Abbildungen der Seite
PDF
EPUB

appeared as clear and as transparent as a piece of the finest rock Crystal.' P. 61.

We are not, however, convinced that the circumstances in that experiment will bear him fully through his conclufion. Steam we know will becoine transparent when confined, and clouds ensue on the admillion of cold air ; but the clouds fee il to arise from the refraction of light through fluids of ditferent densities, as the transparent ítearn and cold air must be, and no conclufion can be drawn, without farther trial, how much the one is cooled and the other heated. When the ice was put in this traniparent steam, the melting thowed that steam really communicated its heat; but it formed water, not air ; and the water, by continued heat, produced steam in a transparent vapour, so that as no fluid differing in density from the fieam was produced, no cloudiness ensued. In other respects, we find no reason to retract the objections we once made to our author's opinions in reviewing the above-mentioned papers. The objections, however, relate chiefly to count Rumford's (then fir Benjamin Thompfon's) explanations. With regard to this part of the subject, we are inclined to think, that steam is a conductor of heat ; for it coinmunicates its own heat to surrounding bodies, and may therefore communicate additional heat. The count supposes, that tteam cannot be chemically changed by additional heat; but, if Saussure's experiments are, faithfully related, the water appears to be decompounded and to become air, either by additional heat or by evaporating from different metals.

The count contends that flame is not a conductor of heat, and that its activity, when impelled by the blow-pipe, arifes from its impinging in successive ecklies. He found that whatever kind of air was forced through the blow-pipe, the effects were the same ; but we believe that chemists in general, who employ vital air with this instrument, think differently. On the whole, this is not fully proved. Flame is only red hot vapour, and must, in its different affinities, be the faine as the va. pour not ignited; and the question is of less importance, as the practical conclufion, that the greatest heat is at the apex, not the side of the flame, is sufficiently established by common experience.

The experiments with boilers of different kinds, we find it difficult to analyse with advantage. Those who would derive benefit from them must read the whole. In general, dry wood is more economical thun moist; the bottoms of the boilers must be as thin as is convenient with strength; the flame thould be confined to the bottom of the boiler, and though in general large boilers :re more æconomical than fmall ones, there is a maximum in the size above which they become disa 'advantageous.

For the account of different kitchens constructed by the author, and the particular advantages of many of his contrivances, we refer to the work itself. But we cannot refrain from ranscribing the following account of a military kitchen, and of a portable boiler. The former is on the plan of closed kitchens for poor houses.

• I lately had an opportunity of fitting up a kitchen on these principles, in the construction of which there was not a particle of iron used, or of any other metal, except for the boiler. On i he approach of the French army under general Moreau in August lait, ihe Bavarian troops being assembled at Munich (under my command) for the defence of the capital, the town was fo full of soldiers, that feveral regiments were obliged to be quartered in public buildings, and encamped on the ramparts, where they had no conveniences for cooking. For the accommodation of a part of them, four large oblong square boilers, composed of very thin sheet coppers well tinned, were fitted up in a mass of brick-work in the form of a cross; each boiler with its separate fire place, communicating by double canals, furnithed with dampers, with one common chimney, which stands in the centre of the cross. The dampers are thin flat tiles; the grates on which the fuel is burned are composed of common bricks, placed edgewise; and the passages leading to the fire-place, and to the afh-pit, are closed by bricks which are made to slide in grooves.

• Under the bottom of each boiler, which is quite flat, there are three flues, in the direction of its lengih; that in the middle, which is as wide as both the others, being occupied by the burning fuel. The opening by which the fuel is introduced is at the end of the boiler fartheit from the chimney; and the flame running along the middle flue to the end of it, divides there, and returning in the two fide fiues to the hither end of the boiler, there rises up into two other fiues, in which it palles along the outside of the boiler into the chimney. The boilers are furnished with wooden covers divided into two equal parts, united by hinges. In order that the four boilers may be transported with greater facility from place to place, (from one camp to another for instance) they are not all precisely of the same fize, but one is so much less than the other, that they may be packed one in the other. The largest of them, which contains the three others, is packed in a wooden chest, which is made just large enough to receive it. In the smallest may be packed a circular teni, sufficiently large to cover them all. In the middle of the tent there must be a hole through which the chimney must pass. The four boilers, together with the tent, and all the apparatus and menhis necessary for a kitchen on this construction for a regiment covfisting of a thousand men, might eally be transported from place to place on an Irish car drawn by a single horse. P. 154.

• There is one more invention for the use of armies in the field,

Wlrich I wifh to recommend, and that is a portable boiler of a ligh and cheap construction, in which victuals may be cooked on a march. There are so many occasions when it would be very desirable to be able to give soldiers, harassed and fatigued with severe service, a warm meal, when it is impossible to stop to light fires and boil the pot, that I cannot help flattering myself that a contrivance, by which the pot actually boiling may be made to keep pace with the troops as they advance, will be an acceptable present to every humane officer and wise and prudent general. Many a battle has undoubiedly been lost for the want of a good comfortable meal of warm victuals to recruit the strength and raise the spirits of troops fainting with hunger and excessive fatigue.' P. 157.

Piates are added, with proper explanations. The description of the perpetual lime-kiln offers some valuable improvements.

The subject of the seventh essay was intended to be the construction of kitchen fire-places and kitchen utensils; but, as all the experiments were not finished, the author has filled it with observations on the manner in which heat is propagated in fluids ; on a remarkable law found to prevail in the condensation of water with cold, when near the freezing point, with its effects in the economy of nature: to which are added

conjectures respecting the final cause of the saltness of the sea.''

