A Discourse on the Plague

by Richard Mead

I Have been thus particular in tracing the Plague up to its first Origine, in order to remove, as much as possible, all Objection against what I shall say of the Causes, which excite and propagate it among us. This is done by Contagion. Those who are Strangers to the full Power of this, that is, those who do not understand how subtile it is, and how widely the Distemper may be spread by Infection, ascribe the Rise of it wholly to the malignant Quality of the Air in all Places, wherever it happens; and, on the other hand, some have thought that the Consideration of the infectious Nature of the Disease must exclude all regard to the Influence of the Air: Whereas the Contagion accompanying the Disease, and the Disposition 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 communicated from the Sick to others: and this not done after the same manner in all. The Hydrophobia is communicated no other way than by mixing the morbid Juices of the diseased Animal immediately with the Blood of the sound, 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 Measles, Small-Pox, and Plague are caught by a near Approach only to the Sick: for in these three last Diseases Persons are render’d obnoxious to them only by residing in the same House, and conversing 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, into 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 Highth 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 subtile and active Particles. And this Discharge will be chiefly made upon those Glands of the Body, in which the Secretions are the most copious, and the most easily increased: such are those of the Mouth and Skin. From these therefore the Air will be impregnated with pestiferous Atoms: which being taken into the Body of a sound Person will, in the Nature of a Ferment, put the Fluids there into the like Agitation and Disorder.

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 Respiration does always communicate to the Blood some Parts from the Air: Which is proved from this Observation, that the same Quantity of Air will not suffice long for breathing, though it be deprived of none 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 confirm’d by what the learned Dr. Halley has inform’d me, that when he was several Fathom under Water in his Diving Engine, and breathing an Air much more condensed than the natural, he observed himself to breath more slowly than usual: Which makes it more than probable, that this conveying to the Blood some subtile Parts from the Air, is the chief Use of Respiration; since when a greater Quantity of Air than usual was taken in at a time, and consequently more of these subtile Parts received at once by the Blood, a less frequent Respiration sufficed.

As to the Skin, since there is a continual Discharge made thro’ its innumerable Pores, of the matter of insensible Perspiration and Sweat; it is very possible that the same Passages may admit subtile Corpuscles, which may penetrate into the inward Parts. Nay it is very plain that they do so, from what we observe upon the outward Application of Ointments and warm Bathings: which have their Effects by their finest and most active Parts insinuating themselves into the Blood.

It is commonly thought, that the Blood only is affected 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 Sensation, and has a great Agency in all the glandular Secretions, and in the Circulation of the Blood itself; any considerable Alteration made in it must be attended with dangerous Consequences. It is not possible that the whole Mass of Blood should be corrupted in so short a Time as that, in which the fatal Symptoms, 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 seized with Rigors, Tremblings, Heart-Sickness, Vomitings, Giddiness and Heaviness of the Head, an universal Languor and Inquietude; the Pulse low and unequal: and Death insued sometimes in a few Hours.

Effects so sudden must be owing to the Action of some Corpuscles of great Force insinuated into, and changing the Properties of, another subtle and active 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 minutest 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 supposes 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[50].