May, 1903.] 



KNOWLEDGE. 



107 



evolutionary history of our system was left in the air, 

 destitute of even the most fragile prop of observed fact. 



From this precarious position it was rescued, partly by 

 the cogent reasonings of Whewell and Herbert Spencer, 

 finally and triumphantly by Sir William Huggins's 

 spectroscopic discovery of the cosmic gas " nebulium." 

 Since August, 1864, there has been no possibdity of 

 denying that the heavens contain ample stores of just the 

 kind of material Laplace wanted, though whether it played 

 just the part he assigned to it, in the manner that he 

 supposed, is a question to be answered with profound and 

 growing reserve. 



An objection of late m-ged against the Nebular Theory, 

 from the standpoint of the kinetic doctrine of gaseous 

 constitution, is of much speculative interest. From a 

 gaseous nebula equal in ma<s to the sun and planets, and 

 distended sufficiently to fill the orbit of Neptune, there 

 should have been, if the prevalent opinion be correct, a 

 rapid leakage into space of its lighter ingredients. Of 

 hydrogen and helium, we are told, it should infallibly have 

 become depleted ; yet there is no lack of either in the sun 

 of the twentieth century. Their retention, it must be 

 admitted, is, on the hypothetical conditions, difficult to 

 account for. The "critical velocity" at the limiting surface 

 of the supposed nebida would have been 48 miles a second. 

 Tliis is, in fact, at the distance of Neptune, parabolic 

 speed. The planet itself, if it could attain to it, would 

 break the bonds that bind it to the sun, and seek its 

 fortunes under some different allegiance. Similarly, any 

 particle <if the primitive nebula thus accelerated should 

 have become an irreclaimable vagrant. Now the velocity 

 of hydrogen-molecules at the zero of Centigrade is, in the 

 mean, about 1 J miles a second, Init attains, in the extreme, 

 to above seven miles. Hydrogen could not then have been 

 permanently retained by the solar nebula; and the escape 

 of helium would have more slowly ensued. Yet these 

 results, though seemingly inevitable, did not, in actual 

 fact, come to pass ; either Ijecause the generating body 

 was diiferently constituted from what has been supposed, 

 or because countervailing influences were brought to bear. 

 It is, for instance, amply possible that the dynamical con- 

 dition of gases may be essentially modified by rarefaction 

 carried to a degree transcending the range of experimental 

 investigation. The progress of science affords many warn- 

 ings against trusting implicitly to the rule of continuity. 

 Curves of change seldom preserve indefinitely a uniform 

 character. Their unexplored sections may include quite 

 unlooked-for peculiarities of Hexure ; and the possil>ility 

 seriously undermines confidence in inferences depending 

 upon " extrapolation." The presence of hydrogen and 

 helium in our system cannot, then, be ranked among fads 

 incontestably contradictory of the Nebular Hypothesis. 



The concerted advance of mathematical astronomy during 

 the eighteenth century was effected with the confident 

 serenity of irresistible power. One after another, the 

 obstacles barring its path went down before repeated and 

 skilful onslaughts, the uubrol;en succession of wliidi lends 

 a certain exultant sameness to the story of the heroic ago of 

 analysis. The Mecanique Ci' teste attested " victory all along 

 the line.'' There were no more worlds to conquer that 

 Laplace knew of; the reign of gravitational law was firmly 

 establisheii throughout the solar dominions ; menaced 

 revolts hal beau appeased; anomalies removed; no extant 

 obiervations any longer impaired the perfect harmony 

 between what was anl what had been antic ipate<l. Nature 

 for th'; moment submitted readily t) the trammels put 

 upon her l)y human tliought: her intricacies no longer 

 seem()d to defy uuravehnent ; her moles of procedure 

 looked straightforward and iutalUgible. As they were 

 julgjl to bs in the present, so th^y might bs prjsamei to 



have been in the past ; and the temptation was irresistible 

 to adventure backward speculation, inferring initial con- 

 ditions from the elaborated product laid open to scrutiny. 



It was an epoch of peremptory renewals. The formula 

 of equality promised to regenerate society ; a political 

 panacea had been found by the creation of a Republic 

 "one and indivisible"; and the success of the guillotine in 

 securing its supremacy was almost outdone by the triumphs 

 of the calculus in vindicating the ixoimpeded sway of 

 gravitation. 



Humanity had made a fresh start ; science should do 

 likewise. The sanguine spirit of a rejuvenated world 

 animated all forms pf, human endeavour. It has long 

 since evaporated. Tiir biiiiyaiit hopes i.if a century back 

 have been crusln'il ; tin- future .■!■ civili/ati.m looks dim; 

 the future of lijnowlf-il^'f in ^'oiupiouii-Ml hv its uncertainty. 

 But we, at any rait, no loiij^'er ilflmlo omselyes with the 

 idea that he who runs niiiy rcu'l the secrets of the 

 universe; «e have learned by coaviuciug experience how 

 much, and how variously, "the subtlety uf nature trans- 

 cends the subtlety of sense and intellect'; we are vividly 

 aware that there is no single and simple recipe for the 

 " cosmification " of chaos. 



That devised by Laplaee has ceased to 1« satisfactory. 

 Its simplicity, at first sight so seductive, leaves it at a 

 disadvantage compared with the intricacy of the effects it 

 was designed to elicit. The relations claiming explanation 

 have multiplied with the progress of research. Those of 

 the dynamical order were alone attended to by the 

 geometers of the eighteenth century, and even they 

 have grown recalcitrant ; while those of a physical and 

 chemical kind have proved wholly unmanageable. It has 

 indeed become abundantly clear that the series oi opera- 

 tions described liy Laplace could scarcely, under the most 

 favourable circumstances, have been accomplished, and 

 in a thin nebulous medium would have been eutirely 

 impossible. The Nebular Cosmogony has not then stood 

 " Foursquare to all the winds that blew." 



Its towers and liattlements have crumbled liefore the 

 storms of adverse criticism. It survives only as a wreck, 

 its distinctive features obliterated, although with the old 

 flag still Hying on the keep. In the nextfhapter we shall 

 attempt a survey of the works set on foot for ita 

 reconstruction. 



Itetters, 



[The Editors do not hold themselves responsible for the opinions 

 or statements of correspondents.] 



MAN'S PLACE IN THE UNIVERSE. 



TO THE EDITORS OF KNOWLXDGK. 



Sirs, — As I do not wish your readers to suppose that I 

 have altered the words of an author whom I quote in order 

 to make them agree more closely with my own opinions, 

 will you permit me to state, in reply to Mr. Maunder's 

 criticism of my article in the Fortnigttttij Review, that 

 when that article was sent to press I had not seen 

 Prof. Newcoml)'s book on "The Stars"; but I quoted 

 from his "Chapters on the Stars," which appeared in The 

 Pojyular Science Moiillity. The quotation beginning "If 

 we should blot out," is verbatim, as at page 323 of that 

 periodical for January, 1001, except that I have, inadver- 

 tently, substituted " Milky Way ' for " galaxy." 



A "writer in the Daity Neirs of March 31st has so well 

 answered Mr. Maunder's criticisms that it is imnecessarr 

 for me to refer to them here. I am, however, indebted 

 both to Mr. Maunder and to Prof. H. H. Turner, who has 

 criticised my article in this mouth's Fortnightly, b.^cause. 



