new iron. And, the false back being plastered and whitewashed, and the hearth reddened, the whole will make a pretty appearance. Before the black lead is laid on, it would not be amiss to wash the plates with strong lie and a brush, or soap and water, to cleanse them from any spots of grease or filth that may be on them. If any grease should afterwards come on them, a little wet ashes will get it out.

If it be well set up, and in a tolerably good chimney, smoke will draw in from as far as the fore part of the bottom plate, as you may try by a bit of burning

paper.

People are at first apt to make their rooms too warm, not imagining how little a fire will be sufficient. When the plates are no hotter than that one may just bear the hand on them, the room will generally be as warm as you desire it.*

* When this pamphlet was first printed, a copy of it was sent by Mr. Cadwallader Colden to the celebrated Gronovius, with a letter from which the following is an extract.

"I send with this a curious and new invention for warming a room with a small fire, and more effectually than can be done by a large fire, in the common method, and is free of the inconveniences which attend the Dutch and German stoves, because by this contrivance there is a continual supply of fresh warm air. It may be particularly useful to you and Dr. Linnæus, by preserving your health while it keeps you warm at your studies. It is the invention of Mr. Benjamin Franklin of Philadelphia, the printer of it, a very ingenious man."

To this passage Gronovius replied as follows, in a letter dated at Leyden, July 9th, 1745.

"I am very much obliged to you for Mr. Franklin's book, which I don't doubt the next letter shall bring to you translated into Dutch.' - EDITOR.

I received your favor of the 20th past, with your medical piece enclosed, the reading of which gave me a great deal of pleasure. I showed it to our friend Mr. Bartram, 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 gentleman 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 occurred to me in reading of it, choosing rather to be blamed for not writing to the purpose, than for not writing at all.

I am extremely pleased with your doctrine of the absorbent vessels intermixed with the perspiratory ducts, both on the external and internal superficies of the body. After I had read Sanctorius, I imagined 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 occasioned 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 helped me a little, when he informed me, in his Vegetable Statics, 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

VOL. VI.

9

F*

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.

sorbents.

I must own, however, that I have one objection to the explanation you give of the operation of these abThat 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

communicating vessel c shall be a perspirant, and d an absorbent; and the contrary, if it moves from b to ɑ. 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