lib. 6, cap. 7. Sandy's Travels, lib. 2. cap. 97.

It will I think be needless to show, that the Distempers here treated on are propagated by Contagion; But it may not be altogether unnecessary to explain by what Methods these Alterations in the Animal Oeconomy are brought about, and especially as the Means by which they are chiefly communicated, have not that I know of been fully examin'd.

CHAP. IV.

Of a Pestilence by Contagion.

The Contagious Particles whether they be generated in the Air, or produced by the Effluvia of Morbid Bodies, being sustain'd in it, are thereby applied to the Surface of our Bodies, with a Force equal to the Pressure of the Incumbent Atmosphere. This Pressure upon the External Superficies of a human Body of a middle Size, has been demonstrated to be equal to 39900 Pounds Troy-weight, and consequently supposing the Body in every Part encompassed with these Particles, the whole Force with which all these Particles are on this Account propell'd into the Body, will amount to the aforesaid Sum. But every single Particle is only applied with the Force of a Column of Air of the Height of the Atmosphere, and whose Base is equal to the Surface of that Side of the intruding Particle, opposite to the cutting Angle. Now the Contagious Particles from their extream Smallness and pungent Angles, may not only be consider'd as Bodies applied to us with the preceeding Force, but likewise as so many small Knives or Lancets, acting upon and penetrating the Coats of the Lungs and Surface of our Bodies, with a Force proportional to the Smallness of their cutting Angles. This appears not only from several Propositions in Mechanics, but even to our Senses, by the strong Contraction of a Cord or Fiddle-string in moist Weather. The Particles of Water from their exceeding Smallness, being protruded into the Cord, with a Force capable of Raising the greatest Weights. Now if to these be likewise added the strong attractive force of these small Volatil Particles, occasioned from their Exiguity, it will be no difficult matter to conceive, that they are capable of penetrating the Vessels of our Bodies. Thus the Attractive Force of the Magnet is greater in Proportion to its Bulk, in Small ones, than in those of a larger Size, from the greater Proximity of all its Particles to each other. And 'tis on this Account that Sir Isaac Newton computes the Attractive Force of the Particles of Light, to be to that of other Bodies, as 1000000000000000 to 1, in Proportion to the Quantity of Matter contained in them [p].

This I think is sufficient to shew, that these Acrid and Pungent Particles are able to penetrate the Surface of our Bodies, and get into the Blood that way; And indeed Experience it self confirms it in all other Pungent and Acrid Substances, as Garlic, Cantharides, Arsenic, and all Pungent and Corroding Bodies.

The Pressure of the Air on the Internal Surface of the Lungs in Breathing determined.

But tho' the whole Superficies of our Bodies are Penetrable by these Poisonous Particles, yet the principal Mischief is communicated to the Blood in its Passage thro' the Lungs. For considering the prodigious Number of the Pulmonary Vesicles, into all which the Air enters in Respiration, and likewise the vast Increase of their Surfaces on that Account, and also the greater Force by which these Particles are applied to the Internal Surfaces of the Vesicles in Expiration, in Proportion to that whereby they are applied to other Parts of the Body of equal Superficies; it will evidently appear, that the Contagion is chiefly communicated by these Vessels to the Blood. For it appears by the Barometer, that every Inch Square upon the Surface of our Bodies is pressed upon by a Weight nearly equal to 1800 Drams, when the Mercury stands highest in the Barometer. Now supposing with Dr. Kiel [q] that both the Lobes of our Lungs contain 226 solid Inches, of which only 1/3 or 75 Inches are Vesicles. Supposing also the Diameter of a Vesicle to be 1/50 Part of an Inch, the Surface of the Vesicle will be .001256 and the Solidity .0000043, by which if we divide 75 the Space fill'd by the Vesicles, the Quotient, viz. 17441860 X .001256 the Surface of a Vesicle, gives the Sum of the Surfaces of all the Vesicles, = 21906.976 square Inches. Which Sum being multiplied by 1800, the Number of Drams which every square Inch of the Surface of our Bodies sustains, gives the Weight which the whole Internal Surface of the Lungs sustains by the sole Pressure of the Atmosphere, when the Mercury stands highest in the Barometer, equal to 39442556.800 Drams, equal to 410859.966 + 64/96 lib. Troy weight, as appears from the known Laws of Hydrostaticks [r].