by being exposed to 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 plaster might lie on the body a week, or a mercurial unguent be used 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, through the different ducts destined 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 pass 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 an artery or a vein, if the stream moves

d

communicating vessel c shall be a perspirant, and d an absorbent; and the 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.

It is a siphon made of two large joints of Carolina

the siphon and the two glass tubes with water, and, when it is 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 it will then be time enough when the experiment is tried, 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 assigned any other cause of the motion. of the blood through its whole circle, than the contractile force of the heart, by which that fluid is driven with violence into the arteries, and so continually propelled 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 judged from the

valves nature has placed 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 allowed, 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 increased resistance, there must, as more is constantly pressed on behind, arise a kind of crowding in the extremities of those vessels, which will naturally force out what is contained 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 discharged 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 it is 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 supplies 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, which used to raise your skin in ridges, will be (with the skin) sunk into channels; the blood being drawn out of them, and their sides pressed so closely together that it is with difficulty and slowly that the blood afterwards enters them, when the compressed artery is relieved. If the blood

was not drawn by the heart, the compression of an artery would not empty a vein, and I conjecture that the pricking pain is occasioned by the sides of the small vessels being pressed together.

I am not without apprehension, 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 enclosed paper on a kindred subject, serve to show the great confidence I place in your candor, since to you I so freely hazard myself (ultra crepidam) in meddling 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, it is 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 mosquitos to me (now I am in the humor 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.

TO CADWALLADER COLDEN.

On the Circulation of the Blood. - Proposal for publishing a Philosophical Miscellany.

SIR,

Philadelphia, 28 November, 1745.

I shall be very willing and ready, when you think proper to publish your piece on gravitation, to print it at my own expense and risk. If I can be the means of communicating any thing valuable to the world, I do not always think of gaining, nor even of saving, by my business; but a piece of that kind, as it must excite the curiosity of all the learned, can hardly fail of bearing its own expense.

I must not pretend to dispute with you on any part of the animal economy. You are quite too strong for me. I shall just mention two or three little things, that I am not quite clear in.

If there is no contrivance in the frame of the auricles or ventricles of the heart, by which they dilate themselves, I cannot conceive how they are dilated. It is said, by the force of the venal blood rushing into them. But, if that blood has no force which was not first given to it by the contraction of the heart, how can it (diminished as it must be by the resisting friction of the vessels it has passed through) be strong enough to over