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PHILOSOPHICAL PAPERS.

OBSERVATIONS on the MEANS of confining HEAT, and directing its OPERATIONS.

[From the Fourth NUMBER of COUNT RUMFORD'S EXPERIMENTAL ESSAYS, POLITICAL, ECONOMICAL, and PHILOSOPHICAL.]

“TH

HAT heat paffes more freely through fome bodies than through others, is a fact well known; but the cause of this difference in the conducting powers of bodies, with refpect to heat, has not yet been difcovered.

"The utility of giving a wooden handle to a tea-pot or coffee-pot of metal, or of covering its metallic handle with leather, or with wood, is well known: but the difference in the conducting powers of various bodies with regard to heat, may be fhown by a great number of very fimple experiments;-fuch as are in the power of every one to make at all times and in all places, and almost without either trouble or ex

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will grow fo hot as to render it impoffible to hold it in the hand without being burnt; but the wood may be held any length of time in the fame fituation without the leaft inconvenience; and, even after it has taken fire, it may be held till it is almoft entirely confumed; for the uninflamed wood will not grow hot, and, till the flame actually comes in contact with the fingers, they will not be burnt. If a small flip or tube of glafs be held in the flame of the candle in the fame manner, the end of the glafs by which it is held will be found to be more heated than the wood, but incomparably lefs fo than the pin or nail of metal;

and among all the various bodies that can be tried in this manner, no two of them will be found to give a paffage to heat through their fubftances with exactly the fame degree of facility.

"To confine heat is nothing more than to prevent its efcape out of the hot body in which it exifts, and in which it is required to be retained; and this can only be done by furrounding the hot body by fome covering compofed of a fub

ftance

ftance through which heat cannot país, or through which it paffes with great difficulty. If a covering could be found perfectly impervious to heat, there is reafon to believe that a hot body, completely furrounded by it, would remain hot for ever; but we are acquainted with no fuch fubftance; nor is it probable that any fuch exifts.

"Thole bodies in which heat paffes freely or rapidly, are called conductors of heat; thofe in which it makes its way with great difficulty, or very flowly, non-conductors, or bad conductors of heat. The epithets, good, bad, indifferent, excellent, &c. are applied indifferently to conductors and to non-conductors. A good conductor, for inftance, is one in which heat paffes very freely; a good nonconductor is one in which it paffes with great difficulty; and an indifferent conductor may likewife be called, without any impropriety, an indifferent non-conductor.

"Thofe bodies which are the worst conductors, or rather the best non-conductors of heat, are beft adapted for forming coverings for confining heat.

"All the metals are remarkably good conductors of heat ;-wood, and in general all light, dry, and fpungy bodies, are non-conductors: glafs, though a very hard and compact body, is a non-conductor. Mercury, water, and liquids of all kinds, are conductors; but air, and in general all elastic fluids, fteam not even excepted, are non-conductors.

"Some experiments which I have lately made, and which have not yet been published, have induced me to fufpect, that water, mercury, and all other non-elaftic fluids, do not permit heat to pass through them from particle to particle, as it undoubtedly paffes through folid bodies, but that their apparent con

ducting powers depend effentially upon the extreme mobility of their parts; in fhort, that they rather tranfport heat than allow it a paffage. But I will not anticipate a fubje&t which I propofe to treat more fully at fome future period.

"The conducting power of any folid body in one folid mafs, is much greater than that of the fame body reduced to a powder, or divided into many smaller pieces: an iron bar, or an iron plate, for instance, is a much better conductor of heat than iron filings; and faw-duft is a better non-conductor than wood, Dry wood-afhes is a better nonconductor than either; and very dry charcoal reduced to a fine powder is one of the beft non-conductors known; and as charcoal is perfectly incombuftible when confined in a space where freth air can have no accefs, it is admirably well calculated for forming a barrier for confining heat, where the heat to be confined is intenfe.

"But among all the various fubftances of which coverings may be formed for confining heat, none can be employed with greater advantage than common atmospheric air. It is what nature employs for that purpose; and we cannot do better than to imitate her.

