678 POPULAR SCIENCE MONTHLY, 



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 com- 

 mon 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 to- 

 ward 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 tem- 

 peratures ; 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 experi- 

 ments of M. De Margay on the vaporization of metals in vacuum 

 at temperatures below their melting points, and especially the re- 

 searches of M. Spring on the direct uniting of metals, either of the 

 same or of different species. We conclude from all these evi- 

 dences 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 them- 

 selves 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 de- 

 veloped that texture, doubtless very fine but still very resisting, 

 which covers all solid bodies and is also very difiicult 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 adher- 

 ing 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 



