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ftance through which heat cannot ducting powers depend essentially pals, or through which it pailes with upon the extreme mobility of their great difficulty. If a covering could parts; in fhort, that they rather be found perfectly impervious to transport heat than allow it a parheat, there is reason to believe that fage. But I will not anticipate a a hot body, completely surrounded fubje& which I propose to treat by it, would remain hot for ever ; more fully at some future period. but we are acquainted with no
“ The conducting power of any such substance ; nor is it probable solid body in one folid mass, is much that any such exists.
greater than that of the same body 46 Thole bodies in which heat reduced to a powder, or divided passes freely or rapidly, are called into many smaller pieces : an iron conductors of heat; thoie in which it bar, or an iron plate, for instance, is makes its way with great difficulty, a much better conductor of heat or very flowly, non-condu&ors, or bad than iron filings; and saw-duft is a conductors of heat. The epithets, better non-conductor than wood, good, bad, indifferent, excellent, &c. Dry wood-athes is a better nonare applied indifferently to conductors conductor than either; and very and to non-conductors. A good con- dry charcoal reduced to
a fine ductor, for instance, is one in which powder is one of the best non-conheat passes very freely ; a good non- ducors known; and as charcoal is conductor is one in which it passes perfectly incombustible when conwith great difficulty; and an indif- fined in a space where freth air can ferent conductor may likewise be have no access, it is admirably well called, without any impropriety, an calculated for forming a barrier for indifferent non-conductor.
confining heat, where the heat to be “ Those bodies which are the confined is intense. worst conductors, or rather the best “ But among all the various subnon-conductors of heat, are best ad- ftances of which coverings may be apted for forming coverings for con- formed for confining heat, none can fining heat.
be employed with greater advan“ All the metals are remarkably tage than common atmospheric air. good conductors of heat ;-wood, It is what nature employs for that and in general all light, dry, and purpose; and we cannot do better fpungy bodies, are non-conductors : than to imitate her. glass, though a very hard and com “ The warmth of the wool and pact body, is a non-conductor. Mer- fur of beasts, and of the feathers of cury, water, and liquids of all kinds, birds, is undoubtedly owing to the are conductors; but air, and in ge- air in their interstices; which air, neral all elaitic fluids, ftcan not being strongly attracted by these even excepted, are non-conductors. subitances, is confined, and forms a
“ Some experiments which I have barrier which not only prevents the lately made, and which have not yet cold winds from approaching the been published, have induced me to body of the animal, but which opsuspect, that water, mercury, and poses an almost insurmountable oball other non-elastic fluids, do not Itacle to the escape of the heat of permit heat to pass through them the animal into the atmosphere. from particle to particle, as it un- And in the same manner the air in doubtedly passes through folid bo- snow serves to preserve the heat of dies, but that their apparent con
the earth in winter. The warmth
of all kinds of artificial clothing it is the confined air fhut up bemay be thown to depend on the tween the two windows, and not 1ame cause; and were this circuin- tlie double glass plates, that renders stance more generally known, and thie paffage of heat through them to more attended to, very important difficult. Were it owing to the inimprovements in the management creased thicknefs of the glass, a finof heat could not fail to result from gle pane of glass twice as thick it. A great part of our lives is would answer the fame purpose; ipent in guarding ourselves against but the increased thickness of the the extremes of heat and of cold, glass of which a window is formed, and in operations in which the use is not found to have any sensible of fire is indispensable ; and yet how effect in rendering a room Farmer. little progress has been made in that “ But air is not only a non-conmost useful and most important ductor of heat, but its non-conductof the arts, — the management of ing power may be greatly increald. heat !
