Pure atmospheric air is composed of three gaseous substances only, but is perpetually contaminated by a variety of exhalations from the earth. “The atmosphere is a vast laboratory,” says Fourcroy, “in which nature operates immense analyses, solutions, precipitations, and combinations: it is a grand reservoir, in which all the attenuated and volatilized productions of terrestrial bodies are received, mingled, agitated, combined, and separated. Notwithstanding this mixture, of which it seems impossible for us to ascertain the nature, atmospheric air is sensibly the same, with regard to its intimate qualities, wherever we examine it.” Hence, whatever may be the nature of the aërial fluid, when absolutely pure, that which we breathe, and which commonly goes under the name of _air_, must be considered as an exceedingly heterogeneous mixture, various at various times, and which it is by no means possible to analyze with accuracy. The whole mass of it contains a great deal of water, together with the vast collection of particles raised from all bodies of matter on the surface of the earth by effluvia, exhalations, &c, so that it may be termed a _chaos_ of the particles of all sorts of matter confusedly mingled together. And hence it has been considered as a large chemical vessel, in which the matter of all kinds of bodies is copiously floating; and thus exposed to the continual action of that immense surface, the sun, from whence proceed innumerable operations, sublimations, separations, compositions, digestions, fermentations, putrefications, &c.