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UNIVERSE

bipolar-spiral, judging from its branches and from two principal star-drifts in opposite directions. The vast system calls for an explanation of its origin: a stellar cosmogony. Here again, as in planetary cosmogony, our terrestrial home seems to present a singularity. Science urges the conclusion that one half of the stars, if not most of them, broke up into components as they condensed, a manner of evolu- tion which would incapacitate them from becoming centres of planetary systems. Stellar cosmogony must leave the question open, whether the mechanism of our own system was not the outcome of special and peculiar design, fitting it to be the abode of life.

(d) Yet even the stellar agglomeration of the Milky Way is a tiny spot in the abysmal cosmos. From near its centre, where we find ourselves at present, the heavens appear studded with similar groupings of mas-ses, partly gaseous, partly condensed into fluid streams or solar clusters. Since Herschel gauged the heavens, more than thirteen thousand of these objects have been catalogued, and hundreds of thousands are suspected. Classifying them into diffused, spiral, and planetary nebula?, Herschel considered them as so many simultaneous exponents of gradual cosmic evolution, thus showing his belief in the possibility of some universal cosmogony. The belief has since been strengthened by a wider knowledge of the ultra- sidereal world. Photography shows the heavens almost covered with nebulous matter, spectrum analysis reveals the general identity of cosmical ele- ments, and has moreover disclosed the fact that planetary nebula? move at great speed with reference to the stellar system, while the diffused Orion nebula remains at rest. The necessity of some comprehen- sive cosmogony is apparent. Attempts in that direction have not been wanting, as we shall presently see.

(2) The classification of cosmogonies by the origin which they ascribe to the world, may appropriately rest on certain celestial objects, from which they took their inspiration. These are Saturn's rings, at first believed to be coherent masses, whether gaseous or fluid or .solid; then the same rings as recognized (by Bond, 1S.")1) to be a swarm of minute sateUites; and finally the spiral nebula?. The differences in the inspiring t>'pes led to corresponding differences in the predominant ideas of cosmogonists. Coherent rings demanded hydrodynamic treatment, pulverulent rings suggested meteoric theories, and spiral nebula; prompted balhstic speculations. Hydrodynamic cos- mogonies confined themselves to the solar system; mefporic cosmogonies made faint attempts towards the stellar system, and only ballistic cosmogonies have dared to speculate on the undivided cosmos.

(a) F'irst among the hydrodynamic cosmogonies is the "nebular hypothesis", imagined by Kant (17.5.5), partly in contradiction with mechanical principles. The application of the hydrodynamic laws was reserved to Laplace (1798). His mechanism is too simple, however, for the complex problem. Objec- tions were raised by Babinet (1.861), Kirkwood (1869), Moulton (1900), and others. Roche (Montpellier, 1873) even fixed a limit for each primary planet, within which a liquid satellite cotild not revolve intact. .Saturn's rings lying inside (he limits, thus failed to accomplish what K.ant and Laplace had expected. The field of cosmogonic possibilities was widened by Darwin and Poincarc'- (1879-188.5), when they intro- duced planetary tides, pear-.shaped hydrodynamic surfaces and .sulcUite fission; and again by C. Braun flSS7- 190.5), when he pointed to plurality of condensa- tion centres, to excentric collisions, and to the result- ant fffeet between resisting mrdium and hydrostatic pressure. The applicability of Darwin's .speculation to our lunar-terrestrial system, and to binary systems in general, h.as been questioned by Moulton!

(b) The baaes of meteoric cosmogoniea are the

asteroidal composition of Saturn's rings and the affinity between meteors and comets, discovered by Schiaparelli. Meteors were no longer the debris of ruined worlds, they became the embryos. Nebula^, stars, comets, zodiac light, solar corona, all originated from meteoric clouds. Life was brought into the chaos of cosmic dust, cold and dark as it was from the first, by a devious variety of motions, after the fashion of Descartes's vortices, resulting in collisions, evapora- tions, condensations, and consequent production of heat. Sims were forming, Newton's gravitational law set in, and the masses began to comport them- selves in the manner imagined by Laplace. Meteoric cosmogonies thus distinguished two periods: the Car- tesian and the Newtoni.an. The quiet machinery of Laplace's annulation is preceded by a primeval whirlpool period. Representatives of meteoric cos- mogony are Faye (1884), Lockyer (1887), and Li- gondes (1897), while Kirkwood, Wolf, and Braun oppose it. Darwin tried to support it by applying the kinetic theory of gases to cosmic matter in mete- oric condition, treating its particles as molecules on an enormously magnifled scale. Belot (1911) has recently imagined a Cartesian whirlpool of cylindrical form, shooting like a torpedo into an amorphous nebular mass, in the direction of the solar apex. The effects of the impact on the cyhnder are longitu- dinal vibrations with nodes, the embryos of the future planets.

(c) Ballistic cosmogonies take their pattern from nebulise. The spiral form of most nebula-, with inter- spersed nuclear condensations, opened the widest field for coUision, ejection, and capture theories. Herschel did not venture on any hjfjjothesis, although he believed in stellar growth from chaotic nebulous matter by a process through diffused spiral and planetary nebulae. Even to-day science has not proved the transition from the nebular to the stellar condition of any celestial object. It is true, the bijiolar structure of spiral nebula>, disclosed in recent years by photography, has greatly strengthened the idea of violent cosmogonic formations. Collision theories were propounded by Chamberlin and Moul- ton (1905) and by Arrhenius (1907). The process in a nebula- begins with nuclear condensations, which are followed by excentric collisions or disruptive approaches. Bipolar systems of streams are thus produced and, if combined with concurrent rotation, spiral nebula? are formed. Collisions are repeated on a smaller scale by the accretion of scattered materi.al around denser nuclei. The further development partly overlaps with the hypothesis mentioned next.

An ejection theory is mentioned by Laplace, as due to Buffon, who supposed comets to fall into the sun and spla.sh solar matter into space. A more scientific form is given to the theory by Wilde (1910). In the right and left .spiral streams of nebulx, in their inter- spersed stellar condensations, and in the elongated fi.ijsions of their convolutions, he recognized processes like the eruption of solar protuberances, or the up- lifting of terrestri.al continents, or the impacts at- tested by lunar craters. Planets and satellites are ejerttimenla from exploded primaries. A capture theory was invented by See (1910). The parent of the solar system is a spiral nebula. Sun, planets, satellites, and comets originate from nuclear condensa- tions, but their grouping into regular order is due to the capturing of the smaller by the larger. The out- lying wisps of the solar nebula appear as comets.

What precedes is more a cla.ssification than a description of the various cosmogonies. None of them has foimd universal acceptance, and none has escaped criticism.

B. Cosmoilysii. — This is the proposed name for all the hypotheses on tlie future of the world, as explained in the introduction to section II. The literature on cosmodysy is far lesa extensive than that on cosmog-