pofition of the air to promote that contagion, ought equally to be confidered; both being neceffary to give the diftemper full force. The defign therefore of this chapter is to make a proper balance between these two, and to fet juft limits to the effects of each.

For this purpose, I fhall reduce the causes which fpread the plague, to three; difeafed perfons, goods tranfported from infected places, and a corrupted ftate of air.

There are several diseases which will be communicated from the fick to others; and this not done after the fame manner in all. The hydrophobia is communicated no other way than by mixing the morbid juices of the difeafed animal immediately with the blood of the found, by a bite, or what is analogous thereto; the itch is given by fimple contact; the lues venerea not without a clofer contact; but the meafles, small-pox, and plague, are caught by a near approach only to the fick : for in these three last difeafes perfons are rendered obnoxious to them only by refiding in the fame houfe, and converfing with the fick.

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 perfons, into their veins, that the whole mafs 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 fo 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 obferved that fermenting liquors do at the latter end of their inteftine motion throw off a great quantity of their most fubtile VOL. II.

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and

and active particles. And this discharge will be chiefly made upon thofe glands of the body, in which the fecretions are the moft copious, and the most eafily increased: fuch are thofe 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 perfon will, in the nature of a ferment, put the fluids there into the like agitation and diforder.

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

I think it certain that refpiration does always communicate 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 none of thofe qualities, by which it is fitted to inflate the lungs and agitate the blood, the ufes commonly afcribed to it. And this is farther confirmed by what the learned Dr Halley has informed me, that when he was feveral fathom under water in his diving-engine, and breathing an air much more condenfed than the natural, he observed himself to breathe more flowly than ufual which makes it more than probable, that this conveying to the blood fome fubtile parts from the air, is the chief ufe of refpiration fince when a greater quantity of air than ufual was taken in at a time, and confequently more of thefe fubtile parts received at once by the blood, a lefs frequent refpiration fufficed.

As to the fkin, fince there is a continual discharge made through its innumerable pores, of the matter of infenfible perfpiration and fweat; it is very poffible that the fame paffages may admit fubtile corpufcles,

which may penetrate into the inward parts.

Nay, it

is very plain that they do fo, from what we obferve upon the outward application of ointments and warm bathings; which have their effects by their fineft and most active parts infinuating themfelves into the blood.

It is commonly thought, that the blood only is affected in thefe cafes 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, ufually called the animal fpirits. As this is the immediate inftrument of all motion and fenfation, and has a great agency in all the glandular fecretions, and in the circulation of the blood itself; any confiderable alteration made in it must be attend. ed with dangerous confequences. It is not poffible that the whole mafs of blood fhould be corrupted in fo fhort a time as that in which the fatal fymptoms, in fome cafes, difcover themselves. Thofe patients of the first class, mentioned in the beginning of this difcourfe, particularly the porters who opened the infected bales of goods in the lazaretto's of Marfeilles, died upon the first appearance of infection, as it were by a fudden stroke; being feized with rigours, tremblings, heart-fickness, vomitings, giddinefs and heavinefs of the head, an univerfal languor and inquietude; the pulfe low and unequal and death enfued fometimes in a few hours.

Effects fo fudden must be owing to the action of fome corpufcles of great force infinuated into, and changing the properties of, another fubtile and active

fluid in the body; and fuch 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 univerfe, and of the laws of attractions, repulfions, 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 philofopher, whofe furprising difcoveries have exceeded the utmost expectations of the moft penetrating minds, has demonstrated that there is diffufed through the universe a fubtile and elastic fluid of great force and activity. This he fuppofes to be the caufe 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 fpirits are a thin liquor, feparated in the brain, and from thence derived into the nerves, of fuch a nature that it admits, and has incorporated with it, a great quantity of this elastic fluid; which makes it a vital subftance of great energy. And a liquor of this kind must be very fufceptible 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 fpirits upon their being put together, fall in

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

to

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

If therefore we allow the effluvia or exhalations from a corrupted mafs of humours in a body that has the plague to be volatile and fiery particles, carrying with them the qualities of thofe fermenting juices from which they proceed; it will not be hard to conceive 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 œconomy. But of this more in another place.

This is one means by which the plague, when once bred, is fpread and increased: but the fecond of the forementioned causes, namely, goods from infected places, extends the mifchief much wider. By the preceding caufe, the plague may be fpread 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 remoteft regions. From hence the trading parts of Europe have their principal apprehenfions, and univerfally have recourse to quarantines for their fecurity. The univerfality of which practice is a strong argument, that merchandise will communicate infection: for one cannot imagine, that fo many countries should agree in fuch a custom without the most weighty reafons. But befides, there is not wanting exprefs proof of this, from particular examples, where this injury has been done by feveral forts of goods carried from infected places to others. Some of these I shall hereafter be obliged to mention; at prefent I fhall çonfine myfelf to three inftances only. The first shall be

of