From the circumstance of this vitiated air arising from persons in disease, and assisting in the propagation of the same malady, it has all along been regarded as a specific contagious animal poison in an atmospheric menstruum; and thus has been created the perplexing and entangled web of confusion and vagueness that has been wove around the principles, viz. contagious poisons, and vitiated air arising from effluvia from persons in disease, and from their excretions. From this circumstance, these principles, viz. specific contagious poisons, and vitiated air arising from persons in disease, have been erroneously classed together, and a supposed analogy has been created. But these principles are widely different in their nature, and in the laws by which they are regulated. The specific contagious poisons produce the same diseases as those with which the bodies, whence they arose, were affected, and them only; and their operation is marked by uniform effects, observing stated and unvarying periods. Vitiated air, of the kind under examination, though it arises from persons in a state of disease, and is sometimes known to operate in the production or propagation of the same distemper, does not always induce disease, does not induce that disease only, whence it sprung, but various others; and, in short, its effects are not uniform, and do not observe stated and unvarying periods. The specific contagious poisons produce their peculiar diseases, as their proper and only effects, without the cooperation of other influences; but vitiated air, when the same disease extends, whence it arose, cannot be said to be causing its proper, only, and peculiar effects, as the same disease does not invariably follow its action. In general, the effluvia which proceed from a sick person, where they prove hurtful, cause the same distemper as that with which he is affected; for instance, the effluvia arising from a person affected with typhus fever, produce that disease again:—but that is not always the case, and an instance will be presently detailed, where the effluvia which proceeded from a body dead of one disease, produced another of a very different nature. The reason that the presence of vitiated air is generally attended with the same disease as that with which the body is affected, whence it sprung, is, that there is existing at the time, a disposition to that particular malady: and the vitiated air only gives it form by acting as an ordinary exciting cause upon individuals prepared for its invasion.