"The warmth of the wool and fur of beafts, and of the feathers of birds, is undoubtedly owing to the air in their interftices; which air, being ftrongly attracted by thefe fubftances, is confined, and forms a barrier which not only prevents the cold winds from approaching the body of the animal, but which oppofes an almoft infurmountable obftacle to the efcape of the heat of the animal into the atmosphere. And in the fame manner the air in fnow ferves to preferve the heat of the earth in winter. The warmth

of

of all kinds of artificial clothing may be shown to depend on the fame caufe; and were this circumstance more generally known, and more attended to, very important improvements in the management of heat could not fail to refult from it. A great part of our lives is ipent in guarding ourselves againft the extremes of heat and of cold, and in operations in which the ufe of fire is indifpenfable; and yet how little progrefs has been made in that moft useful and most important of the arts, the management of heat!

"Double windows have been in ufe many years in most of the northern parts of Europe, and their great utility, in rendering the houses furnifhed with them warm and comfortable in winter, is univerfally acknowledged, but I have never heard that any body has thought of employing them in hot countries to keep their apartments cool in fuminer; yet how eafy and natural is this application of fo fimple and ufeful an invention! If a double window can prevent the heat which is in a room from paffing out of it, one would imagine it could require no great effort of genius to difcover that it would be equally efficacious for preventing the heat without from coming in. But natural as this conclufion may appear, I believe it has never yet occurred to any body; at leaft, I am quite certain that I have never seen a double window either in Italy, or in any other hot country I have had occa. fion to vifit.

"But the utility of double windows and double walls, in hot as well as in cold countries, is a matter of so much importance that I fhall take occafion to treat it more fully in another place. In the mean time, fhall only obferve here, that

it is the confined air fhut up bɛtween the two windows, and not the double glafs plates, that renders the paffage of heat through them fo difficult. Were it owing to the increafed thickness of the glafs, a fingle pane of glafs twice as thick would answer the fame purpofe; but the increafed thickness of the glafs of which a window is formed, is not found to have any fenfible effect in rendering a room warmer.

"But air is not only a non-conductor of heat, but its non-conducting power may be greatly increased. To be able to form a juft idea of the manner in whieh air may be rendered a worfe conductor of heat, or, which is the fame thing, a better non-conductor of it than it is in its natural unconfined state, it will be neceffary to confider the manner in which heat paffes through air. Now it appears, from the refult of a number of experiments which I made with a view to the investigation of this fubject, and which are published in a paper read before the Royal Society, that though the particles of air, each, particle for itself, can receive heat from other bodies, or communicate it to them, yet there is no communication of heat between one particle of air and another particle of air. And from hence it follows, that though air may, and certainly does, carry off heat, and tranfport it from one place, or from one body to another, yet a mafs of air in a quiefcent fiate, or with all its particles at reft, could it remain in this ftate,would be totally impervious to heat; or fuch a mafs of air would be a perfect non-conductor.

"Now if heat paffes in a mass of air merely in confequence of the motion it occafions in that air,—if it is tranfported, not suffered to pafs, in that cafe, it is clear that

what

whatever can obftruct and impede the internal motion of the air, muft tend to diminish its conducting power and this I have found to be the cafe in fact. I found that a certain quantity of heat which was able to make its way through a wall, or rather a fheet of confined air an inch thick in 9 minutes, required 21 minutes to make its way through the fame wall, when the internal motion of this air was impeded by mixing with it part of its bulk of eider-down, of very fine fur, or of fine filk, as fpun by the worm.

"But in mixing bodies with air, in order to impede its internal motion, and render it more fit for confining heat, fuch bodies only muft be chofen as are themselves nonconductors of heat, otherwife they will do more harm than good, as I have found by experience. When, inftead of making ufe of eiderdown, fur, or fine filk, for impeding the internal motion of the confined air, I used an equal volume of exceedingly fine filver-wire flatted, (being the ravellings of gold or filver lace,) the paffage of the heat through the barrier, fo far from being impeded, was remarkably facilitated by this addition; the heat paffing through this compound of air and fine threads of metal much fooner than it would have made its way through the air alone.

"Another circumftance to be attended to in the choice of a fubftance to be mixed with air, in order to form a covering or barrier for confining heat, is the fineness or fubtility of its parts; for the finer they are, the greater will be their furface in proportion to their folidity, and the more will they impede the motions of the particles of the air. Coarse horse-hair would be found to answer much worfe for this purpose than the fine fur of a beav

er, though it is not probable that there is any effential difference in the chymical properties of those two kinds of hair.

