From Benjamin Franklin to Cadwallader Colden, 15 August 1745
To Cadwallader Colden
ALS: New-York Historical Society
Philada. Augt. 15. 1745
Sir
I receiv’d your Favour of the 20th past, with your medical Piece enclos’d,1 the Reading of which gave me a great deal of Pleasure. I show’d it to our Friend Mr. Bertram, who carried it home, and, as he since tells me, is taking a Copy of it; His Keeping of it for that End has prevented my Showing it to any other Gentlemen as you desired; and hitherto prevented my Writing to you upon it as I intended. But lest you should conclude me the very worst Correspondent in the World, I shall delay no longer giving you some Thoughts that occur’d to me in Reading of it; chusing rather to be blam’d for not writing to the Purpose, than for not Writing at all.
I am extreamly pleas’d with your Doctrine of the absorbent Vessels intermix’d with the perspiratory Ducts both on the external and internal Superficies of the Body. After I had read Sanctorius,2 I imagin’d a constant Stream of the perspirable Matter issuing at every Pore in the Skin: But then I was puzzled to account for the Effects of mercurial Unctions, for the Strangury sometimes occasion’d by an outward Application of the Flies, and the like, since whatever Virtue or Quality might be in a Medicine laid upon the Skin, if it would enter the Body it must go against Wind and Tide, (as one may say). Dr. Hales help’d me a little, when he inform’d me, (in his Vegetable Statics)3 that the Body is not always in a perspirable but sometimes in an imbibing State, as he expresses it; and will at Times actually grow heavier by being expos’d to a moist Air. But this did not quite remove my Difficulty, since, as these Fits of Imbibing did not appear to be regular or frequent, a Blistering Plaister might lie on the Skin a Week, or a mercurial Unguent be us’d a Month, to no purpose, if the Body should so long continue in a perspirable State. Your Doctrine, which was quite new to me, makes all easy, since the Body may perspire and absorb at the same Time, thro’ the different Ducts destin’d to those different Ends.
I must own, however, that I have one Objection to the Explanation you give of the Operation of these Absorbents. That They should communicate with the Veins, and the Perspirants with the Arteries only, seems natural enough; but as all Fluids by the hydrostatical Law press equally in all Directions, I question whether the mere Direction of one of those minute Vessels (where it joins with a Vein or Artery) with or against the Stream of Blood in the larger Vessel, would be sufficient to produce such contrary Effects as perspiring and absorbing. If it would, both Perspirants 
and Absorbents might proceed from the Arteries only, or from the Veins only, or from both indifferently; as by the Figure in the Margin, whether the Vessel a b is a Vein or an Artery, if the Stream moves from a to b, the minute communicating Vessel c shall be a Perspirant, and d an Absorbent, and contrary if it moves from b to a. Yet I cannot say, I am certain the mere Direction of the Vessels will have no Effect; I only suspect it, and am making a little Machine to try an Experiment with for Satisfaction. ’Tis a Syphon made of two large Joints of Carolina 
Cane united at e, into which two small glass Tubes f and g are to be inserted one on the descending and the other on the ascending Side. I propose to fill the Syphon and the two glass Tubes with Water, and, when ’tis playing, unstop at the same Instant the Tops of both glass Tubes, observing in which the Water sinks fastest. You shall know the Success. I conceive the Pressure of the Atmosphere on the Apertures of the two glass Tubes to be no way different from the Pressure of the same on the Mouths of the Perspirants and Absorbents; and if the Water sinks equally in the two Tubes, notwithstanding the Direction of one against and the other with the Stream, I shall be ready to think we must look out for another Solution. You will say, perhaps, that ’twill then be time enough, when the Experiment is try’d and succeeds as I suspect; yet I cannot forbear attempting at one beforehand, while some Thoughts are present in my Mind. If a new Solution should be found necessary, this may be ready for Consideration.
