Though, in this view, the atmosphere seems to be a kind of sink or common sewer, where all the poisonous effluvia arising from putrid and corrupted matter is deposited; yet it has a wonderful facility of purifying itself, and one way or other, of depositing those vapors contained in it; so that it never becomes noxious, except in particular places, and for a short time; the general mass remaining, upon all occasions, pretty much the same.[59] The way in which this purification is effected, is different according to the nature of the vapor with which the air is loaded. Aqueous vapor ascends; and also much of that vapor arising from decayed and putrid animal and vegetable substances, (and which, by some modern philosophers, is called _phlogiston_, attaching itself to the aqueous vapor,) ascends along with it; and probably descends again with the rain; whence the fertilizing qualities of rain-water above those of any other: while another part is absorbed by vegetables; for the phlogistic vapor is probably the food for plants. But sulphureous, acid, and metalline exhalations, produced principally by volcanos; vapors, arising from houses where lead and other metals are smelted; descend, in consequence of their specific gravity, and suffocate and spread destruction around them, poisoning not only animals, but vegetables also. From all these, the air seems not capable of purifying itself, otherwise than by winds, or by letting them subside by their superior gravity, till they are absorbed either by the earth or water, according as it is their nature to unite with one or other of these elements. Of this kind also seem to be the vapors which are properly called pestilential. The contagion of the plague itself seems to be of a heavy, sluggish nature, incapable of rising in the air, but attaching itself to the walls of houses, bed-clothes, and wearing apparel. Hence, scarcely any constitution of the atmosphere can dispel these noxious effluvia; nor does it seem probable that pestilential distempers ever cease until the contagion has operated so long, and been so frequently communicated from one to another, that, like a ferment much exposed to the air, it becomes vapid, communicates a milder infection, and at last loses its strength altogether.