Note 14 — Chapter III

“Laplace conjectures that in the original condition of the solar system, the sun revolved upon his axis, surrounded by an atmosphere which, in virtue of an excessive heat, extended far beyond the orbits of all the planets, the planets as yet having no existence. The heat gradually diminished, and as the solar atmosphere contracted by cooling, the rapidity of its rotation increased by the laws of rotatory motion; and an exterior zone of vapour was detached from the rest, the central attraction being no longer able to overcome the increased centrifugal force. This zone of vapour might in some cases retain its form, as we see it in Saturn’s ring; but more usually the ring of vapour would break into several masses, and these would generally coalesce into one mass, which would revolve about the sun,”—Whewell’s Bridgewater Treatise.

The following passage is translated by the same author from Laplace:—

“The anterior state (a state of cloudy brightness) was itself preceded by other states, in which the nebulous matter was more and more diffuse, the nucleus being less and less luminous. We arrive in this manner at a nebulosity so diffuse, that its existence could scarce be suspected. Such is in fact the first state of the nebula which Herschel carefully observed by means of his telescope.”

Sir William Herschel has the following observations on these remarkable masses:—

“The nature of planetary nebulæ, which has hitherto been involved in much darkness, may now be explained with some degree of satisfaction, since the uniform and very considerable brightness of their apparent disc accords remarkably well with a much condensed, luminous fluid; whereas, to suppose them to consist of clustering stars will not so completely account for the milkiness or soft tint of their light, to produce which it would be required that the condensation of the stars should be carried to an almost inconceivable degree of accumulation.

“How far the light that is perpetually emitted from millions of suns may be concerned in this shining fluid, it might be presumptuous to attempt to determine; but notwithstanding the inconceivable subtilty of the particles of light, when the number of the emitting bodies is almost infinitely great, and the time of the continual emission indefinitely long, the quantity of emitted particles may well become adequate to the constitution of a shining fluid or luminous matter, provided a cause can be found that may retain them from flying off, or reunite them.”—Observations on Nebulous Stars: Philosophical Transactions, vol. lxxxi. a.d. 1791.

In addition, the following Memoirs on the same subject, by Sir William Herschel, have been published in the Philosophical Transactions:—Catalogue of 1000 Nebulæ and Clusters of Stars, vol. lxxvi.; Catalogue of another 1000, with remarks on the Heavens, vol. lxxix.; Catalogue of 500 more, with remarks as above, vol. xcii.; Of such as have a cometary appearance, vol. ci.; Of planetary nebulæ, ibid.; Of stellar nebulæ, ibid.; On the sidereal part of the heavens, and its connection with the nebulous, vol. civ.; On the relative distances of clusters of nebulous stars, vol. cviii.