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position of the air to promote that contagion, ought equally to be considered; both being necessary to give the distemper full force. The design therefore of this chapter is to make a proper balance between these two, and to set just limits to the effects of each.

For this purpose, I shall reduce the causes which spread the plague, to three ; diseased persons, goods transported from infected places, and a corrupted state of air.

There are several diseases which will be commitnicated from the fick to others; and this not done after the same manner in all. The hydrophobia is cominunicated no other way than by mixing the morbid juices of the diseased animal immediately with the blood of the found, by a bite, or what is analogous thereto ; the itch is given by simple contact ; the lues venerea not without a closer contact ; but the mealles, small-pox, and plague, are caught by a near approach only to the fick :- for in these three last difeases persons are rendered obnoxious to them only by residing in the same house, and converfing with the sick.

Now, it appears by the experiments mentioned in the preface, of giving the plague to dogs by putting the bile, blood, or urine from infected persons, intó their veins, that the whole mass of the animal fluids in this disease is highly corrupted and putrefied. It is therefore easy to conceive how the effluvia or fumes from liquors so affected may taint the ambient air. And this will more especially happen, when the humours are in the greatest fermentation, that is, at the height of the fever : as it is observed that fermenting liquors do at the latter end of their intestine motion throw off a great quantity of their most fubtile VOL. II.

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and active particles. 'And this discharge will be chiefly made upon those glands of the body, in which the fecretions are the moft copious, and the most easily increased : such are those of the mouth and skin. From these therefore the air will be impregnated with peftiferous atoms : which being taken into the body of a found perfoo will, in the nature of a ferment, put the fluids there into the like agitation and diforder.

The body, I suppose, receives them these two ways, by the breath, and by the skin; but chiefly by the former.

I think it certain that refpiration does always commonicate to the blood fome parts from the air : which is proved from this obfervation, that the fame quantity of air will not fuffice long for breathing, though it be deprived of nove of those qualities, by which it is fitted to inflate the lungs and agitate the blood, the uses commonly ascribed to it. And this is farther confirmed by what the learned Dr Halley has informed me, that when he was several fathom under water in his diving-engine, and breathing an air much more condenfed than the natural, he observed himself to breathe more Nowly than usual : which makes it more than probable, that this conveying to the blood fome fubtile parts from the air, is the chief use of re{piration : fince when a greater quantity of air than pfual was taken in at a time, and consequently more of these fubtile parts received at once by the blood, a less frequent respiration fufficed.

As to the skin, since there is a continual discharge made through its innumerable pores, of the matter of insensible perfpiration and fweat ; it is very possible that the fame passages may admit fubtile corpuscles, which may penetrate into the inward parts. Nay, it is very plain that they do fo, from what we observe upon the outward application of ointments and warm bathings; which have their effects by their fineft and most active parts insinuating themselves into the blood.

It is commonly thought, that the blood only is af. fected in these cases by the morbific effluvia. But I am of opinion, that there is another fluid in the body, which is, especially in the beginning, equally, if not more, concerned in this affair : I mean the liquid of the nerves, usually called the animal spirits. As this is the immediate instrument of all motion and senfation, and has a great agency in all the glandular secretions, and in the circulation of the blood itself, any confiderable alteration made in it mult be attend. ed with dangerous consequences. It is not possible that the whole mass of blood should be corrupted in fo short a time as that in which the fatal fymptoms, in some cases, discover themselves. Those patients of the first class, mentioned in the beginning of this discourse, particularly the porters who opened the infected bales of goods in the lazaretto's of Marseilles, died upon the first appearance of infection, as it were by a sudden stroke; being feized with rigours, tremblings, heart-lickness, vomitings, giddiness and heaviness of the head, an universal languor and inquietude ; the pulse low and unequal : and death ensued fometimes in a few hours.

Effects fo fudden must be owing to the action of some corpuscles of great force insinuated into, and changing the properties of, another fubtile and active G 2

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fluid in the body; and such an one, no doubt, is the nervous liquor.

It is not to be expected that we should be able to explain the particular manner by which this is brought about. We know too little of the frame of the universe, and of the laws of attractions, repulsions, and cohesions among the minuteft parcels of matter, to be able to determine all the ways by which they affect one another, especially within animal bodies, the most delicate and complicated of all the known works of nature. But we may perhaps make a probable conjecture upon the matter. Our great philosopher, whose surprising discoveries have exceeded the utmost expectations of the most penetrating minds, has demonstrated that there is diffused through the universe a subtile and elastic fluid of great force and activity. This he fupposes to be the cause of the refraction and reflection of the rays of light; and that by its vibrations light communicates heat to bodies; and, moreover, that this readily pervading all bodies, produces many of their effects upon one another *.

Now, it is not improbable that the animal spirits are a thin liquor, separated in the brain, and from thence derived into the nerves, of such a nature that it admits, and has incorporated with it, a great quantity of this elastic fluid ; which makes it a vital substance of great energy. And a liquor of this kind must be very susceptible of alterations from other active bodies of a different nature from it, if they approach to and are mixed with it: as we fee fome chemical spirits upon their being put together, fall in.

* Newton's optics, qu. 18. to 24.

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to a fermentation, and make a composition of a quite different kind.

If therefore we allow the effi uvia or exhalations from a corrupted mass of humours in a body that has the plague to be volatile and fiery particles, carrying with them the qualities of those fermenting juices from which they proceed ; it will not be hard to corceive how these may, when received into the nervous fluid of a found perfon, excite in it fuch inteftine motions as may make it to partake of their own properties, and become more unfit for the purposes of the animal economy.

But of this more in another place.

This is one means by which the plague, when once bred, is spread and increafed : but the fecond of the forementioned causes, namely, goods from infected places, extends the mischief much wider. By the preceding cause, the plague may be spread from perfon to perfon, from house to house, or perhaps from town to town, though not to any great distance; but this carries it into the remotest regions. From hence the trading parts of Europe have their principal apprehensions, and universally have recourse to quarantines for their security. The universality of which practice is a strong argument, that merchandise will communicate infection : for one cannot imagine, that so many countries should agree in such a custom without the most weighty reasons. But besides, there is not wanting express proof of this, from particular examples, where this injury has been done by several sorts of goods carried from infected places to others. Some of these I shall hereafter be obliged to mention ; at present I shall confine myself to three instances only. The first shall be

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