NEBULAR HYPOTHESIS, the celebrated
speculation of Sir William Herschel, adopted and
developed by Laplace, assigning the genesis of
the heavenly bodies to the gradual aggregation
and condensation of a highly attenuated
self-luminous substance diffused through space. (See “Philosophical Transactions,” 1802 and 1811.) To this hypothesis Herschel was led by his conclusion that there were nebulosities not composed of stars. The Rosse telescope having decomposed nebulæ hitherto considered to be irresolvable, and exhibited symptoms of resolvability in others still more intractable, it was assumed that all nebulæ are stellar, their nebulosity being solely a question of distance; and thus, the basis of Herschel's reasoning failing, the fabric of his hypothesis was thought to be demolished. Mr. Herbert Spencer came to its support in the “Westminster Review,” No. cxxxvii. (July, 1858). The argument in its favor is substantially as follows. The assumption that all nebulæ are remote galaxies does not invalidate the indications furnished by the structure of the solar system, which still points to a nebular origin just as significantly as before. But the assumption is inadmissible. The mode of distribution of the nebulæ furnishes evidence of a physical connection with our stellar system; and this
evidence is confirmed by the fact of their
resolvability with telescopic power which fails
to make individually visible the most distant
stars of our own milky way. If they are
remote galaxies, it may be assumed that, speaking
generally, the largest are the nearest, and
therefore the most resolvable. But the fact
is, the smallest are the most resolvable. Another difficulty is presented by the Magellanic clouds. (See .) Sir John Herschel, considering the structure of the larger of these clouds, concludes that “it must be taken as a demonstrated fact that stars of the seventh or eighth magnitude, and irresolvable nebula, may coexist within limits of distance not differing in proportion more than as 9 to 10.” (“Outlines of Astronomy,” London, 1851, p. 615.) This clearly supplies a reductio ad absurdum of the popular doctrine. Assuming, for the sake of the argument, a rare, homogeneous, nebulous matter, widely diffused through space, the following successive changes will, on physical principles, take place in it: 1, mutual gravitation of its atoms; 2, atomic repulsion; 3, evolution of heat, by overcoming this repulsion; 4, molecular combination, at a certain stage of condensation, followed by, 5, sudden and great disengagement of heat; 6, lowering of temperature by radiation and consequent precipitation of binary atoms, aggregating into irregular flocculi and floating in the rarer medium, just as water when precipitated from air collects into clouds; 7, each flocculus will move toward the common centre of gravity of all; but being an irregular mass in a resisting medium, this motion will be out of the rectilinear, that is to say, not directly toward the common centre of gravity, but toward one or other side of it; and thus, 8, a spiral movement will ensue, which will be communicated to the rarer medium through which the flocculus is moving; and, 9, a preponderating momentum and rotation of the whole mass in some one direction, converging in spirals toward the common centre of gravity. Certain subordinate actions are to be noticed also. Mutual attraction will tend to produce groups of flocculi concentrating around local centres of gravity, and acquiring a subordinate vertical movement. These conclusions are shown to be in entire harmony with the observed phenomena. In this genetic process, when the precipitated matter is aggregating into flocculi, there will be found here and there detached portions, like shreds of cloud in a summer sky, which will not coalesce with the larger internal masses, but will slowly follow without overtaking them. These fragments will assume characteristics of motion strikingly correspondent to those of the comets, whose physical constitution and distribution are seen to be completely accordant with the hypothesis. — The physical characters resulting from the hypothesis are found totally with the facts. In a rotating spheroid of aëriform matter in the latter stages of