B. _Solid globes colliding under the influence of gravity alone._—As we have already seen, the view has been adopted by Sir W. Thomson that the solar nebula may have resulted from the colliding of cold, solid globes with the velocity due to their mutual gravitation alone. He states his views as follows: “Suppose, now, that 29,000,000 cold, solid globes, each of about the same mass as the moon, and amounting in all to a total mass equal to the sun’s, are scattered as uniformly as possible on a spherical surface of radius equal to one hundred times the radius of the earth’s orbit, and that they are left absolutely at rest in that position. They will all commence falling towards the centre of the sphere, and will meet there in 250 years, and every one of the 29,000,000 globes will then, in the course of half an hour, be melted, and raised to a temperature of a few hundred thousand or a million degrees Centigrade. The fluid mass thus formed will, by this prodigious heat, be exploded outwards in vapour or gas all round. Its boundary will reach to a distance considerably less than one hundred times the radius of the earth’s orbit on first flying out to its extreme limit. A diminishing series of out-and-in oscillations will follow, and the incandescent globe, thus contracting and expanding alternately, in the course, it may be, of 300 or 400 years, will settle to a radius of forty times the radius of the earth’s orbit.”[17] [Footnote 17: _Proceedings of Royal Institution_, vol. xii. p. 16.] The reason which he assigns for the incandescent globe settling down at a radius forty times that of the earth’s orbit is as follows: “The radius of a steady globular gaseous nebula of any homogeneous gas is 40 per cent. of the radius of the spherical surface from which its ingredients must fall to their actual positions in the nebula to have the same kinetic energy as the nebula has.” If the solar nebula thus produced would be swelled out into a spherical incandescent mass with a radius 40 times the radius of the earth’s orbit, simply because the globes fell from a distance of 100 times the radius of that orbit, then for a similar reason the mass would have a radius of 400 times that of the earth’s orbit had the globes fallen from a distance of 1,000 times the radius, and 400,000 times if the globes had fallen from a distance of 1,000,000 times the radius, and two-fifths of any conceivable distance from which they may have fallen. Supposing all this to be _physically_ possible, which it undoubtedly is not, still the heat generated would not be sufficient; for, whatever the radius of the nebula might be, its entire energy, both kinetic and potential, is simply what is obtained from gravitation, and this, as we have seen, is insufficient.