“Docebo causas, quas diligentiâ experientiâque comperi.” Experience has taught us that the phenomena of epidemic pestilences or diseases are various and dissimilar, observing no regular course or succession, but commencing and ceasing at periods influenced by certain changes of the seasons, and modified by various circumstances, especially such as locality and habit of body. The chronicles of all nations are replete with notices respecting the remarkable commotions of nature, which have proved from time to time so inimical both to the animal and the vegetable kingdoms; and on carefully reviewing the facts detailed in the histories of epidemic pestilences or diseases, it will be readily perceived that, during certain periods or seasons, ELEMENTAL DISTURBANCE has enveloped, as it were, the entire globe, carrying death and misery into every quarter. I am, therefore, of opinion that the grand phenomena of nature, exhibited in the commotions of our physical world, supply us with abundant materials for the explication of all epidemic pestilences or diseases; that the latter are consequently assignable to natural causes, without searching for or hunting after mysterious agencies, to the neglect of those which Nature is constantly presenting to our view. I am fully aware that ascertaining the causes of maladies of any description is a matter attended with much difficulty, not only from the variety and impalpable nature of the causes themselves, but also from the peculiar functions and various idiosyncrasies of the living system on which such causes act; numerous conditions being found capable of modifying the effects of a common cause of disease, viz. the season of the year, peculiar state of the atmosphere, especially as regards heat and humidity, circumstances of locality, the different temperaments of individuals exposed to their agency, the influence of previous ailments,--all these concurring more or less in the production of diversified forms of disease.