Facts accounted for by it.For it follows from the nebular hypothesis that the large planets should rotate rapidly, and the small ones more slowly; that the outer planets and satellites should be larger than the inner ones. Of the satellites of Saturn, the largest is the outermost; of those of Jupiter, the largest is the outermost save one. Of the planets themselves, Jupiter is the largest, and outermost save three. These cannot be coincidences, [282] but must be due to law. The number of satellites of each planet, with the doubtful exception of Venus, might be foreseen, the presence of satellites and their number being determined by the centrifugal force of their primary. The hypothesis also points out the time of revolution of the planets in their orbits, and of the satellites in theirs; it furnishes a reason for the genesis and existence of Saturn's rings, which are indeed its remaining witnesses—their position and movements answering to its requirements. It accounts for the physical state of the sun, and also for the physical state of the earth and moon as indicated by their geology. It is also not without furnishing reasons for the existence of comets as integrant members of our system; for their singular physical state; for the eccentric, almost parabolic orbits of so many of them; for the fact that there are as many of them with a retrograde as with a direct motion; for their more frequent occurrence about the axis of the solar system than in its plane; and for their general antithetical relations to planets.
Whether nebulæ actually exist.If these and very many other apparently disconnected facts follow as the mechanical necessities of the admission of a gravitating nebula—a very simple postulate—it becomes important to ascertain whether, by actual observation, the existence of such material forms may be demonstrated in any part of the universe. It was the actual telescopic observation of such objects that led Herschel to the nebular hypothesis. He concluded that there are two distinct kinds of nebulæ, one consisting of clusters of stars so remote that they could not be discerned individually, but that these may be discerned by sufficient telescopic power; the other being of a hazy nature, and incapable of resolution. Nebulæ do not occur at random in the heavens: the regions poorest in stars are richest in them; they are few in the plane of our sidereal system, but numerous about its poles, in that respect answering to the occurrence of comets in the solar system. The resolution of many of these hazy patches of light into stars by no means disproves the truly nebulous condition of many others.
Fortunately, however, other means than telescopic observation for the settlement of this question are [283] available. In 1846, it was discovered by the author of this book that the spectrum of an ignited solid is continuous, that is, has neither dark nor bright fixed lines. Fraunhofer had previously made known that the spectrum of ignited gases is discontinuous. Here, then, is the means of determining whether the light emitted by a given nebula comes from an incandescent gas, or from a congeries of ignited solids, stars, or suns. If its spectrum be discontinuous, it is a true nebula or gas; if continuous, a congeries of stars.
In 1864, Mr. Huggins made this examination in the case of a nebula in the constellation Draco. It proved to be gaseous.
Subsequent observations have shown that of sixty nebulæ examined, nineteen give discontinuous or gaseous spectra; the remainder continuous ones.
It may, therefore, be admitted that physical evidence has at length been obtained, demonstrating the existence of vast masses of matter in a gaseous condition, and at a temperature of incandescence. The hypothesis of Laplace has thus a firm basis.
Opposition to the nebular hypothesis.Notwithstanding the great authority of the astronomers who introduced it, the nebular hypothesis has encountered much adverse criticism; not so much, however, from its obvious scientific defects, such as its inability to deal with the cases of Uranus and Neptune, as from moral and extraneous considerations. There is a line in Aristophanes which points out precisely the difficulty:
Ὁ Ζεὺς οὐκ ὤν, ἀλλ' ἀντ' αὐτοῦ Δῖνος νυνὶ βασιλεύων.
A reluctance to acknowledge the presidency of law in the existing constitution and movements of the solar system has been yielded only to be succeeded by a reluctance to acknowledge the presidency of law in its genesis. And yet whoever will reflect on the subject will be drawn to the conclusion that the principle involved was really settled by Newton in his "Principia"—that is to say, when it became geometrically certain that Kepler's laws originate in a mathematical necessity.
As matters now stand, the nebular hypothesis may [284] be regarded as the first superficial, and therefore imperfect, glimpse of a series of the grandest problems soon to present themselves for solution—the mathematical distribution of matter and force in space, and the variations of that distribution in time.