I do not remember that any Anatomist that has fallen in my Way, has assign’d any other Cause of the Motion of the Blood, thro’ its whole Circle, than the contractile Force of the Heart, by which that Fluid is driven with Violence into the Arteries, and so continually propell’d by Repetitions of the same Force, till it arrives at the Heart again. May we, for our present Purpose, suppose another Cause, producing half the Effect; and say, that the Ventricles of the Heart, like Syringes, draw when they dilate, as well as force when they contract? That this is not unlikely, may be judg’d from the Valves Nature has plac’d in the Arteries to prevent the Drawing back of the Blood in those Vessels when the Heart dilates, while no such Obstacles prevent its Sucking (to use the vulgar Expression) from the Veins. If this be allow’d, and the Insertion of the Absorbents into the Veins, and of the Perspirants into the Arteries, be agreed to, it will be of no Importance in what Direction they are inserted: For, as the Branches of the Arteries are continually lessening in their Diameters, and the Motion of the Blood decreasing, by means of the encreas’d Resistance, there must, as more is continually press’d on behind, arise a kind of Crouding in the Extremities of those Vessels, which will naturally force out what is contain’d in the Perspirants that communicate with them. This lessens the Quantity of Blood, so that the Heart cannot receive again by the Veins all it had discharg’d into the Arteries, which occasions it to draw strongly upon the Absorbents that communicate with them. And thus the Body is continually perspiring and imbibing. Hence, after long Fasting, the Body is more liable to receive Infection from bad Air; and Food, before ’tis sufficiently chylified, is drawn crude into the Blood, by the Absorbents that open into the Bowels. To confirm this Position, that the Heart draws as well as drives the Blood, let me add this Particular. If you sit or lean long in such a Manner as to compress the principal Artery that supplys a Limb with Blood, so that it does not furnish a due Quantity, you will be sensible of a pricking Pain in the Extremities like that of a thousand Needles; and the Veins, that us’d to raise your Skin in Ridges, will be (with the Skin) sunk into Channels; the Blood being drawn out of them, and their Sides press’d so closely together, that ’tis with Difficulty and slowly that the Blood afterwards enters them when the compress’d Artery is reliev’d. If the Blood was not drawn by the Heart, the Compression of an Artery could not empty a Vein; and I conjecture that the pricking Pain is occasion’d by the Sides of the small Vessels being press’d together.
I am not without Apprehensions, that this Hypothesis is either not new, or, if it is new, not good for any Thing. It may however, in this Letter, (with the enclos’d Paper on a kindred Subject)4 serve to show the great Confidence I place in your Candour, since to you I so freely hazard myself (ultra crepidam) in Medling with Matters directly pertaining to your Profession, and entirely out of the way of my own. If you give yourself the Trouble of Reading them, ’tis all I can modestly expect. Your Silence about them afterwards will be sufficient to convince me, that I am in the wrong; and that I ought to study the Sciences I dabble in, before I presume to set Pen to Paper. I will endeavour however to make you some Amends, by procuring you from better Judges some better Remarks on the Rest of your Piece; and shall observe your Caution not to let them know from whom I had it.
The Piece on Fluxions I purpose shortly to read again, and that on the several Species of Matter, when you shall have what little I shall be able to say about them.
The Members of our Society here are very idle Gentlemen; they will take no Pains. I must, I believe, alter the Scheme, and proceed with the Papers I have and may receive, in the Manner you advise in one of your former Letters. The mention of your former Letters puts me in mind how much I am in Arrear with you: Like some honest insolvent Debtors, I must resolve to pay ready Money for what I have hereafter, and discharge the old Debt by little and little as I am able.
The Impertinence of these Moskito’s to me, (now I am in the Humour of Writing) prevents a great deal of mine to you, so that for once they are of some Use in the World. I am Sir Your most humble Servant
B Franklin
Endorsed: Benj Franklin
[Enclosure]
A Guess at the Cause of the Heat of the Blood in Health and of the hot and cold5 Fits of some Fevers.
The Parts of Fluids are so smooth, and roll among one another with so little Friction, that they will not by any (mechanical) Agitation grow warmer. A Phial half full of Water shook with Violence and long continued, the Water neither heats itself nor warms the Phial. Therefore the Blood does not acquire its Heat either from the Motion and Friction of its own Parts, or its Friction against the Sides of its Vessels.
