NEBULAR HYPOTHESIS 201 (contrary to the usual statement in our text books of astronomy) that many nebulae form parts of our own star system. At a later stage of his labors he advanced the hypothesis com- monly known as Herschel's nebular hypothe- sis, which however related only to certain or- ders of nebulae. At this stage Herschel for the first time indicated his ideas respecting the arrangement of all orders of stellar aggrega- tions and nebulous matter. At the lower ex- tremity of the scale he placed widely spread luminosity, such as he had first described in 1802. He passed from this irregularly spread luminosity, through all the orders of gaseous nebulae (irregular nebulae, planetary nebulae, nebulous stars) formed by the gradual conden- sation of the gaseous matter, until the star it- self is formed ; then he entered on the part of the series he had before recognized, passing on to the various orders of stellar aggregation, diffused clusters, ordinary stellar nebulae, and more and more condensed groups of stars, up to the richest star clusters. At this period (1814) we no longer find him speaking of ex- ternal nebulae ; not, it is to be presumed, that he no longer recognized the probability that other stellar galaxies besides our own exist, but that he no longer found it possible to dis- criminate those nebulae which are external from the far greater number which unques- tionably form component parts of our own sidereal system. The researches of the pres- ent writer into the subject dispose him to believe that our sidereal system extends far beyond the limits which have ordinarily been assigned to it, and that there are no nebulae which can be regarded as external to it. NEBULAR HYPOTHESIS, the celebrated spec- ulation of Sir William Herschel, adopted and developed by Laplace, assigning the genesis of the heavenly bodies to the gradual aggrega- tion 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 nebulae hitherto consider- ed to be irresolvable, and exhibited symptoms of resolvability in others still more intracta- ble, it was assumed that all nebulae are stellar, their nebulosity being solely a question of dis- tance ; and thus, the basis of Herschel's rea- soning failing, the fabric of his hypothesis was thought to be demolished. Mr. Herbert Spen- cer came to its support in the " Westminster Review," No. cxxxvii. (July, 1858). The ar- gument in its favor is substantially as follows. The assumption that all nebulae 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 nebulae furnishes evidence of a physical connection with our stellar system; and this evidence is confirmed by the fat of their re- solvability with telescopic power which fails to make individually visible the most distant stars of our own milky way. If they are re- mote galaxies, it may be assumed that, speak- ing generally, the largest are the nearest, and therefore the most resolvable. But the fact is, the smallest are the most resolvable. An- other difficulty is presented by the Magellanic clouds. (See NEBULA.) 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 dif- fering in proportion more than as 9 to 10." (" Outlines of Astronomy," London, 1851, p. 615.) This clearly supplies a reductio ad ab- surdum of the popular doctrine. Assuming, for the sake of the argument, a rare, homo- geneous, 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 con- sequent precipitation of binary atoms, aggre- gating into irregular flocculi and floating in the rarer medium, just as water when precip- itated 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 pre- ponderating momentum and rotation of the whole mass in some one direction, converging in spirals toward the common centre of grav- ity. 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 con- clusions are shown to be in entire harmony with the observed phenomena. In this ge- netic 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 coa- lesce with the larger internal masses, but will slowly follow without overtaking them. These fragments will assume characteristics of mo- tion strikingly correspondent to those of the comets, whose physical constitution and dis- tribution are seen to be completely accordant with the hypothesis. The physical characters resulting from the hypothesis are found to tally with the facts. In a rotating spheroid of aeriform matter in the latter stages of
