those influences and radiations which go to place upon their surface the generative elements of life. Yet while these stars present so strict analogies in formation and nature, they do not by any means indicate the same degree of advancement in what may be called geological or rather planetary evolution, or that which tends to the appearance and development of life on their surface. Here the conditions of mass, of distance from the sun, and doubtless other circumstances still unknown, come in to order the epoch and the extent of these developments; but we can affirm, without going beyond the inductions permitted by the condition of science, that if life has not yet been established directly on the surface of any of the planets, we have very strong reasons for assuming its existence on some of them. We may regard this conclusion as gained from the long labors of antiquity and modern discoveries.
We say that, while the problem has not been directly resolved by the eyes, it has been worked out by an aggregate of facts, analogies, and rigorous deductions that leave no room for doubt. This is the mature and perfect fruit of science. It is the view of intelligence, as certain an authority and of even a higher and nobler order than the senses. I say further that what we know of the unity of the chemical composition of the matter of the sun, the stars, and the nebulæ permits us to make new inductions respecting the part performed by the bodies which are on the earth the most important factors of the phenomena of life. It is thus infinitely probable that hydrogen, oxygen, nitrogen, carbon, and especially water, which on the earth are the indispensable constituents of vegetable and animal life, fill a like office not only in the planets of our system, but throughout the universe.
Water in particular, by virtue of its chemical functions and the properties with which it is endowed in the solid, liquid, and gaseous states—properties which are so admirably fit for the accomplishment of physiological processes—is a unique substance, and the search through the whole series of chemical compounds for any body that can take its place has been in vain. The discovery of the spectrum of the vapor of water permits us to deduce and assert its presence in the atmospheres of the planets and in those of a whole class of stars as well. Drawing from these results the fact of the presence of hydrogen, one of the generative gases of water, in nearly all the stars, we are justified in supposing an extreme diffusion of that important element from the point of view of the unity of the phenomena which control the production and maintenance of life.
Thus, the more science advances, the more is that great law confirmed and established of unity in the material elements, in
