depraved Condition, the rest of the Animal Juices must degenerate in Proportion thereto, and the Nervous Fluid, as it consists of the most volatil and subtil Parts, be extreamly acrid and pungent, as well as unequal in its Texture and Fluidity, from the more viscous Parts contain'd in it.

Putrid Fevers how produced.

This then being the State of the Blood and other juices of the Body, it is easy to perceive how from very Slight, and otherwise trivial Occasions, a Fever of a very Malignant Nature may be produced. Thus the perspirable Matter from a slight Cold taken being retained, or the Vessels any otherwise filled by Irregularities in Diet, or others of the Non-naturals, the Weight of the moving Fluid will be increased, and the Circulation be more languid and slow. Whence the intestine Motion of the Particles of the Blood being diminished, the viscous Parts will cohere more strongly and in greater Quantities than before, and obstruct the Capillary Arteries, especially in the Extremities, and a Coldness, Stretching, Yawning, Torpor, &c. necessarily succeed, the constant Attendants of a beginning Fever; All which will bear a Proportion to the Quantity retain'd, and the Viscosity of the moving Fluid.

These Disorders will necessarily be increased on account of the Air's Spring being weakned by its Heat, the Vessels of the Lungs being less inflated, and the Globules of the Blood less broken and divided, and the more especially in a humid Air, Heat and Moisture necessarily relaxing the Tone of the Fibres and Vessels, and rendring them less Springy and Elastic. Hence then the Quantity of Spirits being diminish'd, and their Motion more slow, the Contraction of the Heart and other Muscles will be more weak and languid, and being stimulated by the Acrimony of the Circulating Liquors, must contract more frequently than in a Natural State; The Consequence of which is Weakness, Faintness, Thirst, and Dejection of Spirits.

These and the preceeding Symptoms will necessarily continue, 'til such time as the gross and viscous Matter, being shook and loosen'd by the Action of the Capillary Vessels, is washed away into the Veins by the force of the Circulating Fluids, and there continues its Course with the rest, 'til it be either attenuated and secreted, or lodged again in the Capillaries to excite new Disorders.

Malignant Fevers.

Now if to this evil Disposition of the Air be added a number of pungent stimulating Particles, whether bred in the Body or floating in the Air, and thereby communicated to the Blood, which are apt to coagulate the Animal Juices, so as to form Moleculæ of such Shapes and Sizes as more firmly obstruct the Capillary Vessels, and at the same time stimulate and corrode the nervous Parts; It will necessarily happen, that the preceeding Symptoms must be highly exasperated, and a Fever of a much worse Nature produc'd.

Hence then must follow a violent Hurry and Colluctation of the Fluids, the Viscid and coagulated Parts of the Blood in some Parts obstructing the Circulation of the Juices, and the Acrid, Volatil, and Fiery Parts, rarefying and dissolving others of the more Liquid, to the greatest degree of Pungency and Volatility imaginable.

Hence it is easy to perceive how the Motion of the Blood must necessarily be in some Parts more languid, by the Cohesion of the more Viscous parts, in others quicker, join'd with a pungent and stimulating heat, from the increased Velocity and Acrimony of the moving Fluid, and the various Actions of the Particles upon each other, and their Impulses on the containing Vessels; As also how these are capable of almost infinite Variations, in Proportion to the different Quantities and Qualities of the constituent Particles. Hence then appears the Reason of that wandering and uncertain Heat and Coldness, in different Parts of the Body at the same time. Hence appears the Reason of that great Inquietude and Anxiety, of those uncertain and partial Sweats, Watchings, Tremors, stretching Pains of the head, and the like, as will be more fully shown hereafter.

But before I proceed to explain the Nature of a Fever truly Pestilential, it will be necessary to observe, that notwithstanding the foremention'd putrid Disposition be generally a Prelude to a Pestilential Constitution of the Air, yet it has never that I know of been observ'd, that these Causes alone at their first Onset, produced a real Plague or Pestilential Contagion, without the Concurrence of some preceeding Infection, either brought from abroad, or gradually augmented from the increased Putrifaction of the Air, and Poisonous Steams of Morbid Bodies.

Thus the putrid Air of Camps in hot Countries is frequently found to produce Pestilential Fevers, but this never happens at their first Onset; The Diseases first appearing being Fluxes, Putrid, and afterwards Malignant Fevers; which being exasperated and propagated by the Virulent Effluvia of Diseased Bodies, and the increased Putrifaction of the Air, grow up gradually to those of a Pestilential, and exceedingly infectious Disposition.

Of Putrifaction & Fermentation.

Now Putrifaction being only a kind of Fermentation, wherein the Particles of a putrifying Body are put into an intestine Motion, and by their Action and Attrition broken and divided, and since all fermenting Substances do emit vast Quantities of small separable Parts, it will necessarily follow, that the most subtil and active Particles of the Purifying Body will be elevated into the Air, and float in it.

Effluvia from putrified Bodies what.

These Effluvia consist of the finest and most volatil saline and oleaginous Particles, highly attenuated and set at Liberty from the gross Oil and Terrestrious Part, as appears from the Distillation of such Substances, all which afford great Quantities of a pungent and volatil Salt. It is likewise observable, that the subtil Oleaginous Particles being specifically lighter, as well as more easily attenuated and divided than those of a Saline Nature, will be thrown off in greater Proportion in the beginning of the Fermentation or Putrifaction than the heavier Salts will be, which must either be more attenuated and volatilised, or require a greater force to raise them into, and sustain them in the Air than the former, and consequently the greatest Emission of these saline Particles will be after the Fermentation has been for some time continued; As we find it happens in all fermenting Liquors, as Wine, Beer, Cyder, and the like. All which emit, during the Fermentation, greater Quantities of Particles of an active attenuated Oil or Spirit for some time, than of a Saline Nature, which requiring a longer time in order to attenuate them, are not raised till the former are in a manner quite exhaled, as appears from collecting the Steams of fermenting Liquors, and of those which are turn'd sower by Distillation, and consequently the Exhalations arising from putrifying Bodies will after some time consist mostly of Saline Particles highly attenuated and volatilised, and those not wrapt up and sheathed in the Oily ones, and thereby render'd innocuous, and often useful to the Body, but naked and exceedingly acrid and poignant.

How unfit an Air stock'd with these kind of Particles is for Respiration, appears from several of Mr. Boyl's Experiments on Animals, shut up with putrified Air in the Receiver, most of which with