Here is then an agency, of whose existence there is no evidence of a sufficient nature, and here there is reason to believe that the same circumstances are operating widely, and upon the relatives, the attendants, and visitors of the sick, which have already produced the disease in those visited. These circumstances, in general, are ascertained to be acting upon these individuals, and where they cannot, from their obscure nature, be recognised, there is reason, from the very circumstance of the sick having been affected, to conclude that they are operating, though perhaps in an insidious way. Now, a question arises, whether it is wisest to attribute the prevalence of disease among those holding communication with the sick, to the operation of atmospheric contagion, or to those circumstances and agencies which caused the disease originally, and which there is room to believe are exerting their influence over them also. It has been said in the preceding chapter, that, during the prevalence of widely spread or epidemic disease, there are generally found circumstances of an unwholesome tendency, favouring its career, and that the range of their action corresponds with that of pestilence. That being the case, as it undoubtedly is, it would be proper, before admitting the operation of atmospheric contagion, to shew that no such circumstances were in operation. An inquiry would be necessary; and their presence being proven, it would not be short of imbecility to attribute to that agency, effects such as are wont to follow their action. It would be to call in a principle whose existence has never been proven, and which, therefore, must be held as at least doubtful, to account for phenomena, the ordinary results of circumstances present, which would indeed be absurd. Before the operation of atmospheric contagion could with propriety be entertained, it behoved to shew that those circumstances which induced disease in the visited, were not operating with those holding communication with them. But in all those cases in which atmospheric contagion is held as acting, no attempt is made to prove such absence, and the belief in its presence is not the less strong because these circumstances can be proved to be present. It is a self-evident truth that some agency or agencies, totally independent of atmospheric contagion must have been in operation, and acting as the cause of disease in the first case or cases that occurred. For this ample reason, that, for atmospheric contagion to exist at all, it is obviously necessary that disease pre-existed, since it is the product of disease, and of disease only.