In most of the epidemics recorded, some such agencies or circumstances were in operation. They were known to be so—and in almost all that have come under our own observation, and they have neither been few nor carelessly noted—there have, on nearly every occasion, been found the influences to which we allude. We are led to believe an agent to be the cause of an effect when the one follows upon the operation or presence of the other, uniformly, and on every occasion, when the latter bears some relation in its amount to the force and length of duration of the former—and when the effect ceases with the removal of the agent. Such a close connection, as subsists in that case, entitles the former or agent to be held as the cause of the effect observed. For those very reasons day-light is said to be the effect of the sun that comes with it, remains with it, and goes with it. Let us see if the same connection holds with disease and those agencies and circumstances we have cursorily referred to. Those agencies and circumstances, relating to food, drink, air, heat, contagious matter, &c. &c. are known to present themselves, and with them are presented diseases. They are known to remain, and with them are known to continue diseases. They are known to disappear, and with them all the world knows diseases disappear also. These being the causes of epidemic or widely spread disease, as such a connection proves, it is altogether superfluous to admit the operation of atmospheric contagion, whose existence has never been known, but by the very circumstances which it is said to bring about. It is surely most unwise, when we see disease arising with the existence of unwholesome circumstances, such as scarcity of food, unwholesome quality of it, great vicissitudes of weather, uncommon conditions of the atmosphere, want of sufficient clothing or incommensurate with the severities of the season, the operation of depressing passions, and the like,—growing with their intensity—extending where they extend—abating where they decrease—and finally disappearing when they disappear—to refuse to grant the relation as cause and effect, and to plunge into the tide of difficulties such ill-timed incredulity creates, with nothing but the appearance, nothing but the assurance, of an object to grasp at. In general, such a connection can be made out between the existence of wide-spread disease and such circumstances.