through it. At first nothing is visible. But very soon in the path of the luminous ray we perceive a delicate blue color. It is because the gas is decomposed by the luminous waves, and the invisible particles of sulphur set free decompose the light in turn. The blue of the vapor deepens, then it turns whitish, and at last a white cloud is produced. The particles composing this cloud are still each by itself invisible, even under strong microscopes, and yet they are infinitely more coarse than the primitive atoms that occasioned the sky-blue tint at first seen in the receiver. In this experiment we pass in steady progress from the free atom of sulphur parted from the oxygen-atom by the ether-waves to a mass apparent to the senses; but, if this mass is made up of free molecules which defy the strongest magnifiers, what must be the particles which have produced those very molecules!
A last instance of another kind will complete the proof as to the minuteness of the elements of matter. When a clear solution of sulphate of aluminum is poured into an equally clear solution of sulphate of potassa, the mixture at once grows turbid, and after a few seconds myriads of little crystals, sparkling like diamonds, make their appearance in the liquid, which are nothing else than crystals of alum. If we suppose the diameter of these crystals to be 125 of an inch, it will follow from this experiment that in the lapse of a few seconds crystals had the power of producing themselves containing tens of millions of molecules, each composed of 94 atoms, grouped in admirable harmony. The motions of these chemical atoms take place under the influence of the same forces that guide the motions of those enormous agglomerations of atoms called stars. The revolution of one sun around another takes a thousand years, while these atoms in course of combination perform hundreds of millions of such revolutions in the millionth part of a second!
By varied and delicate calculations, Thomson has succeeded in establishing the fact that, in liquids and transparent or translucent solids, the mean distance between the centres of two contiguous atoms is comprised between the ten-millionth and the two-hundred-millionth part of 125 of an inch. It is not easy to form an exact conception of dimensions so small, of which nothing, among the objects that affect our senses, can convey any idea. Thomson judges that the following comparison may aid us to appreciate them: If we imagine a sphere as large as a pea magnified, so as almost to equal the earth's volume, and the atoms of that sphere enlarged in the same proportion, they will then have a diameter greater than that of a shot, and less than that of an orange. In other words, an atom is to a globe the size of a pea what an apple is to the terrestrial globe. By arguments of quite another kind, drawn in part from the study of chemical molecules, in part from the phenomena of capillarity, Gaudin has ascertained, for the dimension of the smallest particles of matter, figures very nearly the same as Thomson's. The maximum distance apart of the chemical
