incredible Inquietude die sooner than in Vacuo, as also from the pernicious Effects of the Steams of Vaults, Mines, the Grotto de Cane, and such like. But besides this Inaptitude of such Air to expand the Pulmonary Vessels, these minute and pungent Particles may be considered as so many Stimuli or Lancets, acting upon and penetrating the Coats of the Stomach, Lungs, and other Vessels. On which Account they are not only capable of creating great Disordes, as Inflamation, Pain, Sickness, Anxiety, Vomiting, &c. in the Stomach and Nervous Parts; But likewise being carried immediately into the Blood, will there stimulate the ultimate Vessels, ferment, dissolve, or coagulate the circulating Juices according to the particular Qualities and Quantity of the Contagious Particles. Nor is it unlikely, that from the various Action of the Particles upon each other, and their different Combinations in a stagnating Air, Particles may be formed of Qualities vastly differing from, and in their Force almost infinitely exceeding those of their Primogenial Salts and first Principles, as in Sublimate, some Preparations of Antimony, &c. Instances of which those versed in Chymistry are no Strangers to.

Infectious Particles how produced.

Now supposing the Blood Saturated with these kind of Particles, and a Malignant Fever produced by their means, we all know that the Blood in this State throws off vast Quantities of subtil and active Particles thro' the Perspirable, Salival, and other Excretory Ducts of the Body, which not only must load the adjoining Air with great Quantities of them, and render it capable of producing more dismal Effects than the preceeding, but also the Particles thus thrown off must be endued with a more acrid and pungent Disposition than the former, inasmuch as they are more subtily divided and attenuated by the Force of the Fever, than those in the preceeding Disposition of the Air, where so powerful an Agent was wanting, And consequently produce a Fever of a most infectious and deleterious Nature; And especially when the Infection is taken toward the latter end of the Disease, at which time the Saline Particles will be more exalted and volatilised, as well as thrown off in greater Quantities, and thereby made more capable of producing an infectious Contagion.

For the Blood in these Circumstances may not unaptly be compared, as was before hinted, to a fermenting Liquor, whose Parts being constantly in Motion, are continually throwing off great Quantities of subtil and active Spirits, capable of exciting the same Fermentation, and producing the same Qualities in those of the like Species, as appears from our manner of fermenting Ale, Beer, &c. with Yeast, which is a Spirituous Ferment, and also from the Sower Ferments used in making Vinegar, &c.

Analogous to this we may observe, that the Blood in different Diseases, as well as different Animals, throws off great Quantities of active Particles, which when mixed with the Blood of a Healthful Person, are capable of exciting the same Fermentation and Disorder in the Animal Juices, with those of the Morbid Animal from which they exhale, as we find in the Small-Pox, Measles, Saliva of a Mad-dog, and the like.

This then being the Disposition of the Blood and other Juices, in those Fevers which we call Pestilential, it is evident, that whatever the particular Substance of the Contagious Particles may be, they must be endued with such Qualities as will Coagulate the Animal Juices, Stimulate the Fibres to frequent Vibrations, cause Obstructions in the Capillary Vessels, and render the Blood and other Juices of the Body exceedingly Acrid and Pungent, as appears from hence and the foregoing Propositions; The Symptoms and Consequences (cæteris paribus) being the same, whether the Disease has gradually grown up to this Height, or took its Rise only from Contagious Particles brought from abroad.

How propagated.

This is the Method by which I suppose these Contagious and Pestilential Particles to be first generated and produced, in those places which are most subject to them, and thence propagated first into the Neighbourhood, and afterwards to greater Distances by way of Intercourse and Commerce. The Pestilential Effluvia being pack'd up and conveyed in Goods of a soft and loose Texture, as Silk, Wool, Cotton, and the like; And so much the more easily, as the Air into which these infested Materials are brought, is predisposed to act in full Concert with them; as happens in all Places at some times more than others; At which time if these infectious Particles be communicated, they exert their Rage with the utmost Violence, but frequently are either dissipated and lost, or produce Diseases of less fatal Consequence, in an opposite Disposition of the Air.

Why the Plague ceases.

Thus hard Frost, strong cold and Northerly Winds, are found frequently to put an End to, or at least bridle the Fury of Contagious Diseases, and render them more mild and curable, as was observable in the Beginning of the last great Plague in London [m], and frequently taken Notice of in other places by the Writers on this Subject. Consonant to this we find in Ægypt, that the Rising of the Nile by giving a fresh Motion to, and altering the Disposition of their stagnating and putrid Air, by the mild Vapours and Nitrous Exhalations [n] issuing from it, immediately checks the Raging of the Plague, and reduces it to a Fever of a more mild and curable Nature; insomuch that as Purchas and others inform us, if there die in Grand Caire 500 Persons of the Plague the Day before, yet upon the Increase of the River it ceases to be Pestilential, and none die of it [o]. And indeed it can hardly be imagin'd, how the Plague when it has once got establish'd in any Place, shou'd cease but with the Destruction of all or most of the Inhabitants, was it not checked by some Alteration in the Disposition of the Air, and gradually reduced to a Fever of a more mild and curable Disposition.