which being the most capacious, as well as most apt to constitute a Fluid Body, by touching in the fewest Points The farther any Particles deviate from this Figure, the more likely they will be to obstruct the Vessels, and vice versa.

Prop. IV.

The Contagious Particles being admitted into the Blood, do there coagulate its Parts, and form Moleculæ of a larger Size than ordinary.

Demonstration

The Force of the Heart and Cavities of the Canals being the same, when the Infection is first taken, as before, the Blood would pass with the same Facility thro' the Vessels as at other Times, and Obstructions could not be formed, were not the Moleculæ thus increased; as our Senses show they are by the Eruption of Pustules in the Small-Pox, by the great Inflammations, Mortifications, Buboes, and Carbuncles in Malignant and Pestilential Fevers; and consequently the Contagious Particles do increase the Bulk of several of the constituent Parts of the Blood, by altering the Figures of its Particles, and forming Moleculæ of a larger Size than in a Natural State.

CHAP. II.

It has been the constant Observation of Physicians, as well ancient as modern, and confirm'd by numerous Instances, that a hot and moist Constitution of the Air, joyn'd with southerly Winds, was generally a Fore-runner of malignant and Pestilential Fevers. Thus Hippocrates observes, that the Constitution of the Air preceeding that malignant Fever describ'd in the 3d Book of his Epidemics, 'was calm, moist, and southerly, and succeeded a hot, and dry Season; the Winter, calm, cloudy, rainy, warm, southerly; some Showers, and Northerly Winds about the Equinox; the Spring, calm and southerly, with great Rains; the Summer very hot, with little Wind, and much Rain about the Dog-days [a]. Some Authors led by the Title of this Book of his Epidemics, viz. Κατάστασις Λοιμώδης or the Pestilential Constitution, have imagin'd the Diseases here spoken of, to be the same with that terrible Plague describ'd by Thucydides, which taking its Rise in Æthiopia, and passing thence thro' Lybia and Ægypt, miserably harass'd all Persia, Phœnicia, Judea, Greece, and Cœle Syria, and was one of the most dreadful Calamities of this kind that ever appeared in the World. But whosoever will give himself the Trouble to compare the Symptoms of the Fevers here described by Hippocrates, with those related by that accurate Historian [b], who both had it himself, and visited many others in it, will find that there is not the least Similitude between them. The one being highly infectious, and not the least Appearance of Contagion in the other: Galen also the best Interpreter of Hippocrates, in his Comment on this Book of his Epidemic's suspects, this Title to be spurious, tho' both he and others observe much the same Constitution of the Air to be the Fore-runner of these Diseases.

What Places most Subject.

Pestilential and Malignant Fevers, are likewise observ'd to be the most frequent in those Places where the Climate is hot and scorching, and especially when Rains fall in such Seasons of the Year. Thus in Ægypt and some other Parts of Africa, if Rains fall during the Months of July and August, the Plague usually breaks out the September following [c].

This is still more remarkable in such Places, as not only are Situated in the forementioned Climate, but are likewise deprived of a constant Succession of pure and clear Air. An Instance of this we have in Grand Caire, which besides being subject to the common Disadvantages of the Country, (as are a Climate hot and scorching, a Situation low and flat, exposed chiefly to the warm Winds, their Water fetid and stagnating, being reserv'd in Vaults and Canals, which are Annually fill'd by the Overflowing of the River, the Air abounding with putrid Steams and Exhalations, arising from the Parts of Animals, Vegetables, and other Substances brought down and there deposited by the River), lies close under the Hill of the Castle, by which all Wind and Air is intercepted, which causes such a stifling Heat there, as ingenders many Diseases. [d]

That these may justly be esteem'd the Causes of the greater Frequency of these Diseases in this Place, than others in the same Climate, appears from their being so rarely known in those Places, which tho' equally hot, enjoy an Air free from Vapours. [e] Thus in Numidia and some other Parts of Africa, the Plague is scarce to be found once in a hundred Years, and hardly at all in the Land of Negroe. [f]