KNOWLEDGE 



[March, 1903. 



the effects on tlieir development of tbe plasticity of the 

 heavenly bodies. He published in 1754, in a Kiini^'sherg 

 paper, bv way of preliminary to his forthcoming " Natural 

 History," an outline of the woi-kings of tidal friction in 

 the earth-moon system. He saw clearly that it had acted 

 in the jiast to reduce our satellite's rotation to its present 

 minimum rate ; and that it even now, by very slow degrees, 

 tended to retard the spinning of the earth. This brilliant 

 intuition remained unnoticed for well-nigh a century. 



The assertion, however, that Kant's Cosmogony was an 

 anticipatory " Meteoritic Hypothesis " lacks foundation. 

 It is only true in the sense that his building-materials were 

 pulverulent, not " fluid." Laplace's primitive nebula was 

 a coherent mass. It rotated as a whole ; it divided only 

 utider considerable strain ; its separated parts showed 

 concentrative power and individual unity. Kant's elemental 

 matter, on the contrary, was a loose aggregate of in- 

 dependent particles, each pursuing its way, disturbed 

 indeed by its neighbours, but essentially isolated from 

 them. They were, in short, genuine Lueretian atoms, 

 intended to stand for the ultimates of Nature, so far as 

 such obscui-e entities could be evoked by fancy. The chaos 

 that they formed was in nowise a "meteoritic plenum,' 

 unless the phrase be emptied of all distinctive meaning. 

 Meteorites, so far from being primordial units, have the 

 show and semblance of advanced cosmical ju-oducts. They 

 raise special questions in chemistry, mineralogy, geology, 

 and physics, claiming to be dealt with by experts in each 

 branch. Before serving for explanatory purposes, in fact, 

 they tiiemselves need to be explained. 



Laplace enounced his Hypothesis in 1796, and re- 

 published it with supplementary details in 1808. Herschel 

 had meanwhile ascertained the retrograde movement of the 

 Uranian satellite-system, a cii'cumstance highly damaging 

 to the validity of the adopted line of reasoning ; yet its 

 author was content to leave it in jeopardy. He must, to 

 be sure, have regretted that Nature had seen fit to mar the 

 admirable symmetry indicative of her presumed plan of 

 action, running counter thereby to the plaiaiest teachings 

 of the doctrine of probabilities. But he kept his own 

 counsel on the subject, preferring that it should be 

 discussed, as it has been, by posterity ; and posterity has, 

 at any rate, learned that the seeming caprices of Nature 

 are often more instructive than her most harmonious 

 regularity, and has derived a warning from her frequent 

 breaches of continuity against the undue extension of 

 apparently well-grovmded inferences. 



Nevertheless, the constructive scheme handed on by the 

 eighteenth to the nineteenth century has not, so far, been 

 consigned to the Limbo of Vanities. It accorded too pro- 

 foundly with imdoubted realities to be thus summarily 

 disposed of. No one then living had studied the mechanism 

 of the solar system so attentively, or was so intimately 

 acquainted with its workings, as Pierre Simon Laplace. 

 None knew better how admirable, yet how far from inevit- 

 able were the adjustments by which its stability was secured. 

 Long meditation upon their poise and jdan persuaded him 

 that the subsisting congruities of arrangement must have 

 had their source in community of origin. He thus acquired 

 the settled conviction that the sun engendered his cortege, 

 or was together with it engendered from one parent-mass. 

 And this virtually new truth (for Kant's speculation had 

 attracted the minimum of notice) was set forth by him 

 with a directness and lucidity which won for it an imme- 

 diate place among the permanent acquisitions of the 

 human intellect. Few perhaps any longer believe that 

 planetary formation took the precise course laid down for 

 it in the "Systeme du Monde"; but fewer still doubt that 

 the entire ambit of the solar system was once occupied by 

 an inchoate sun, and that its component bodies came into 



being incidentally to that sun's progressive contraction, 

 In favour of this view Laplace could allege no clinching 

 argument ; it recommended itself to him solely through 

 its inherent probability. Unexpected confirmation has, 

 however. l>een afforded to it by the modern theorem of the 

 conservation of energy, applied by Helmholtz with widely 

 illuminative eft'ect to solve the problem of the mainten- 

 ance of solar heat. Laplace assumed an enormously high 

 initial temperature. It was the only way open to him, 

 and he took it. But a transcendentally hot nebula is not 

 easily conceivable; an exalted thermal state seems, and 

 probably is, incompatible with a high degree of attenua- 

 tion. The key to the enigma was given by the demon- 

 stration that a diffuse mass, although actually cold, 

 might contain vast stores of potential heat. There was 

 then no need to postulate a primitive "fire-mist"; the 

 surrendered energy of position amply sufficed to meet the 

 requirements of the case. The temperature of the nebula 

 necessarily rose as it contracted through gravitational 

 stress ; shrinkage and heat-evolution proceeded together ; 

 and they proceed together still. We live by the collapse 

 of the sun. If its piarticles ceased to descend, their 

 incandescence would quickly fail, and terrestrial vitality 

 would become extinct. 



Their number, however, being finite, the store of energy 

 they can supply in falling, even from an infinite distance, 

 is also finite. The process of solar sustentation is then 

 terminable ; it had a beginning, and it will assuredly 

 come to an end. Now the t-^rminus ad quern is of a 

 calculable remoteness; it can be located within certain 

 limits of lime. But the terminvs a quo depends upon too 

 many conditions to be satisfactorily defined It is only 

 certain that the sun is to-day slightly more condensed 

 than it was a year ago. It might, a few millenniums 

 back, have been measurably larger, had modern micro- 

 metrical methods been available in the Stone Age ; while, 

 looking into the geological past, we discern a continually 

 more diffuse globe, filling the orbit of Mercury when the 

 earth was perhaps still red-hot, then successively ampler 

 spheres, out to, and beyond that of Neptune. And here 

 we are confronted with Lap)lace's nebula. The state of 

 things he imagined results, accordingly, from two opposite 

 modes of enquiry, by tracing forward the development of 

 a tenuous rotating mass, and by pursuing backward the 

 surel}' indicated, unceasing and inevitable distension of 

 the sun. Hence, no sooner was it acknowledged that 

 energy may be transformed, but cannot be destroyed, than 

 the Nebular Cosmogony assumed a new and authoritative 

 aspect. 



Its scofie, during the interim, had prodigiously widened. 

 Five years before its promulgation at Paris, Herschel gave 

 at Slough the first hint of a corresponding scheme of 

 sidereal evolution. The discovery of a nebulous star in 

 Taurus (N.G.C. 1514) set him pondering; and he found 

 himself, as the upshot of his meditations, reduced to the 

 dilemma either of concluding nucleus and chevelure to be 

 alike stellar, though composed of stars differing enormously 

 in real magnitude, or of admitting the possession by the 

 star of a voluminous ajipendage constituted of a peculiar 

 and unknown " shining fluid." He chose the latter 

 alternative, adding the jiregnant remark: "The shining 

 fluid might exist independently of stars," and " seems 

 more fit to produce a star by its condensation than to 

 depend on the star for its existence." * 



Thus tentatively , and under the compulsion of phenomena, 

 rather than by the deliberate choice of its inventor, the 

 universal theory of the genesis of stars from nebulae took 

 its rise. Herschel shaped it definitively in 1811 and 1814 



* Shil. Trans., Tol. LXXXI., p. 85. 



