Jakuaky, 1904.] 



KNOWLEDGE. 



{ilane of tlie svsttMii. whicli would thus oonsist of a rotatinrr 

 inioleus eni'oinpassfd bv a wido zone of indejiondently 

 ciix-uhitiusj metforitos. But tliis mode of dt>velo|iniout is 

 not even ap}>roxiinately followed l>v ooinets. It would be 

 pt)ssible only if they were isolated in space, and. in point 

 of faet. their revolutions roiuid thosuuareof overwhelming 

 importanee to their destinies. The suu"s repulsive energy 

 eauses theni to waste aud diffuse with expansion of splendid 

 plumage. Under the sun's unequal attraction at close 

 quarters they are subject to disruption, and the upshot of 

 the tidal stresses acting upon them is the dispersal of their 

 constituent particles along the wide ambit of their oval 

 tracks. 



We are, however, invited to look further afield. C'ometary 

 meteor-swarms may be only miniature specimens of the 

 contents of space. Why should not remote sidereal regions 

 be thronged with similar assemblages, colossal in their 

 jiroportions, countless in number? And may they not 

 supply the long-sought desideratum of a suitable "world- 

 stuff" for the construction of suns and planets? From some 

 such initial considerations as these. Sir Norn\an Lockyer 

 developed, in 1887, an universal Meteoritic Hypothesis, 

 designeil on the widest possible lines, based on promising 

 evidenc«, and professing to supply a key to the baifling 

 enigma of cosraical growth aud diversification. The 

 meteoric affinities of comets formed its starting point ; 

 comets were assimilated to nebulw ; and from nebuhe were 

 derived, by gradual processes of change, all the species of 

 suns accessible to observation. The view was of far-reaching 

 import and magnificent generality, but its value avowedly 

 rested on a marshalled collection of facts of a special kind. 

 In this it differed from the crowd of ambitious speculations 

 regarding the origin of things by which it had been 

 preceded. In this, it attained an immeasurable superiority 

 over them, if only the testimony appealed to could be 

 proved valid. Indeed, it is scarcely too much to say that, 

 whether it were valid or not, the mere circumstance of 

 having called the spectroscope as a witness in the high 

 court of Cosmogony constituted an innovation both 

 meritorious and significant. 



The spectrum of the nebula; was a standing puzzle. A 

 theory which set out by making its meaning plain secured 

 at once a privileged position. This was seemingly accom- 

 plished by Sir Norman Lockyer through the means of 

 some simple laboratory e.xperiments on the spectra of 

 meteorites. Certain "low temperature" lines of magnetism 

 given out by the vapours of ston}' aerolotic fragments 

 were shown to fall suspiciously close to the chief nebular 

 lines previously classed as " unknown." The coincidences, 

 it is true, were determined with low dispersion, and were 

 published for what they were worth ; but they looked 

 hopeful. Their substantiation, had it been possible, 

 would have marked the beginning of a new stadium of 

 progress. Nature, however, proved recalcitrant. The 

 suggested agreements avowed themselves, on closer enquirj', 

 as approximate only ; magnesium-light makes no part of 

 the nebular glow, and nebulium, its main source, evades 

 terrestrial recognition. The light of cosmic clouds is, in 

 fact, gui yeneris ; it includes no metallic emissions ; while 

 the fundamental constituents of meteorites are metals 

 variously assorted and combined. 



The decipherment of the nebular hieroglyphics was the 

 crucial test ; its failure to meet it left the hypothesis 

 seriously discredited ; for coincidences between spectral rays 

 common to nearly all the heavenly bodies naturally counted 

 for nothing. Yet the investigation had its uses. The energy 

 with which it was prosecuted, the ingenuity and resource 

 with which it was directed, told for progress. There has 

 been a clash of arms and a reorganisation of forces. 

 Thought was stirred, observation and experiment received 



a strong stimulus, fresh affluents to the great stream of 

 science began to be navigated. Efforts to prove what had 

 been asserted wert> fruitful in some directions, and the 

 work of refutation had inestimabUi value in defining what 

 was ailiuissible, aud establishing unmistakable landmarks 

 in astrophysi('S. 



The discussion, however, threw very little light on the 

 part played by m(>teorites in Cosmogony. Their world- 

 building function remains largely speculative. Doubts of 

 many kinds qualify its possibility, and lend it a fantastic 

 air of unreality. 15ut this may in jiai't be due to a defect 

 of imaginative power with which the universe is not con- 

 cerned. 



Waiving, then, i>reliiuinary objections, we find ourselves 

 confronted with the essential (piestion : Given a meteor- 

 swarm of the requisite mass and dimensions, is there 

 any chance of its coiulensiug into a planetary system ? 

 Sir Norman Lockyer sot aside this branch of his suliject. 

 His hypothesis was in fact " pre-iu^buhxr." He assumed 

 that the small soli<l bodies with which it started would, in 

 course of time, become completely volatilised by the heat 

 of their mutual impacts, and that the resulting ga.seous 

 mass would thenceforward comport itself after the fashion 

 prescribed by Laplace. Professi>r Darwin regarded the 

 matter otherwise. It seemed to him possible to combine 

 the postulates of the meteoric and nebular theories in a 

 system planned on an original principle. F'or this purpose 

 it was necessary to excogitate a means of rendering the 

 kinetic theory of gases availabh; for a meteor-swarm. 

 " The very essence," he wrote,* " of the nebular hypothesis 

 is the conception of fluid pressuris since without- it the 

 idea of a figure of equilibrium becomes inapplicable." 

 M. Faye abandoned this idea; he built up his planets out 

 of incoherent materials, thereby avoiding the incongruities, 

 but forfeiting the logical precision, of Laplace's stricter 

 procedure. Prof. Darwin consented to forfeit nothing; he 

 stood forward as a syncretist, his object being to " point out 

 that by a certain interpretation of the meteoric theory wo 

 may obtain a reconciliation of these two orders tif ideas, and 

 may Injld that the origin of stellar and planetary systems 

 is meteoric, whilst retaining the conception of fluid 

 pressure." For the compassing of this end, he ado[>ted a 

 bold expedient. Fluid pressure in a gas is " the average 

 result of the impacts of molecules." Fluid pressure in a 

 meteor- swann might, he conceived, be the net product, of 

 innumerable collisions lietweeu liodies to be regarded as 

 molecules on an enormously inagiwAed scale. The sup- 

 ])osition is, indeed, as Kepler said of the distances of the 

 fixed stars, " a liig ])ill to swallow." From molecules to 

 meteorites is a long leap in the dark. The machinery of 

 gaseous impacts is obscure. It can be set in motion only 

 by ascribing to the particles concerned properties of a 

 most enigmatical character. These i>articles are, however, 

 unthinkably minute ; and in sub-sensible regions of 

 research, the responsibilities of reason somehow become 

 relaxed. We are far more critical as to the behaviour of 

 gross, palpable matter, because experience can there be 

 consulted, and is not unlikely to interpose its veto. 

 Meteorites are (hjubtless totally dissimilar from molecules, 

 however many million-fold- enlarged ; and they would 

 infallibly be shattered by collisions which only serve to 

 elicit from molecules their distinctive vibrations. More- 

 over, the advance of the shattering process would admit- 

 tedly end the prevalence of fluid pressure. So that the 

 desired condition, even if initially attained, would be 

 transitory. There is, besides, a ra<lical difference between a 

 group of bodies in orbital circulation and a collection of 

 particles moving at hap-hazard, unconstrained by any 



* Proceedingn of the Rot/al Society, Vol. XIV., p. i. 



