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whatever can obftruct and impede the internal motion of the air, muft tend to diminish its conducting pow. er: 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 213 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 themfelves 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 pail ing through this compound of air and fine threads of metal much fuoner 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 covèring or barrier for confining heat, is the fireness 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.

Coarfe horse-hair would be found to answer much worfe for this purpose than the fine fur of a bea

ver, though it is not probable that there is any effential difference in the chymical properties of thofe 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 ufe than others; and this is a certain attraction which fubfifts be tween 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 affift very pow erfully 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 ftil more hidden cause which renders one. fubftance better than another for confining heat. I have fhewn 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 most probably pervades all folid bodies with the greateft facility, and which must 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 exist 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 men tioned, published in the year 1786 in the ixxvith 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 Tranfactious 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 pow. er of which was to that of air as 576 to 1284; or as 448 to 1000. Now, from the refult of this last-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 fpace (which now contained 55 parts of air and 1 part of filk) more impervious to heat than even a Torricellian vacuum,- The filk muft there fore 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 ethereal 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 448. But I forbear to enlarge upon this fubject, being fenfible of

the danger of reafoning upon the properties of a fluid whole 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 puriue 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 enabies us to take our measures with certainty and with facility for con. fining heat, and directing its opera tions to useful 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 elastic 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 glass, with a narrow neck, and its bottom drawn inward fo as to form a hollow hemisphere 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 fheet 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,

and

and the hollow formed by the bottom of the bottle was filled with clouds of steam, which after circulating in it with furprifing rapidity 4 or 5 minutes, and after forcing out a good deal of air from under the bottle, began gradually to clear up. At the end of 8 or 10 minutes (when, as I fuppofed, the air remaining with the fteam in the hollow cavity formed by the bottom of the bottle, had acquired nearly the fame temperature as that of the fteam) these clouds totally difappeared; and, though the water continued to boil with the utmost violence, the contents of this hollow cavity became fo perfectly invifible, and fo little appearance was there of fteam, that had it not been for the ftreams of water which were continually running down its fides, I fhould almost have been tempted to doubt whether any fteam was actually generated.

"Upon lifting up for an inftant one fide of the bottle, and letting in a fmaller quantity of cold air, the cloud inftantly returned, and continued circulating feveral minutes with great rapidity, and then gradually disappeared as before. This experiment was repeated feveral times, and always with the fame refult; the fteam always becoming visible when cold air was mixed with it, and afterwards recovering its tranf parency when, part of this air being expelled, that which remained had acquired the temperature of the fteam.

"Finding that cold air introduced under the bottle caufed the fteam to be partially condensed, and clouds to be formed, I was defirous of feeing what visible effects would be procured by introducing a cold folid body under the bottle. Iimagined that if steam was a conductor of heat, fome part of the heat in the 1797.

fteam paffing out of it into the cold body, clouds would of course be formed: but I thought if steam was a non-conductor of heat, that is to fay, if one particle of fteam could not communicate any part of its heat to its neighbouring particles, in that cafe, as the cold body could only affect the particles of fteam actually in contact with it, no cloud would appear; and the refult of the experiment fhewed that fteam is in fact a non-conductor of heat; for, notwithstanding the cold body used in this experiment was very large and very cold, being a folid lump of ice, nearly as large as an hen's egg, placed in the middle of the hollow cavity under the bottle upon a fmall tripod or ftand made of iron wire; yet as foon as the clouds which were formed in confequence of the unavoidable introduction of cold air in lifting up the botte to introduce the ice, were ditipated, which foon happened, the tream became fo perfectly tranfparent and invifible, that not the fmalleft appearance of cloudinefs was to be feen any where, not even about the ice, which, as it went on to melt, appeared as clear and as tranfparent as a piece of the fineft rock cryftal.

"This experiment, which I first made at Florence, in the month of November, 1793, was repeated fe veral times in the prefence of lord Palmerston, who was then at Florence, and Monf. de Fontana.

"In these experiments the air was not entirely expelled from under the bottle; on the contrary, a confiderable quantity of it remained mixed with the steam even after the clouds had totally difappeared, as I found by a particular experiment made with a view to ascertain that fact; but that circumstance does not render the refult of this experiment lefs curious, on the contrary I think

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it

it tends to make it more furprising. It fhould feem that neither the mais of steam, nor that of air, were at all cooled by the body of ice which they furrounded, for if the air had been cooled (in mafs), it feems highly probable that the clouds would have returned.

"The refult of thefe experiments compared with those formerly alluded to, in which I had endeavoured to ascertain the most advantageous forms for boilers, opened to me an entirely new field for fpeculation and for improvement in the management of fire. They fhewed me that not only cold air,

but alfo hot air, and hot fteam, and hot mixtures of air and fteam, are non-conductors of heat; confequently that the hot vapour which rifes from burning fuel, and even the flame itself, is a non-conductor of heat.

