A Source Book in Physics

Author: Benjamin Thompson

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Convection of Heat

It is certain, that there is nothing more dangerous, in philosophical investigations, than to take any thing for granted, however unquestionable it may appear, till it has been proved by direct and decisive experiment.

I have very often, in the course of my philosophical researches, had occasion to lament the consequences of my inattention to this most necessary precaution.

There is not, perhaps, any phenomenon that more frequently falls under our observation, than the Propagation of Heat. The changes of the temperature of sensible bodies—of solids—liquids—and elastic fluids, are going on perpetually under our eyes; and there is no fact which one would not as soon think of calling in question, as to doubt of the free passage of Heat, in all directions, through all kinds of bodies. But, however obviously this conclusion appears to flow, from all that we observe and experience in the common course of life, yet it is certainly not true;—and to the erroneous opinion respecting this matter; which has been universally entertained—by the learned, and by the unlearned—and which has, I believe, never even been called in question, may be attributed the little progress that has been made in the investigation of the science of Heat:—a science, assuredly, of the utmost importance to mankind!

Under the influence of this opinion, I, many years ago, began my experiments on Heat; and had not an accidental discovery drawn my attention with irresistible force, and fixed it on the subject, I probably never should have entertained a doubt of the free passage of Heat through air; and even after I had found reason to conclude, from the results of experiments which to me appeared to be perfectly decisive, that air is a non-conductor of Heat; or that Heat cannot pass through it, without being transported by its particles; which, in this process, act individually, or independently of each other; yet, so far from pursuing the subject, and contriving experiments to ascertain the manner in which Heat is communicated in other bodies, I was not sufficiently awakened to suspect it to be even possible, that this quality could extend farther than to elastic Fluids.

With regard to liquids, so entirely persuaded was I, that Heat could pass freely, in them, in all directions, that I was perfectly blinded by this prepossession, and rendered incapable of seeing the most striking and most evident proofs of the fallacy of this opinion.

I have already given an account, in one of my late publications, of the manner in which I was led to discover, that steam and flame are non-conductors of Heat: I shall now lay before the Public an account of a number of experiments I have lately made, which seem to show that water—and probably all other liquids,—and Fluids of every kind, possess the same property. That is to say, that although the particles of any Fluid, individually, can receive heat from other bodies, or communicate it to them; yet, among these particles themselves all interchange and communication of Heat is absolutely impossible.

It may, perhaps, be thought not altogether uninteresting, to be acquainted with the various steps by which I was led to an experimental investigation of this curious subject of inquiry.

When dining, I had often observed that some particular dishes retained their Heat much longer than others; and that apple pies, and apples and almonds mixed, (a dish in great repute in England) remained hot a surprising length of time. Much struck with this extraordinary quality of retaining Heat, which apples appeared to possess, it frequently occurred to my recollection; and I never burnt my mouth with them, or saw others meet with the same misfortune, without endeavouring, but in vain, to find out some way of accounting, in a satisfactory manner, for this surprising phenomenon.

About four years ago, a similar accident awakened my attention, and excited my curiosity still more; being engaged in an experiment which I could not leave, in a room heated by an iron stove, my dinner, which consisted of a bowl of thick rice soup, was brought into the room; and as I happened to be too much engaged at the time to eat it, in order that it might not grow cold, I ordered it to be set down on the top of the stove; about an hour afterwards, as near as I can remember, beginning to grow hungry, and seeing my dinner standing on the stove, I went up to it, and took a spoonful of the soup, which I found almost cold, and quite thick. Going, by accident, deeper with the spoon the second time, this second spoonful burnt my mouth. (It is probable that the stove happened to be nearly cold when the bowl was set down upon it, and that the soup had grown almost cold; when a fresh quantity of fuel being put into the stove, the Heat had been suddenly increased.) This accident recalled very forcibly to my mind the recollection of the hot apples and almonds with which I had so often burned my mouth, a dozen years before, in England; but even this, though it surprised me very much, was not sufficient to open my eyes, and to remove my prejudices respecting the conducting power of water.

Being at Naples, in the beginning of the year 1794, among the many natural curiosities which attracted my attention, I was much struck with several very interesting phenomena which the hot baths of BAIA presented to my observation; and among them there was one, which quite astonished me: standing on the seashore, near the baths, where the hot steam was issuing out of every crevice of the rocks, and even rising up out of the ground, I had the curiosity to put my hand into the water. As the waves which came in from the sea followed each other without intermission, and broke over the even surface of the beach, I was not surprised to find the water cold; but I was more than surprised, when, on running the ends of my fingers through the cold water into the sand, I found the heat so intolerable, that I was obliged instantly to remove my hand. The sand was perfectly wet; and yet, the temperature was so very different at the small distance of two or three inches! I could not reconcile this with the supposed great conducting power of water. I even found that the top of the sand was, to all appearance, quite as cold as the water which flowed over it; and this increased my astonishment still more. I then, for the first time, began to doubt of the conducting power of water, and resolved to set about making experiments to ascertain the fact. I did not however put this resolution into execution till about a month ago; and should perhaps never have done it, had not another unexpected appearance again called my attention to it, and excited afresh all my curiosity.

