HENRY. For the present, the argument drawn from the actual observation of the origin and propagation of disease, against the doctrine of atmospheric contagion, will be waived, and it is proposed here, before going farther, to inquire, whether the case may not be settled by a reference to the history of analogous agents, and to the results of experiment. It is proposed here to inquire if it is likely, judging from their chemical constitution, that palpable contagious poisons, such as the matter of small-pox, may be disseminated through the air, without chemical changes being effected upon them, that must be destructive of their peculiar properties. The palpable contagious poisons are products of the blood, formed therefrom, by the nicest processes. They partake of the nature common to all animal products; are, like them, prone to putrefaction,—and, like them, are of a very compound nature. They are animal products: now it is a well known fact that almost all animal products are fixed—that is, incapable of being volatilized or disseminated in air, unchanged in chemical constitution. Gelatin or animal jelly; albumen, or what is much the same, the white part of an egg; fibrin or muscular fibre, and the like, are never known to be in the vaporic state, or commingled with the air. They are incapable of assuming the aeriform state, not in virtue of a character peculiar to them, but on account of that nature they share in common with almost all animal principles, which precludes the possibility of their being volatilized. No experiment has ever been made which can show that the principles specified may be diffused through the air. When exposed to the air for even a short period, decomposition takes place, and their original nature is totally subverted. Their elements are held together by affinities too feeble to admit of their particles being separated by air, without new combinations being formed. If heat be applied to them, immediate destruction takes place; if they be kept moist, and in merely a moderate temperature, putrefaction or fermentation, in the proper sense of the terms, occurs; if carefully dried and exposed to the atmosphere, they remain little altered, for a considerable time; but at length fundamental changes, though operating slowly, entirely change their nature. It cannot be shewn that contagious poisons are less animalized than the products alluded to. Is it ascertained that contagious poisons, unlike other proximate animal principles, enter into the aeriform state? Putting aside the loose and rash statements current upon the subject, as unworthy of notice, there can be no doubt that, in the whole history of those poisons, no fact is known, that can legitimately be held as proving, that they possess such a property, or of giving the idea any degree of countenance.