright matter separated 

 by dark space, greatly foreshortened by perspective, sur- 

 round 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 therefore of 

 a magnitude greatly beyond our power of adequate compre- 

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

 may be, appears to be distributed, as in so many other neb- 

 ula;, 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 remarkable features brought to light 

 by this photograph as a presumptive indication of a progress 

 of events in cosmical history following broadly upon the 

 lines of Laplace's theory. 



Tlie old view of the original matter of the nebulae, that it 

 consisted 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 tlie conver- 

 sion of the gravitational energy of shrinkagj into molecular 

 motion. 



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

 due to a succession of ignited flashes of gas from the encoun- 

 ters of meteoric stones was suggested by Professor Tait, and 

 was brought to the notice of this association in 1871 by Sir 

 William Thomson in his presidential address. 



Tlie spectrum of the bright-line nebute is certainly not 

 such a spectrum as we should expect from the flashing by 

 collisions of meteorites similar to those which have been an 

 alyzed in our laboratories. The strongest lines of the sub- 

 stances which in the case of such meteorites would first show 

 themselves, iron, sodium, magnesium, nickel, etc., are not 

 those which distinguish the nebular spectrum. On the con- 

 trary, this spectrum is chiefly remarkable for a few brilliant 

 lines, very narrow and defined, upon a background of a 

 faint continuous spectrum, which contains numerous bright 

 lines, and probably some lines of absorption. 



The two most conspicuous lines have not been interpreteil, 

 for though the second line falls near, it is not coincident 

 with a strong double line of iron. It is hardly necessary to 

 say that though the near position of the brightest line to the 

 bright double line of nitrogen, as seen in a small spectro- 

 scope in 1864, naturally suggested at that early time the pos- 

 sibility of the presence of this element in the nebulae, I have 

 been careful to point out, to prevent misapprehension, that 

 in more recent years the nitrogen line and subsequently a 

 lead line have been employed by me solely as fiducial points 

 of reference in the spectrum. 



The third line we know to be the second line of the first 

 spectrum of hydrogen. Mr. Keeler has seen the first hydro- 

 gen spectrum in the red, and photographs show that this 

 hydrogen spectrum is probably present in its complete form, 

 or nearly so, as we first learned to know it in the absorption 

 spectrum of the white stars. 



We are not surprised to find associated with it the lineDg, 

 near the position of the absent sodium lines, probably due to 

 the atom of some unknown gas. which in the sun can only 

 show itself in the outbursts of highest temperature, and for 

 this reason does not reveal itself by absorption in the solar 

 spectrum. 



It is not unreasonable to assume that the two brightest 

 lines, which are of the same order, are produced by sub- 

 stances of a similar nature, in which a vibratory motion 