In the course of a set of experiments on the communication of Heat, in which I had occasion to use thermometers of an uncommon size, (their globular bulbs being above four inches in diameter) filled with various kinds of liquids, having exposed one of them, which was filled with spirits of wine, in as great a heat as it was capable of supporting, I placed it in a window, where the sun happened to be shining, to cool; when, casting my eye on its tube, which was quite naked, (the divisions of its scale being marked in the glass with a diamond) I observed an appearance which surprised me, and at the same time interested me very much indeed. I saw the whole mass of the liquid in the tube in a most rapid motion, running swiftly in two opposite directions, up, and down, at the same time. The bulb of the thermometer, which is of copper, had been made two years before I found leisure to begin my experiments; and having been left unfilled, without being closed with a stopple, some fine particles of dust had found their way into it, and these particles, which were intimately mixed with the spirits of wine, on their being illuminated by the sun’s beams, became perfectly visible, (as the dust in the air of a darkened room is illuminated and rendered visible by the sun-beams which come in through a hole) and by their motion discovered the violent motions by which the spirits of wine in the tube of the thermometer was agitated.

This tube, which is

of an inch in diameter internally, and very thin, is composed of very transparent, colourless glass, which rendered the appearance clear and distinct, and exceedingly beautiful. On examining the motion of the spirits of wine with a lens, I found that the ascending current occupied the axis of the tube, and that it descended by the sides of the tube.

On inclining the tube a little, the rising current moved out of the axis, and occupied the side of the tube which was uppermost, while the descending current occupied the whole of the lower side of it.

When the cooling of the spirits of wine in the tube was hastened by wetting the tube with ice cold water, the velocities of both the ascending and the descending currents were sensibly accelerated.

The velocity of these currents was gradually lessened, as the thermometer was cooled; and when it had acquired nearly the temperature of the air of the room, the motion ceased entirely.

By wrapping up the bulb of the thermometer in furs, or any other warm covering, the motion might be greatly prolonged.

I repeated the experiment with a similar thermometer of equal dimensions, filled with linseed-oil, and the appearances, on setting it in the window to cool, were just the same. The directions of the currents, and the parts they occupied in the tube, were the same; and their motions were to all appearance quite as rapid as those in the thermometer which was filled with spirits of wine.

Having now no longer any doubt with respect to the cause of these appearances, being persuaded that the motion in these liquids was occasioned by their particles going individually, and in succession, to give off their Heat to the cold side of the tube, in the same manner as I have shown in another place, that the particles of air give off their Heat to other bodies, I was led to conclude that these, and probably all other liquids, are in fact non-conductors of Heat; and I went to work immediately to contrive experiments to put the matter out of all doubt.

On considering the subject attentively, it appeared to me, that if liquids were in fact non-conductors of Heat, or if it be propagated in them only in consequence of the internal motions of their particles; in that case, every thing which tends to obstruct those motions, ought certainly to retard the operation, and render the propagation of the Heat slower, and more difficult. I had found that this is actually the case in respect to air; and though (under the influence of a strong and deep-rooted prejudice) I had, from the result of one imperfect experiment, too hastily concluded, that it did not take place in regard to water; yet I now found strong reasons to call in question the result of that experiment, and to give the subject a careful and thorough investigation.

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Chicago: Benjamin Thompson, "Convection of Heat," A Source Book in Physics in A Source Book in Physics, ed. William Frances Magie (Cambridge: Harvard University Press, 1935), 146–150. Original Sources, accessed May 30, 2024, http://www.originalsources.com/Document.aspx?DocID=4Z73YJQ8DYD3VHM.

MLA: Thompson, Benjamin. "Convection of Heat." A Source Book in Physics, Vol. II, in A Source Book in Physics, edited by William Frances Magie, Cambridge, Harvard University Press, 1935, pp. 146–150. Original Sources. 30 May. 2024. http://www.originalsources.com/Document.aspx?DocID=4Z73YJQ8DYD3VHM.

Harvard: Thompson, B, 'Convection of Heat' in A Source Book in Physics. cited in 1935, A Source Book in Physics, ed. , Harvard University Press, Cambridge, pp.146–150. Original Sources, retrieved 30 May 2024, from http://www.originalsources.com/Document.aspx?DocID=4Z73YJQ8DYD3VHM.