kno\vli:dgi:. 



Novemukk, 1912. 



Iiavc no excessive inopi-r nidtions, showing' tliat tlie 

 masses of their nebulae are very small. " The 

 ni'hula of .\nclromeda or the great nebula of Orion 

 must exceed in volume the whole space occui)ied b\- 

 our solar system, many tiiousands, perhaps millions, 

 of times." Such enormous olijects cannot remain at 

 a hii^h temperature, for their heat would be quickly 

 radiated into space ; and consequently the view that 

 the neinilae in general are highly heated masses of 

 fluid in hydrostatic equilibrium is being slowly, but 

 surely, abandoned. Long ago (in 1861) Habinet, of 

 the " Institut," by simple arithmetic showed that 

 no such nebula as Laplace had supposed could have 

 given rise to the Earth and other planets from rings 

 of abandoned matter left behind during its condensa- 

 tion ; for the actual speed of these bodies is much 

 quicker than can be thus accounted for, and " the 

 hyjiothetical solar nebula could not have rotated w ith 

 suflicient speed to detach the masses." Thus, such 

 a doctrine of the evolution of the planets by separa- 

 tion of rings of matter from the central condensation 

 by rotational instability must be abandoned. Some 

 have proposed as an alternative that sccondarv 

 condensation nuclei might be formed by i^njx'ifcjfioitcil 

 instability within the gaseous "nebula": such a 

 doctrine has been outlined by Mr. J. H. Jeans, of 

 Cambridge, in papers which he has contributed to 

 the Pliilosopliical Transactions of f lie Royal Society. 

 Mainly by the work of Professor See during the 

 last few vears a more rational and consistent 

 cosmogonv has been built up. We know that there 

 are vast numbers of spiral nebulae scattered all over 

 the sky, and it is supposed that our solar system 

 has been formed from such, and not from a spherical 

 or ellipsoidal mass gradually condensing and con- 

 tracting. Within such a mass, " a discontinuous 

 cosmical cloud, with vortices formed of streams 

 circulating without exerting hydrostatic pressure 

 between the coils " two or more streams of particles, 

 " cosmical dust," meeting gi\e rise to such a spiral 

 nebula, and it is possible that collision and friction 

 between their parts may give rise to a feeble 

 luniinositv, as suggested by Sir Xorman Lockyer 

 in his '■ Meteoritic Hypothesis." Two opposite 

 branches of the spirals thus originate by " the 

 meeting of separate streams or by the settling of 

 one stream towards the centre, so that the branches 

 coil up as they condense" (See). The varied forms 

 of spiral nebulae represent various stages in the 

 development of these bodies, and we may thus form 

 some estimate of their relative antiquit\'. By actual 

 tneasurement of a number of photographs, however. 

 Professor See has concluded that their forms are 

 only roughly approximate to true geometric spirals, 

 a mixture of the Logarithmic and .\rchimedean 

 spirals giving an approach to the most common 

 forms observed. " The older nebulae tend to 

 approach the form of the Spiral of .Vrchimedes, the 

 newer more nearly resemble that of the Logarithmic 

 Spiral, but neither form is exactly observed." MaTiy 

 irregularities occur, as might perhaps be exi^ccted, 

 so that he concludes that in reality their true forms 



are " chance spirals, and necessaril\- depart from an\' 

 kind of geometric regularity." 



I'rom such a system our own has probably 

 developed. Herbert Spencer was probably one of 

 the earliest to point out that a number of flocculent 

 nebulous masses falling towards a centre would 

 assume the spiral form at a time when few spiral 

 nebulae were known to exist (1858). It is some- 

 what remarkable that Spencer, whose views on manv 

 physical questions were so sharply and justly 

 criticized by Tait, J. F. Moulton and others, should 

 have nevertheless held views on some matters in 

 advance of his critics, specialists in their subjects, 

 just as a century earlier Euler, the great mathematician, 

 held more correct views on man\- physical questions 

 than contemporary physicists. Within thisnebulosity 

 processes tending for division are going on. In 

 many cases we have a division into two more or less 

 nearly equal masses and we have then a double star, 

 of which many examples are to be found in the sk\-. 

 In others the greater part of the material becomes 

 condensed towards the centre, smaller portions onlv 

 elsewhere, and we have a predominant Sun and com- 

 paratively small planets, as in our own system. I5y 

 degrees much of the rest of the material will be 

 gradually sw ept up by the larger bodies, but a portion 

 will remain, the comets, meteorites and matter of 

 the Zodiacal Light (?) Meanwhile this, acting as a 

 resisting medium, will render the orbits of the planets 

 and their satellites more nearly circular, at the same 

 time slightly reducing their distance from the central 

 body. Such an action of resistance was well known 

 to Laplace, who gave a mathematical proof of it in 

 his " Mecanique Celeste" (Book X). It has been 

 supposed that Encke's well-known comet, whose 

 return we have recently witnessed, is gradually draw- 

 ing nearer to the Sun b}- such an action. However 

 this may be, the secular action of a resisting medium 

 affords the most satisfactory- explanation of the 

 I)resent almost perfect circularity of the planetarj- 

 orbits. 



The views we have here brietl\' outlined invest 

 spiral nebulae with a paramount interest, for in 

 their development, movements and condensation 

 we ma\- trace the processes which have resulted 

 in the evolution of our own solar system. Just as 

 in the forest we observe \egetation in all stages 

 of growth, from the nuts and seeds, through the 

 \-oung and tender saplings, to the full-grown adults, 

 next the "giants of the forest," and, lastly, the 

 moss-grown oaks and decaying remains of former 

 life, so, too, in the starry heavens we may expect 

 to find worlds, past, present, and to come. A 

 favourite scale gave the nebulae as worlds coming 

 into being, white stars, such as ^'ega, as youthful 

 orbs, our own Sun as a specimen of a later stage, 

 though still intensely hot and luminous : wd stars 

 older still in the course of developnuiit. tlie planets 

 and *' dark " suns, as hot only in tlu-ir interiors, anti 

 lastK' "dead" worlds, like our own Moon. lUit 

 such an arrangement, though admired and repeated 

 by popular w riters, has little support in facts ; it is 



