The result has been, that from the actual, constant, and minute observation of disease, from an enlarged inquiry into the circumstances coincident therewith, of the pestilential character of many agencies, and a careful comparison with every agent or form in nature with which we are acquainted, bearing any resemblance to what Atmospheric Contagion must be, if it have an existence at all; that where other hurtful influences are operating, Atmospheric Contagion is needlessly called in to account for their effects, and that it (_i. e._ Atmospheric Contagion) has no existence, properly considered, in the light of an atmosphere holding in solution a specific contagious poison. Before commencing the argument, it is proposed to notice shortly its history, and the opinions held at this day respecting its nature and qualities. But as these opinions are very various and conflicting, and as, moreover, from the general confusion of terms, the reader will almost unavoidably become perplexed and unable to understand the merits of the case as treated here or by others, the Author proposes to explain, before going further, what is meant or ought to be meant by contagion, and by contagious air. He is not aware that any plain and uniform method or arrangement of the principles in question is in common use, though some physicians, as will appear in the historical sketch that is to follow, have reduced contagion to two or three distinct kinds, and thus divested the subject of much of its perplexing clashing of terms. They have given fixed meanings to some terms formerly used by all, and even at present by most, with too great latitude. We will consider, _1st_, Contagion. That term is, and with propriety may be, used to denote that property, which matter eliminated in a body suffering under disease, has of producing the same disease when applied to another in a state of health, as the matter of small-pox. Contagion is also used, and will be employed here, to denote the matter itself which we have just defined. Thus it appears that contagion is used to signify both the property of the matter and the matter itself. This should be understood, as confusion may lead to great misconception. In the same way, the term “heat” is used to denote caloric itself, and also its property. Contagion, signifying the matter itself, is said to act in different shapes, but here medical men divide. According to those on whose authority most reliance is to be placed, they are the following—three in number:— _1st_, By the direct application to the body of palpable contagious matter. _2dly_, By the application to the body of clothes, and the like, impregnated with contagious matter. _3dly_, By the application to the body of air holding in solution, contagious matter.