July, 1903.] 



KNOWLEDGE, 



151 



cosmic development as, iu its essence, a thermodynamic 

 process on the grandest scale. Yet the alliance entered 

 into, fruitful and fortifying though it was, had an 

 attendant embarrassment. Time had now to be reckoned 

 with. In the cosmogonies of Kant, Herschel and Laplace 

 the allowance of aeons was unstinted. Because the rate 

 of change was indeterminate, they might be permitted to 

 elapse ad libitum. But it was otherwise when the driving- 

 power came to be defined. "Conservation of force" 

 iinpUes the measurableuess of force. Equivalence cannot 

 be ascertained where no limits are determinable. Know- 

 ledge, accordingly, regarding the source of the sun's heat 

 brought with it the certainty that the source was by no 

 means inexhaustible. The stock of energy rendered 

 available by shrinkage from a primitively diffuse, to its 

 present compact state, was euomioiLs, but not boundless. 

 The task then became incumbent upon cosmogonists of 

 proving its sufficiency, or of eking out its shortcomings. 

 The prolilem is both retrospective and prospective. We 

 look back towards the birth of the sun, we look forward 

 to its demise; and each eveut has, if possi])le, to be located 

 on our time-scale. Helmholtz assigned terms of twentv- 

 twd millions of years m the past, and seventeen millions 

 in the future, for the shining of our luminary with its 

 actual intensity. Geologists and biologists, however, 

 claimed a much more extended leisure for the succession 

 of phenomena on this globe; and efforts on the part of 

 physicists to meet their demands barely availed to tone 

 down without removing the discrepancy. M. Faye then 

 came to the rescue. His suggestion that the earth toot 

 separate form while the sun was still nebulous, was 

 designed to conciliate the demands of those who needed 

 all but eternity for the slow accumulation into specific 

 differences of infinitesimal variations. In this way, a start 

 was gained upon the sun ; the preparations for vitality 

 on our planet were going forward long before the lavish 

 radiative expenditure designed to nurture its development 

 had begun. The earth, in fact, was shaping itself for its 

 destiny in advance of the epoch when time began to count 

 for the sun. 



This supposed relation of jjrecedence cannot, indeed, be 

 insisted upon ; it was imagined to save a difficult situation, 

 and intimates a design more or less academic. The 

 expedient, however, was significant as regards the effect of 

 the introduction into modern thought of the principle of 

 the conservation of energy. It gave definiteness and a 

 kind of solidity to s]5eculation by widening the basis upon 

 which it was made to rest. At the same time it necessi- 

 tated adjustments between the exigencies of the various 

 sciences, and lirought into prominent view apparent 

 incompatibilities only to be removed by prolonged investi- 

 gations of wide scope and intricate bearings. Modern 

 cosmogony, in short, whde disposing of enlarged means 

 has to meet multiplied exigencies. 



Sir Robert Ball, nevertheless, regards the origin of the 

 solar system chiefly under its mechanical aspect. Like 

 Helmholtz and Faye, he chooses pulverulent materials to 

 work with ; his nebula is a " white nebula." Looking still 

 further back, however, he discerns as its parent an irregular 

 " green" nebula, the jostling movements in which becoming 

 regularised by the elimination of colliding particles, it 

 became flattened <lown into the " ]>lane of maximum 

 areas" — the fundamental i)lane conformed to more and 

 more closely as the energy of a system inevitably wastes. 

 He dispenses with the troublesome process of annulation, 

 and starts his planets virtually by Kant's method of 

 accidental nuclear condensation.* A spiral structure, 

 moreover, would be imparted to the entire nebula by the 



'The Earth's IBofjiiitiinija," [i. 'liT. 



gradual propagation outward of the central acceleration 

 due to contraction. 



But would it have contracted? It had, by supposition, 

 reached the stage of approximate unanimity in movement. 

 The great bulk of its constituent bodies circulated in the 

 same direction, in nearly the same plane, and presumably 

 in orbits not deviating much from circularity. Their 

 aggregate condition might then be regarded as permanent 

 and stable The central mass would, accordinglv, no 

 longer be fed by the engulfment of particles brought to 

 rest by their mutual impacts ; motion being unimpeded, 

 heat could not be evolved ; and the imagined transforma- 

 tion of a disc-like meteoric formation into a sun and 

 planets would fail to come to pass. 



What then, we may ask ourselves, is the upshot of these 

 various efforts at reconstruction ? They establish, certainlv, 

 the unassailable unity of the solar world ; and the solar 

 world must be understood to embrace comets and cometarv 

 meteors. The arguments fav<3uring this unity have gained 

 enormously in cogency through modern discoveries. For 

 those depending upon structural coincidences and har- 

 monies of movement have been reinforced by others of a 

 totally different nature, furni-shed liy the doctrine of the 

 conservation of energy and the teachings of spectrum 

 analysis. The sun is hot because it was anciently 

 exjjanded ; the energy of position formerly Ijelonging to 

 its particles incontestably provided its present thermal 

 energy ; and this amounts to saying that a sphere inde- 

 finitely great was once filled by our inchoate system. The 

 conclusion that it arose from an undivided whole through 

 the gradual differentiation of its parts is further ratified 

 by the identity oE solar and terrestrial chemistry. Thus, 

 the earth once made an integral part of the substance of 

 the sun ; and what is true of the earth is no less true of 

 its sister planets. 



Regarding the modus operandi, however, of cosmic 

 change, there is no consensus of opinion. Pave alone has 

 striven to elaborate a process enduring the modern tests 

 of feasibility, and his theoiy has been well-nigh torn to 

 pieces by adverse criticism. That there was, iu the 

 beginning, a solar nebula, all are agreed ; but whether it 

 was gaseous or pulverulent, whether it shone with inter- 

 rupted or continuous Ught, how it became ordered and 

 organised. Low it collected into spheres, leaving wide 

 interspaces clear, the wisest are perplexed to decide. 



Mr. Moulton concludes from his careful examination of 

 the subject, that " the solar nebula was heterogeneous to a 

 degree not heretofore considered as lieiug probable, and 

 that it may have been in a state " resembling that exhibited 

 in recent photographs of spiral nebula?.* But, even if all 

 the facts do not chime in with this tempting aualogv. 

 there can lie little reason to dissent from his intimated 

 opinion that " the Laplacian hypothesis is only partially 

 true, and that we do not yet know the precise mode of the 

 development of the solar system." 



WHAT IS THE MILKY WAY? 



By C. E.vsTu.v, D.sc. 



The Milky Way, a "bow iu the lieAveus" in monochrome, 

 and permanent, is unfolded on the celestial vault as the most 

 strange and most amazing of optical errors. Supposina; 

 that our retina — an admirable instrument, the most sensi- 

 tive organ with which God has gifted our body, but yet far 

 from being perfect — supposing that it were improved to 

 such a degree that the extremities of its nervous ramifica- 

 tions presented surfaces that were tea or a hundred times 



• AHroph. Jnurn,,/. Vol. \I , p. 130. 



