The results of numerous and delicate observations show also that germs of plants and animals exist as universally in the air as in water; and when favorable conditions of light and temperature come, these germs at once grow, multiply, and become visible under the microscope.
Approaching the relations of our air particles with solids, we meet the question of what these minute bodies can have in common with compact masses of invariable form, incomparably denser than they, and all the particles of which seem to be too dense to permit the access of our gaseous particles. This, the hitherto prevalent idea of the structure of solid bodies, does not conform to the real condition; for, just as the superficial parts of liquids tend to diffusion in the ambient air, a like habit exists in the molecules of solids of being repelled from the interior toward the exterior, and they separate from one another, but only in an extremely thin exterior layer. Thus camphor, iodine, ice, and some other substances change into vapor at ordinary temperatures; and the exhalation of perfumes may be something of the sort.
Many other facts point to an exceptional constitution of the free surface of solid bodies, of which I need cite only the experiments of M. De Marçay on the vaporization of metals in vacuum at temperatures below their melting points, and especially the researches of M. Spring on the direct uniting of metals, either of the same or of different species. We conclude from all these evidences that there exists on the surface of solid bodies an extremely thin layer, the density of which diminishes the more nearly we approach the free surface. Let us assume, consequently, such a special constitution for the superficial layer of solids, and, by a new effort of our imagination, witness the unrelaxing work of our particles of air in the immediate vicinity of some solid body; we might thus see them dashing into the invisible intervals between the extreme molecules of the solid and opening passages for themselves through innumerable spaces, whence there results a whole formed of solid particles and more or less condensed aggregations of certain gases. Possibly this is the way in which has been developed that texture, doubtless very fine but still very resisting, which covers all solid bodies and is also very difficult to take away from them.
You ask, Of what interest to us is this incessant activity of the air? We answer that it has an interest of the very highest importance; for without this protecting layer covering solids, every object brought in contact with another would risk adhering to it so closely that they could not be separated without a great effort. It is this invisible layer that permits the workman to use his tools handily, the reader to turn the leaves of his book
