The community is aware that Medicine is not now the subtle, hidden, affectedly mysterious art, it was at no very remote date; and that its present enlightened professors now seek not the assistance of darkness, of silence, to disguise their ignorance and questionable views, or to heighten the impression of the skill and cunning of their order. Its study is now conducted openly, and its foundation, happily, is laid upon principles established in nature that are as well known to the unprofessional as to the professional man. There is no wish to disguise matters from the public, and, were it attempted, it could not possibly succeed. The utmost care will be observed to lay the evidence impartially down, plainly, and divested of technical phraseology; and, satisfied of the general ability of the public to judge, the author will await their decision with as much anxiety as that of the medical world. It will afford the writer the return he most values, if, by his means, less anxiety and apprehension are felt in future among the public on occasions of disease; if those acting under a sense of duty are enabled to discharge their humane offices with less feeling of danger; if the patient remain unoppressed with alarm for the dear ministering friends around him; if even one sufferer be spared the anguish of bearing wants unanswered, and if in his last hours he is spared the bitterness of soul he must experience, when deserted by those to whom, through life, he looked for comfort and support. The author has also been induced to publish his views upon vitiated air, its nature, sources and effects, with directions for its prevention, avoidance, and correction. Vitiated air has been confounded with atmospheric contagion, has performed the greater part of the work of death attributed to that agency, or supposed agency, and it has been it that has been affected and controlled, when both non-professional people and the medical world thought systems of quarantine, isolation, gens des cordons, (contagion police,) and fumigations, were effecting the objects for which they were established. The immediate objects of these counteracting agents, the destruction of contagion and contagious atmosphere, &c., could not be effected, since, at least, the latter does not exist: but fortunately, though they could not effect the objects immediately proposed, the ultimate ends have been served, by their acting on many occasions upon the efficient causes of disease, viz. vitiated air. They were useful after the fashion of the medicine and charms in olden times, used for the expulsion of evil spirits, devils, and the like, which, by their natural action upon the functions of the body, corrected derangements which were mistaken for the operations of these imaginary beings. The Author has pointed out other causes of pestilence, and has given some directions for their prevention, and for the preservation of health.