To be able to form a juft idea of the “ Double windows have been in manner in wbieh air may be renderuse many years in most of the north- ed a worse conductor of heat, a, ern parts of Europe, and their great which is the same thing, a better utility, in rendering the houses fur- non-conductor of it than it is in its nished with them warm and com- natural unconfined ftate, it will be fortable in winter, is universally ac- neceflary to consider the manner in knowledged, - but I have never which heat passes through air. Now heard that any body has thought of it appears, from the result of a numemploying them in hot countries to ber of experiments which I' made keep their apartments cool in sum- with a view to the investigation of mer ; - yet how easy and natural this subject, and which are publithis this application of to simple and ed in a paper read before the Royal useful an invention !-- If a double Society, that though the particles of .window can prevent the heat which air, each, particle for itselt, can reis in a room from palling out of it, ceive heat from other bodies, er one would imagine it could require communicate it to them, yet there no great effort of genius to discover is no coinmunication of heat bethat it would be equally efficacious tween one particle of air and anofor preventing the heat without ther particle of air. And from from coming in. But natural as hence it follows, that though air this conclusion may appear, I be- may, and certainly does, carry off lieve it has never yet occurred to heat, and transport it from one any body; at least, I am quite cer- place, or from one body to another, tain that I have never seen a double yet a mass of air in a quiescent window either in Italy, or in any fiate, or with all its particles at rett, other hot country I have had occa could it remain in this ftate, fion to visit.
would be totally impervious to heat; “ But the utility of double win or such a mass of air would be a dows and double walls, in hot as perfect non-conductor. well as in cold countries, is a matter “ Now it beat patres in a mass of of so much importance that I thall air merely in consequence of the take occasion to treat it more fully motion it occasions in that air, if in another place. In the mean it is transported, — not suffered to time, I fall only observe here, that pats, - in that case, it is clear that
whatever can obstruct and impede er, though it is not probable that
“ But in mixing bodies with air, tween certain bodies and air. The
vacuum as 1000 to 604, or as 10 to “ Another circumstance to be at- 6, very nearly); but if heat can tended to in the choice of a sub- pass where there is no air, it muli in stance to be mixed with air, in or that care pass by a medium more der to form a covering or barrier for fubtile than air ; -a medium which confining heat, is the fineness or most probably pervades all folid bosubtility of its parts ; for the finer dies with the greatett facility, and they are, the greater will be their which must certainly pervade either Yurface in proportion to their foli- the glass or the mercury employed dity, and the more will they impede in making a Torricellian vacuum. the motions of the particles of the « Now, if there exists a medium air. Coarse horse-hair would be more subtile than air, by which heat found to answer much worse for this may be conducted, is it not possible purpose than the tine fur of a beay- that there may exist a certain affi
nity between that medium and sen- the danger of reasoning upon the lible bodies ? A certain attraction or properties of a faid whose existence cohesion by means of which bodies even is doubtful; and feeling that in general, or fome kinds of bodies our knowledge of the nature of in particular, may, fome how or heat, and of the manner in which other, impede this medium in its it is communicated from one body operations in conducting or tran- to another, is much too imperfeá sporting heat from one place to an- 'and obscure to enable us to pursue other ? — It appeared from the re- these sprculations with ariy prospect sult of several of my experiments, of success or advantage. of which I have given an account “ Whatever may be the manner in detail in my paper before men- in which heat is communicated tioned, published in the year 1786 in from one body to another, I think the lxxvith vol. of the Philosophical it has been sufficiently proved that it Transactions, that the conducting paffes with great difficulty through power of a Torricellian vacuum is confined air ; and the knowledge of to that of air as 604 to 1000:- but this fact is very important, as it enI found by a subsequent experi- ables us to take our measures with ment, (see my second paper on heat, certainty and with facility for compublithed in the Philofophical Tranf- fining heat, and directing its oper3actions for the year 1792,)—that 55 tions to useful purpoles. parts in bulk of air, with 1 part of “ But atmospheric air is not the fine raw filk, formed a covering for only non-conductor of heat. All confining heat, the conducting pow. kinds of air, artificial as well as niaer of which was to that of air as 576 tural, and in