fermentation, viz. decompofing fubftances, are always attended with heat. That fort of decompofition of combustible substances, or of oxygen air, which produces a redness, &c. is commonly called fire; and is gradually propagated from one part to another by its own action. Thus, when the abovementioned ignited particles of fteel are received upon a fubftance of eafy decompofition, fuch as tinder, the touching parts of the tinder are heated and decompofed by that heat, their component particles then, attracting the oxygen of the air, difengage the caloric of that fluid, and this caloric heats and decomposes the contiguous particles of the tinder, which also decompofe more air; and thus the combuftion proceeds and continues as long as there are combuftible fubftances and oxygen air ready for decompofition.

It is evident that the contact of a fubftance actually burning is not abfolutely neceffary for communicating the combuftion to other combustible bodies; it being only neceffary to heat those combustible bodies to a certain degree; and heat is communicable without the actual contact of the ignited body.

Sometimes combuftion is communicated from a burning body to another, which is not fo near as to be heated fufficiently by it. Thus, when a tallow candle just blown out is fituated within a certain distance of the flame of another lighted candle, and in fuch a direction as that the ftream of smoke or

vapour, which proceeds from the former, may pafs. through the flame of the latter; it frequently happens that the former is thereby lighted. But it must be observed, that in this and other fimilar cafes, the stream of smoke and vapour is a real train of combustible matter, which is inflamed, and burns progreffively from the flame of the lighted candle to the wick of the other.

A variety of economical regulations, the established cuftoms of the greatest part of the human fpecies, the operations' of different arts, the comforts and even the actual existence of human life, require an artificial fupply of heat; and the greatest part of that heat can only be obtained from the burning of combustible bodies.

The combustibles, or the fuel for common fires, are either wood or pit-coals; for all the other combustible substances are neither plentiful, nor can they be advantageoufly ufed. Wood for burning is become rather scarce almost all over Europe; coals are not to be found in every country, and even where found their mines must be exhausted in time. These confiderations fuggeft the propriety of ufing with care and œconomy thofe two fpecies of fuel. By proper management a great deal of waste may be prevented, without diminishing the advantages which are derived from the ufe of fires.

In the conftruction of fire-places, and in the application of their heat, fome general rules may be of ufe to the intelligent reader.

1. The materials for the construction of fireplaces ought to be bad conductors of heat, viz. earthen ware or ftone, rather than metal; but where metal cannot be avoided, then the metallic parts ought to be furrounded by bricks or other bad conductors of heat, excepting where the heat may be required to be tranfmitted.

2. The draught of air neceffary for the combustion ought to be juft fufficient, and not too much. The ftream of it must be conveyed in fuch a direction as not to interfere with the veffels, or people, &c. that are to be heated by the fire.-It has been found that in a furnace where ftrong heat is required to be produced, and where bellows are ufed, a large quantity of air thrown in with little velocity, is more useful than a fmaller quantity which is thrown in with greater velocity.

3. When heat is to be conveyed through tubes, paffages, &c. care must be had to furround those tubes with bad conductors of heat.

4. In the conftruction of fire places, furnaces, ovens,' &c. and in the management of heat, it must be likewife remarked, that heat paffes through certain bodies, is reflected by others, and is refracted (viz. its courfe is bent) in paffing through others *. Thefe properties will be briefly explained in the fequel.

For the particular conftruction of Kitchen fire-places, fee Count Rumford's Eflays.

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Of the folids the metallic fubftances are the best conductors of heat, next to them are fome compact ftones. The other earthy bodies conduct lefs and lefs, in proportion as they are lefs compact in their texture, and more mixed with water or oleaginous fubftances. Coals, and other combustible minerals, are very bad conductors of heat. Wood, and other vegetable parts, and fuch bodies as are formed of them, viz. paper, ropes, &c. are fo bad conductors of heat, that you may fafely hold a piece of any of them that is actually burning within less than an inch distance from your fingers. Charcoal and charcoal duft, are very good non-conductors of heat, and on that account very fit to be placed round tubes, partions, &c. wherein heat is to be retained.

Fluids feem to be exceedingly bad conductors, if not perfect non-conductors, of heat. In fhort, heat seems to be propagated through fluids merely in confequence of the internal motion of their particles. Whatever permits or promotes that motion, contributes to the propagation of heat; -- whatever obftructs that motion, retards the propagation of heat through fluids. The particles of air which come in contact with an heated body, being thereby heated and rarefied, become fpecifically lighter than the furrounding air, and of course afcend; other air then comes in contact with the heated body, and this alfo is heated and caufed to afcend, &c. Thus is heat conveyed from the original hot body,

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body, by the air, to a diftance from it; but if that motion of the air be obftructed, as by the interpofition of partitions, of papers, wool, cotton, furs, and the like; then that communication of heat is thereby prevented more or lefs, in proportion to the obstruction to the motion. It is principally on this account that furs, feathers, eider down, cotton, and the like, form warm coverings, viz. because, by preventing in a great measure the motion of the air between their filaments, prevent at the fame time the diffipation of heat.

The like obfervations are applicable to water, and perhaps to all other fluids. When a veffel full of water is placed upon the fire, the particles of water that are close to the bottom of the vessel are first heated and rarified, viz. become fpecifically lighter; hence they afcend, and other colder particles take their place; these are heated next, and likewife rife, &c. This is the cause of the intestine motion of water whilft heating. If the fire be applied to the upper part of the water, the lower water will not thereby be heated; for heated and rarefied water will not defcend.

Count Rumford confined a piece of ice at the bottom of a pretty tall glafs veffel full of water near the boiling point, and noted the time it required to melt the ice. He then repeated the experiment, with this difference, viz. that a fimilar piece of ice was placed on the surface of the hot water. It was found that the ice melted more than eight times

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