"This may be thought a bold affertion, but a little calm reflection, and a careful examination of the phenomena which attend the combuftion of fuel, and the communication of heat by flame, will fhew it to be well founded; and the advantages which may be derived from the knowledge of this fact are of very great importance indeed."

ACCOUNT of a METHOD of making SOAP of WOOL, with OBSERVATIONS refpecting its USE in various ARTS. By M. CHAPTAL.

[From the ANNALES DE CHIMIE, and inferted in the Seventh Volume of the REPERTORY of ARTS and MANUFACTURES.]

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"As that part of the report referred to by M. Chaptal appears to be of general utility, we fhall here give a translation of it.

"A very good way of ufing foap is, to employ it in a liquid ftate; that is, diffolved in water. In confequence of which, M. Chaptal propofes that faponaceous liquors should be prepared, which may be uted inftead of folutions of foap; and in order to be able to procure fuch liquors, at all times, in all places, and at a small expence, he advifes one or the other of the following methods to be practifed. We fhall defcribe them exactly as M. Chaptal communicated them to us, with obfervations thereon, made by himself.

"First Method.

"Take the afhes produced from the combuftion of wood which has not been floated, and make a ley of them, according to the ufual manner; mixing with the ashes a handful or two of quick-lime, well pounded or recently flaked. Let the ley ftand till it is grown clear, by the fettling or fwimming of the foreign fubftances contained therein; then pour it into another veffel, and keep it for ufe. When it is propofed to make use of this ley, take any quantity of oil, and pour upon it thirty or forty times as much of the ley. Immediately a liquor as white as milk will be formed, which, by being well

fhaken,

to my former work, inftructing them how to prepare, as a fubfti

tute for foft-foap, (which is at prefent made use of in fulling almoft

evéry

fhaken, or ftirred, lathers and froths like a good folution of foap. This liquor is to be poured into a washing-tub, or other veffel, and to be diluted with a greater or lefs quantity of water; after which, the linen meant to be washed, is to be fteeped therein, to be rubbed and wrung, in the ufual way.

"Obfervations.

1. It is better that the ley should not be made until the time when it is to be used: if it is left to ftand in open veffels, its power is weakened, and its nature is changed. "2. Fresh wood-afhes are preferable to old ones, particularly if the latter have been exposed to the air; in that cafe, they have no longer their usual power, and we must, in order to make them ferve our purpose, mix with them a greater proportion of quicklime.

"3. Those afhes alfo are preferable which are produced from hard wood: thofe which are left after the burning of floated wood cannot be made ufe of with equal fuccefs.

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4. Fat oils, of a thick confiftence, are moft proper for the purpose here spoken of: fine thin oils are by no means fit for it.

"5. If ftinking oil be made use of, it is apt to give a bad smell to the linen; this may be removed by paffing the linen carefully through a ftrong pure ley; but in general, this fmell goes off as the linen becomes dry.

6. When the mixture of oil with the ley is of a yellow colour, it must be diluted with water.

"7. When the oil rises in the ley, and swims upon the surface of it, in the form of fmall drops, it fhews that the oil is not fit for the purpose, not being thick enough; or elle, that the ley is too ftrong, or not fufficiently cauftic.

8. To prevent the quick-lime from lofing its power, and that we may always have fome to use when we want it, it may be broken into small pieces, and kept in bottles well dried, and well corked.

"Second Method.

"Floated wood, which is made ufe of in many parts of France, produces afhes which contain very little alkaline falt, and which are confequently very improper for making leys; in that cafe, barilla, or potash, may be used inftead of them.

“Take barilla, and break it into pieces about the fize of a walnut; put these into a veffel of any kind, and pour upon them twenty times their weight of water: the water is to be left upon the barilla till it appears, by putting a little upon the tongue, to be flightly falt.

"Some oil is then to be put into an earthen veffel, and forty times as much of the barilla-ley is to be poured upon it: the mixture, which foon becomes milky, is to be well shaken, or ftirred; and, after being diluted with more or lefs clean water, according to its ftrength, and the purpofe for which it is intended, is to be made ufe of like a folution of foap in water.

"Inftead of barilla, pot-afh may be employed, but it requires a fmall quantity of pounded quick-lime to be mixed with it.

“Obfervations.

"1. Alicant or Carthagena barilla may be ufed without any mixture of lime; but the bad barilla of our country requires to have mixed with it a greater or less proportion of lime, according to its degree of ftrength and purity.

“2. When barilla, of whatever kind it may be, is in a state of efflorefcence, it cannot be employed without a mixture of lime.

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"3. If

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