general all elastic to 1284 ; or as 448 to 1000. Now, fluids, fteam not excepted, seem to from the result of this laft-mention- potles this property in as high a ed experiment, it should seem that degree of perfection as atmospheric the introduction into the fpace air. tiirough which the heat passed, of “ That steam is not a condueca so small a quantity of raw silk as să of heat, I proved by the following part of the volume, or capacity of experiment: a large globular bottle that space, rendered that space being provided, of very thin and (which now contained 55 parts of very transparent glais, with a warair and I part of filk) more imper- row neck, and its bottom drawn invious to heat than even a Torricel- ward so as to forin a hollow hemilian vacuum. - The silk must there-sphere about 6 inches in diameter ; fore not only have completely de- this bottle, which was about 8 inches stroyed the conducting power of in diameter externally, being filled the air, but must also at the same with cold water, was placed in a time have very fentibly impaired shallow dishi, or rather plate, about that of the etherial Aluid which pro 10 inches in diameter, with a flat bably occupies the interstices of air, bottom formed of very thin sheet and which ferves to conduct heat brass, and raised upon a tripod, and through a Torriceliian vacuum: for which contained a small quantity a Torricellian vacuum was a better (about të of an inch in depth) of conductor of heat, than this medi- water ; a spirit lamp being the um, in the proportion of 604 to placed under the middle of this 449. But I forbear to enlarge plate, in a very few minutes the upoa this subje&t, being sensible of water in the plate began to boil,
and the hollow formed by the bot- steam passing out of it into the cold tom of the bottle was filled with body, clouds would of course be clouds of steam, which, after circu- formed; but I thought if steam was lating in it with furprising rapidity a non-conductor of heat, - that is to 4 or 5 minutes, and after forcing say, if one particle of steam could out a good deal of air from under not communicate any part of its the bottle, began gradually to clear heat to its neighbouring particles, up. At the end of 8 or 10 minutes in that cafe, as the cold body could (when, as I supposed, the air re- only affeat ihe particles of steam acmaining with the steam in the hol- tually in contact with it, no cloud low cavity formed by the bottom of would appear ; ant the result of the the bottle, had acquired nearly the experiment showed that steam is in same temperature as that of the faè a non-conductor of heat; for, steam) these clouds totally dilap- notwithftanding the cold body ofed peared ; árd, tkough the water con in this experiment wis very large tinued to boil with the utmost vid- and very cold, being a fold lump lence, the contents of this hollow of ice nearly as large as an hen's. cavity became so perfectly invisible, egg, placed in the middle of the and so little appearance was there of hollow cavity under the bottle, upsteam, that, had it not been for the on a small tripod or stand made of streams of water which were con- iron wire; yet as soon as the clouds tinually running down its fides, I which were formed in consequence should almost have been tempted to of the unavoidable introduction of doubt whether any steam was actu- cold air in lifting up the bottle to ally generated.
1 introduce the ice, were dissipated, “ Upon lifting up for an instant which foon happened, the steam beone side of the bottle. and letting in came so perfectly transparent and a smaller quantity of cold air, the invisible, that not the smallest apclouds instantly returned, and con- pearance of cloudiness was to be tinued circulating several minutes seen any where, not even about the with great rapidity, and then gradu- ice, which, as it went on to meit, ally disappeared as before. This ex- appeared as clear and as transparent periment was repeated several times, as a piece of the finest rock cryftal. and always with the same result; “ This experiment, which I first the fteam always becoming vitible made at Florence, in the month of when cold air was mixed with it, November, 1793, was repeated seand afterwards recovering its tranf- veral times in the presence of lord parency when, part of this ait being Palmerston, who was then at Floexpelled, that which remained haŭ rence, and Mont. de Fontana. acquired the temperature of the In thele experiments the air Iteain.
was not entirely expelled from un“ Finding that cold air introduc- der the bottle ; on the contrary, a ed under the bottle cauted the steam considerable quant'ty of it remained to be partially condensed, and clouds mixed with the steam even after the to be formed, I was defirous of fee- clouds had totally disappeared, as I ing what visible effects would be found by a particular experiment procured by introducing a cold fo- made with a view to ascertain that lid body under the bottle. · I ima- fact; but that circumftance does not gined that if steam was a conductor render the result of this experiment of heat, some part of the heat in the leis curious, on the contrary I think