This suggestion was eagerly adopted by the members of the French Academy, who were at that time on the look-out for anything which they thought would help them to account for the existence of the world, while they refused to acknowledge a Creator. It was taken up by one of their number—Laplace—a man who stood in the very foremost rank as a mathematician and physical astronomer, and moulded into shape by him.[Footnote: There is a very full account of Laplace's hypothesis, extracted from the works of Pontecoulant, in Professor Nichol's System of the World, pp. 69—86.]
He assumed, that the Solar System existed at the very earliest period as a shapeless nebula, a vast undefined mass of "fire- mist;" that at some time or other the separate particles of this fire-mist began to move towards their centre of gravity, under the influence of their mutual attractions, and thus assumed a spherical shape; that by some means or other a motion of rotation was originated in this spherical mass, which increased in rapidity as the process of condensation advanced. The effect of this rotation would be a flattening of the sphere; the equatorial diameter would increase while the polar diameter, or axis of rotation, diminished; and when the centrifugal force thus produced had reached a certain point, a ring would detach itself from the equator, but would continue to revolve about the common centre. He supposed that a succession of rings were thus thrown off, which finally broke up and accumulated into one or more spherical masses, forming the planets and their satellites, while the remainder of the original sphere was condensed into the sun. The planets and their satellites would continue to revolve about the centre as the ring from which they were formed had done, while the different original velocities of the particles of which they were formed, some having been in the outer, some in the inner part of the ring, would cause them also to rotate on their axis. As the condensation advanced, the heat which had originally existed in the "fire-mist" would be condensed also, so that all the masses when formed would be in an incandescent state, but the planets and their satellites being comparatively small would soon cool down, while the sun, owing to its greatly superior bulk, still retains its heat.
There is no doubt much to be said in favour of this theory, which may be more advantageously considered hereafter, when we shall have to consider it as supplementary to the Mosaic account. At present we are only concerned with it as it claims to stand alone, and to be accepted as a substitute for that account. Viewed in this light, as a substitute for a Creator, as showing us how the universe might have come into existence spontaneously, it utterly breaks down in three points.
1. It gives us no account whatever of the origin of matter, but assumes that it was already in existence at the time from which the theory takes its point of departure. But some account of it must be given. Either it was created by some higher power, or it was eternal; for the idea of its being self-originated is manifestly untenable. If it was created, there is an end of the theory—the act of creation assumes the existence of a Creator; and the only question left is, whether that Creator did more or less. But the very object of the theory was to dispense with the existence of a Creator. This alternative, then, it must reject, and there is nothing left but to fall back upon the other, and to assume that it existed from all eternity. But it is certainly not less difficult to us to conceive the possibility of inert matter being self-existent and eternal, than it is to recognize the existence of an eternal and all-powerful Spirit. Our own consciousness helps us to realize the possibility of the existence of an Eternal Mind, and of the exercise of power by that mind; but we have nothing to help us to a conception of self-existent matter.
In addition to this, the idea of eternity precludes from its very nature the idea of possible change. If there is change there must be the distinction of before and after, and so of the succession of existence, which involves the idea of time. That which is subject to change, and this theory assumes a change in the condition of matter, cannot be eternal.
2. The next failing point is, that this theory assumes a change, of the origin of which it can give no account. The assumption is, that matter which had existed from all eternity, or for an indefinite time, in a state of perfect rest, suddenly began to move towards its centre of gravity. A body, or a system of particles, can remain at rest only under one of two conditions. Either it must be acted on by no force at all, or all the forces by which it is acted on must be in perfect equilibrium. If matter existed under the first of these conditions, whence did the force suddenly emanate? Force cannot be self-originated any more than matter. But if the other alternative be adopted, how was the equilibrium disturbed? It is a fundamental axiom of mechanics that "a body (or system of bodies) at rest will continue at rest till it be acted upon by some external force." But the theory supplies no such external force, for it could only originate in that which the theory ignores—the will and power of some intelligent Being.
3. The third defect is, that the theory does not give any satisfactory account of the origin of the motions of rotation and revolution. Laplace does not attempt this. He simply assumes that a motion of rotation was set up somehow; but many of his followers, perceiving that the theory broke down here—though they passed the other two defects unnoticed—have attempted to supply the deficiency in this point. Some have attempted to account for this motion by analogy. It has been suggested that it was of the same nature, and produced by the same causes, as the vortex which is formed when a vessel full of fluid is emptied through an orifice in its bottom. Pontecoulant, in his account of the theory, enters more into detail. He assumes that in the process of agglomeration large bodies of matter impinged obliquely on the already formed mass, and so imparted to it a motion of rotation.
A consideration of the mechanical conditions of the problem will show the unsoundness of Pontecoulant's views. It is of course assumed that the forces by which this rotation is said to have been produced are identical in their character with those with which we are familiar, for the introduction of any force peculiar to that time would be equivalent to an admission of a directing power. The following propositions then seem unquestionable:—
1. The nebula must be considered as a system of particles acted on by their mutual attractions, and by no other force.
2. When two particles of matter, a and b, attract each other, it is a fundamental principle of mechanics, (commonly known as the "Third Law of Motion") that whatever amount of momentum is produced in a, an equal and opposite momentum must be produced in b. Hence if the mutual action remain undisturbed, the two particles will approach each other and finally meet. On their union, the two momenta being equal and opposite will neutralize each other, and there will be no tendency to produce motion of any kind. 3. The same law will hold good with reference to any number of particles, and therefore with reference to the supposed nebula. Every single particle will produce a certain momentum in each of the other particles, and at the same time will have impressed upon it by each of the other particles an equal and opposite momentum. Hence when all the particles are collected into a single mass, each individual momentum will be balanced by an equal and opposite one, and there can be no resultant motion.