We have used, with our author, the term conductor of heat, without accurately defining his meaning. It is necessary now to be more clear and definite. When heat, as in metals, is communicated from one particle to another, while these particles are at rest, the body is properly called a conductor of heat; but, when there appears to be no Yor a very night) communication of heat between these particles, while at rest, the body is said to be a non, or a bad, conductor of heat. Probably a perfect non-conductor is not known, but many bodies are very imperfect conductors. Air is certainly fo, and water perhaps does not greatly excel air in that refpe&t. Those who have been accustomed to mix warm and cold water, particularly in tempering baths, must be convinced of this fact ; for it requires long time and frequent agitation before the temparature of the bath is uniform. If water and air then were at rest they would conduct heat very slowly, but they are never in a state of perfect tranquillity, and the motion of the particles on each other, which constitutes their fluidity, enables them to conduct heat so well as they do. The particles by these motions communicate their heat to the surrounding vessel, which in turn heats the contiguous particles of fluid. The first suggestion of this method of communicating heat was, from observing the motion of the particles of spirit of wine in a thermomcier, rendered conspicuous by accidental impurities; and a similar inteftine nicion may be oblerved in boiling fluids. The writer accounts for any pulpy substance, or, in reality, any gluten added to water, rendering the fiuid a worse conductor of heat, by preventing this intestine motion. We formerly explained the effect, froin the difficulty with which a particie of water communicates its heat to a particle of the down or pulp, while the contiguous drop receives the heat with equal difficuliv, fo thai heat is transmitted through such fubfiances with a slownieis proportioned to the number of particles interposed, in other words to the fineness of the down or pulpy. Probably both causes inay have their influence; but whatever may be the reason, the difficulty of communicating heat may be the final cause, as our author alleges, of the viícolity of the fluids of plants. They retain in this way the heat communicated by the ground more tenacioudly, and, for this reafon probably, the viscidity of the fap is increased in winter. We well know, that geraniums and many fucculent plants of warin climates, may be exposed to the common colds of our winters with impunity, by a gradual subtraction of their usual Tupply of water.

Our author brought his doctrine of the communication of heat in water, by the commotion of its particles, to the test of experience, by inixing powilered amber in water. Amber he chose as it resembles water fo nearly in specific gravity; while the excess of gravity in the amber was compensated by increasing that of the water, which for this purpose had some fale dissolved in it. His method was unneceffarily operase, for t!e same effect might have been produced by a more minute division of any heavier body.

It is nevertheless a very pleasing and instructive experiment, selembling that in common uie, by which it is proved that 110 heat is felt at the bottom of a tea-kettle while boiling, though it is perfectly and painfully fenfible when the boiling ceases. Yet the latter part of the phænomena is not peculiar to water, for we found the fame want of communication of heat in boiling mercury, in the torricellian tube. To make tre vacuum more complete, the mercury was boiled in portions from the bottom to the top, before it was inserted into the basin; and, though some defence was required for the hands while the under portion boiled, and the tube was held by the top, the upper portions, when boiling, communicated no heat below. Perhaps the same law takes place in all fluids; at least the inquiry is not unimportant, and our author hints at it in p. 241.

The count's next object was an inquiry into the conimuni. cation of heat from water to ice. This investigation presents

fome very curious and unexpected results. By pouring boiling water on ice, less than part of the heat lost by the water was communicated to the ice; and the ice meiled eighty cines more slowly at the bottom than at the top of the water. This might be supposed in some measure froin the former experiments, fince the heat afcends, and the ice inight have reinaincd unaffecte i under the flip of wood which confined it to the bortom, though not in contact except at the edge of the cake, by absorbing heat faster than the frozen Huid. We knowsbad ice receives bear very slowly. When the ice was covered by a tin plate with a circular bole, it was diffolved only at the part where the cover was perforated. This wis a lingular crent; but it is ingeniously explained trom fir Charles Blagden's experimenis. He found that all fluids were gradually condensed by cold, in proportion to its intentitv, cxcept water, which is condensed only till it has reached its foth degree, when it hegius to expand as we know, from the separation of air. When the hot water then reaches the ice, it is foon cooled to the 40th degree, and then becomes lighter than the water above, though of a higher temperature. The consequence is, that the hotter water defcendis ; but the cover pre, vents it froin touching the ice, except at the point where the perforation is, and by descending through it the excavation is forred, which, when over filled, forms a channel over the furface of the cake down the edges. This might have also been the cause of the ice remaining unchanged under the wood.

These principles lead to an extraordinary fact, viz, that water of 40° will melt as much ice, when standing on its furface, as boiling water ; 2.d our author has confirmelit lyy ingenious and operose experiments. We have preferred giving the foregoing detail in our own languape that we might add the conclusion, and the table which results from it, more at length, The general deduction is, that whatever may be the heat of the water which is poured on ice, no water above 40° can remain in contact while any ice is left; for the moment the water acquires a lower temperature it ascends, fince the rarefaction produced by the emulsion of air is grater than that occafiond by the heat of water under 112", as it must be when brougiis

from the fire. Our author has, however, confirmed it liv ex.periment, and lie finds that 190 grains of ice may be melted by water of 410 in about 1o'. These experiineuis leend very clearly so thow that water is a non-conductor of heat. .

Other miscellaneous experiments were adied, and circum. ftances occurred in those above mentioned which contributed to establish our author's principal pofition. The impuis of the water when poured on the ice muit add to the heat, as we commoniy find in heated air; but it seemed to ackl to the cirect, by increasing the motion of the particles on cach other. When

« ZurückWeiter »