But the Parts of Solids, by Reason of their closer Adhesion, cannot move among themselves without Friction, and that produces Heat. Thus, bend a Plumbet to and fro, and in the Place of Bending it shall soon grow hot. Friction on any Part of our Flesh heats it. Clapping of the Hands warms them. Exercise warms the whole Body.
The Heart is a thick Muscle, continually contracting and dilating near 80 Times in a Minute; By this Motion there must be a constant Interfrication of its constituent solid Parts; That Friction must produce a Heat, and that Heat must consequently be continually communicated to the perfluent Blood.
To this may be added, That every Propulsion of the Blood by the Contraction of the Heart, distends the Arteries, which contract again in the Intermission; and this Distention and Contraction of the Arteries may occasion Heat in them, which they must likewise communicate to the Blood that flows thro’ them.
That these Causes of the Heat of the Blood are sufficient to produce the Effect, may appear probable, if we consider that a Fluid once warm requires no more Heat to be apply’d to it in any Part of Time to keep it warm, than what it shall lose in an equal Part of Time. A smaller Force will keep a Pendulum going than what first set it in Motion.
The Blood thus warm’d in the Heart, carries Warmth with it to the very Extremities of the Body, and communicates it to them; but as by this Means its Heat is gradually diminished, it is return’d again to the Heart by the Veins for a fresh Calefaction.
The Blood communicates its Heat not only to the Solids of our Body, but to our Clothes, and to a Portion of the circumambient Air. Every Breath, tho’ drawn in Cold, is expir’d Warm; and every Particle of the Materia Perspirabilis carries off with it a Portion of Heat.
While the Blood retains a due Fluidity, it passes freely thro’ the minutest Vessels, and communicates a proper Warmth to the Extremities of the Body. But when by any Means it becomes so viscid as not to be capable of passing those minute Vessels, the Extremities, as the Blood can bring no more Heat to them, must grow Cold.
The same Viscidity in the Blood and Juices checks or stops the Perspiration, by clogging the perspiratory Ducts; or, perhaps, by not admitting the perspirable Parts to separate. Paper wet with Size and Water will not dry so soon as if wet with Water only.
A Vessel of hot Water, if the Vapour can freely pass from it, soon cools. If there be just Fire enough under it to add continually the Heat it loses, it retains the same Degree. If the Vessel be clos’d so that the Vapour may be retain’d, there will from the same Fire be a continual Accession of Heat in the Water, till it rises to a great Degree: Or, if no Fire be under it, it will retain the Heat it first had for a long time. I have experienc’d that a Bottle of hot Water stopp’d, and put in my Bed at Night, has retain’d so much Heat 7 or 8 Hours, that I could not in the Morning bear my Foot against it, without some of the Bedclothes intervening.
During the cold Fit then, Perspiration being stop’d, great Part of the Heat of the Blood that us’d to be dissipated, is confin’d and retain’d in the Body; The Heart continues its Motion, and creates a constant Accession to that Heat, the Inward Parts grow very hot, and, by Contact with the Extremities, communicate that Heat to them: The Glue of the Blood is by this Heat dissolved, and the Blood afterwards flows freely as before the Disorder.
1. Probably a version or abstract of a part of Colden’s treatise on “The Animal Œconomy,” composed in 1733 or 1734. The MS is in the Colden Papers in N.-Y. Hist. Soc.
2. Santorio Santorio (1561–1636), called Sanctorius, professor of medicine at Padua; founder of the physiology of metabolism by his discovery that the loss by “insensible perspiration” exceeds that by all other bodily excretions together. His best known work was translated and edited by John Quincy in London, 1712, as De Medicina Statica: being Aphorisms of Sanctorius.
3. Rev. Stephen Hales, Vegetable Staticks: Or, An Account of some Statical Experiments on the Sap in Vegetables (London, 1727).
4. Printed immediately following.
5. In the MS an arabic 2 has been placed under “hot” and an arabic 1 under “cold,” as if the author intended the words should be reversed.