pestilential particles, or the coming of an infected person from some place where it rages, to an uninfected one, and is not epidemic there, unless the constitution of the air favours it. Otherwise I cannot conceive how it should happen, that, when the plague rages violently in one town in the same climate, a neighbouring one should totally escape it, by strictly forbidding all intercourse with the infected places; an instance of which we had some few years ago when the plague raged with extreme violence in most parts of Italy; and yet the Grand Duke, by his vigilance and prudence, entirely prevented its entering the borders of Tuscany.” As to the nature of the disease, when once produced, Dr. Sydenham is of opinion that it is altogether inflammatory; for which he gives the following reasons: 1. The colour of blood taken away that resembles that in pleuritic and rheumatic disorders. 2. The carbuncles resemble the mark of _an actual cautery_. 3. The buboes are equally disposed to inflammation with any other tumours that come to suppuration. 4. The season of the year may be adduced in proof of this; for between spring and summer, inflammatory disorders, as pleurisies, quinsies, &c. are common. Before we put an end to this section, it may now be proper to say a few words by way of apology for the many apparent digressions from the subject which have appeared in it. In the first place, then, the work being intended for general inspection, and not merely for medical readers, it became absolutely necessary to introduce a number of things which for medical readers would have been totally superfluous. It was to be supposed that the book might come into the hands of some who had not read any thing concerning the structure of the body, who had not heard of any of the systems of medicine now prevalent, or the different doctrines they contain. It was impossible to write in an intelligible manner for such people without giving some few hints concerning all these subjects: the same consideration made it necessary to enter pretty largely into the discoveries concerning the composition of the atmosphere and various kinds of elastic fluids, concerning heat, &c. In doing this the writer was under a necessity either to adopt some of the doctrines he took notice of, or to animadvert upon them. If he has ventured freely to give his sentiments, it is not with a view to establish a theory of his own, but to direct the attention of the reader to those natural agents which seem to be at present too much overlooked, principally because they are less accessible to our senses, and of consequence less subject to experiment, than others. If therefore in this treatise it is suggested that the atmosphere acts on the human body by its internal or latent heat, and by its electricity, as well as by its other properties; if the writer is inclined to believe that these are in fact the most powerful parts of it; that we never can act without them, and that in short our life