Considering the insignificance of our earth when viewed in comparison with the millions of other orbs in the universe, considering also the stupendous distances by which the earth is separated from innumerable globes which are very much greater, it is certainly not a little astonishing to learn that the elements from which the various bodies in the universe have been composed are practically the same elements as those of which our earth is built. Is not this a weighty piece of evidence in favour of the theory that earth, sun, and planets are all portions of the same primæval nebula in which these elements were blended?

We do not, of course, mean to affirm that the great primæval nebula was homogeneous throughout its vast extent. The waters of ocean are not strictly the same in all places; even the atmosphere is not absolutely uniform. Nature does not like homogeneity. The original nebula, we may well believe, was irregular in form, and denser in some places than in others. We do not suppose that if we could procure a sample of nebula in one place and another sample from the same nebula, but in a different place, say a hundred million miles distant, the two would show an identity of chemical composition; two samples of rock from different parts of the same quarry will not always be identical. But we may be assured that, in general, whatever elements are present in the nebula will be widely dispersed through its extent. If from different parts of the nebula two globes are formed by condensation, though we should not affirm, and though in fact we could not believe, that those globes would be of identical composition, yet we should reasonably expect that the elementary bodies which entered into their composition would be in substantial agreement. If one element, say iron, was abundant in one globe, we should expect that iron would not be absent from the other. Thus the elements represented in one body should be essentially those which were represented in the other.

It is obvious that if the sun and the earth—to confine our attention solely to those two bodies—had originated from the primæval nebula, they would bear with them, as a mark of their common origin, a resemblance in the elementary bodies of which they were composed. When Laplace framed his theory, he had not, he could not have had, the slightest notion as to the particular elements in the sun. For anything he could tell, those elements might be absolutely different from the elements in the earth. Yet, even without information on this critical point, the evidence for the nebular theory appeared to him so cogent that he gave it the sanction of his name.

It cannot be denied that if spectroscopic analysis had demonstrated that the elements in the sun were totally different from the elements in the earth a serious blow would have been dealt to the nebular theory. The collateral evidence, strong as it undoubtedly is, might hardly have withstood so damaging an admission. If, on the other hand, we find, as we actually have found, that the elements in the sun and the elements in the earth are practically identical, we obtain the most striking corroboration of the truth of the nebular theory. Had Kant and Laplace been aware of this most significant fact, they would probably have cited it as most important testimony. They would have pointed out that the iron so abundant in the earth beneath our feet is also abundant in the sun overhead. They would, I doubt not, if they had known it, have dwelt upon the circumstance that with that element, carbon, which enters into every organic body on this earth, our sun is also richly supplied, and they would have hardly failed to allude to the wide distribution in space of calcium, hydrogen, and many other well-known elements.

Laplace mainly based his belief in the nebular theory on some remarkable deductions from the theory of probabilities. To the consideration of these we proceed in the next three chapters. We may, however, remark at the outset that if the evidence derived from probabilities seemed satisfactory to Laplace one hundred years ago, this same line of evidence, strengthened as it has been by recent discoveries, is enormously more weighty, at the present day.