Those circumstances, connected with the appearance and propagation of disease, on which the doctrine of atmospheric contagion rests, will be inquired into, and their weight and importance duly ascertained. This inquiry will be prosecuted as if no such investigation as the preceding had been made, and as if the existence of that agency was not irreconcilable with well ascertained laws; and, for the sake of argument, the possibility of its existence will be conceded. The facts in the history of disease, which are held as lending countenance to the doctrine of atmospheric contagion—of proving its existence, are, chiefly, the general prevalence of disease at one and the same time among the members of the same family, of the inhabitants of the same town, district, and country,—its affecting the visitors and attendants of the sick,—and its observation in places hitherto healthy, shortly after communication with those ravaged with the distemper. These facts cannot be denied; and all that can be done is to weigh their value, as proofs of the existence of atmospheric contagion, and the first mentioned will occupy our attention. THE EVIDENCE DRAWN FROM THE WIDELY SPREAD AND SIMULTANEOUS PREVALENCE OF DISEASE, IN FAVOUR OF ATMOSPHERIC CONTAGION, CONSIDERED. The widely-spread prevalence of disease at the same time among the inhabitants of a country or district, is almost invariably held as affording proof of the presence and operation of atmospheric contagion. The ravages of pestilence, rapid, wide, and deadly, are noticed in the histories of all nations, and at intervals they have been experienced during the long period of the existence of the world; and the destruction of whole armies, and the annihilation of entire nations, prove how widely spread its operation has sometimes been. Did the circumstance of disease being widely spread prove its propagation by atmospheric contagion, then the matter were at rest; but the propriety of such an inference is questionable. Let it be supposed that there is prevailing, in a district of country, disease to a great amount, that is to say, many cases of the same distemper. That single circumstance proves nothing in reference to atmospheric contagion, more than to any other probable cause of disease. It shews, merely, that there is in wide operation some cause or causes of sickness; and it is totally unwarrantable to conclude that one agency, more than another, is the efficient cause, without further information directly bearing on the subject. It is with the knowledge of the single fact, and in total ignorance of others, or with total blindness to them, that atmospheric contagion is pronounced to be the active agent.