"But it is not only the fineness of the parts of a fubftance, and its being a non-conductor, which render it proper to be employed in the formation of covering to confine heat; there is ftill another property, more occult, which feems to have great influence in rendering fome fubftances better fitted for this use than others; and this is a certain attraction which fubfifts between certain bodies and air. The obftinacy with which air adheres to the fine fur of beafts and to the feathers of birds, is well known; and it may eafily be proved that this attraction muft aflift very powerfully in preventing the motion of the air concealed in the interstices of thofe fubftances, and confequently in impeding the paffage of heat through them.

"Perhaps there may be another ftill more hidden caufe which renders one fubftance better than another for confining heat. I have fhown by a direct and unexceptionable experiment, that heat can pafs through the Torricellian vacuum, though with rather more difficulty than in air (the conducting power of air being to that of a Torricellian vacuum as 1000 to 604, or as 10 to 6, very nearly); but if heat can pafs where there is no air, it must in that cafe pafs by a medium more fubtile than air; -a medium which moft probably pervades all folid bodies with the greateft facility, and which muft certainly pervade either the glafs or the mercury employed in making a Torricellian vacuum.

"Now, if there exists a medium more fubtile than air, by which heat may be conducted, is it not poffible that there may exift a certain affi

nity between that medium and fenfible bodies? A certain attraction or cohesion by means of which bodies in general, or fome kinds of bodies in particular, may, fome how or other, impede this medium in its operations in conducting or tranfporting heat from one place to another?-It appeared from the refult of feveral of my experiments, of which I have given an account in detail in my paper before mentioned, published in the year 1786 in the lxxvith vol. of the Philofophical Tranfactions, that the conducting power of a Torricellian vacuum is to that of air as 604 to 1000:-but I found by a fubfequent experiment, (fee my fecond paper on heat, published in the Philofophical Tranfactions for the year 1792,)-that 55 parts in bulk of air, with 1 part of fine raw filk, formed a covering for confining heat, the conducting power of which was to that of air as 576 to 1284; or as 448 to 1000. Now, from the refult of this laft-mentioned experiment, it fhould feem that the introduction into the space through which the heat paffed, of fo small a quantity of raw filk as, part of the volume, or capacity of that space, rendered that space (which now contained 55 parts of air and 1 part of filk) more impervious to heat than even a Torricellian vacuum. The filk muft therefore not only have completely deftroyed the conducting power of the air, but muft alfo at the fame time have very fenfibly impaired that of the etherial fluid which probably occupies the interstices of air, and which ferves to conduct heat through a Torricellian vacuum: for a Torricellian vacuum was a better conductor of heat, than this medium, in the proportion of 604 to 449. But I forbear to enlarge upon this subject, being fenfible of

the danger of reafoning upon the properties of a fluid whofe exiftence even is doubtful; and feeling that our knowledge of the nature of heat, and of the manner in which it is communicated from one body to another, is much too imperfect and obfcure to enable us to pursue thefe fpeculations with any profpect of fuccefs or advantage.

"Whatever may be the manner in, which heat is communicated from one body to another, I think it has been fufficiently proved that it paffes with great difficulty through confined air; and the knowledge of this fact is very important, as it enables us to take our meafures with certainty and with facility for confining heat, and directing its opera tions to ufeful purposes.

"But atmospheric air is not the only non-conductor of heat. All kinds of air, artificial as well as natural, and in general all elaftic fluids, fteam not excepted, feem to poffefs this property in as high a degree of perfection as atmospheric air.

"That fteam is not a conductor of heat, I proved by the following experiment: a large globular bottle being provided, of very thin and very tranfparent glais, with a narrow neck, and its bottom drawn inward fo as to form a hollow hemifphere about 6 inches in diameter; this bottle, which was about 8 inches in diameter externally, being filled with cold water, was placed in a fhallow difh, or rather plate, about 10 inches in diameter, with a flat bottom formed of very thin sheet brafs, and raised upon a tripod, and which contained a fmall quantity (about of an inch in depth) of water; a fpirit lamp being then placed under the middle of this plate, in a very few minutes the water in the plate began to boil.

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