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given us an excellent compilation containing all the principal facts requisite to be known relative to this most interesting subject.

We shall, in the following pages, endeavour to give a concise view of a few of the principal phenomena of heat, and to explain, in as popular a way as can be done, the causes which produce them. But before we proceed to this part of our subject, we would take a brief view of the history of heat, which, among heathen nations, from its great importance in nature, was considered as worthy the patronage of particular deities, and Vesta and Vulcan were supposed to be, the former-the divinity of heat, as a natural, the latter as an artificial agent; and though from the continual presence and utility of fire in all the processes of art and nature, it would be probable that a people who deified their very vices, and placed among their gods the personifications of their crimes, should also establish deities to represent that agent; yet it seems extraordinary the opinions which were held of its efficacy as a means of purification in all their sacrifices, and in many of the ceremonials of their religion, unless we suppose their acquaintance with those nations, whose ceremonies had received the sanction of divinity itself. Among the philosophers of those days, the opinions respecting heat were as crude and ill-defined as most of their opinions relative to the different branches of natural knowledge, and it is not surprising that those, who would reduce all natural objects to four elementary substances should suppose that heat was one of the chief of these; some of them even going so far as to assert, that fire was the origin or first principle of all things-that by its extinction it produced earth or ashes-that these ashes conceived or collected moisture, whence there ensued a flood of water, which again emitted air, and that therefore fire was the elemental nature of all things.*

Such were the popular and scientific notions which obtained, relative to heat, even 'till very lately, as the alchemists, who were the first investigators of the real constitution of nature were too submissive servants of

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the Aristotelian doctrines to question opinions, which bore the sanction of his disciples, they were content to regard heat as one of the principal means of performing the "great work ;" and though continually conversant with its effects, they made no attempt to investigate its properties, nor to establish the laws that regulate its operations; nor would any advance have been probably made in this subject, had not Lord Bacon, amongst his many other labours for the advancement of knowledge, also applied his mind to the investigation of the nature of heat; his Method of discovering forms in the example of the form of heat" in the Novum Organum will ever remain a perpetual monument of his genius and penetration. In this short sketch he gave suggestions which served to place this part of physical knowledge on its present advanced and eminent position; and when we consider the mass of error he had to combat, we are astonished at that acuteness of mind which enabled him to lay a foundation, which it has required ages to perfect. It was not however immediately that his suggestions were acted on; and it is only within our own times that investigations were entered into, which reduced the abstruse and complicated phenomena of heat to the rank of the physical sciences. And it is with the great names of Black, Watt, Davy, and Leslie, that we connect all the discoveries in a branch of natural knowledge, which, though it has not elevated its investigators to the same rank as mental philosophers with Newton and La Place, has yet given them greater practical rank as the real founders of Great Britain's power and wealth.

The term heat is generally used in two significations, either to express the cause by which a certain sensation is produced, or to signify the sensation itself. We say that we feel heat from a fire, when we only mean that the sensation of heat is produced in us; and in this sense is the word heat ge nerally employed; the confusion arising from using the same word to express two distinct ideas, has induced many writers to make use of the word caloric to express the cause of heat

* Vide History of Natural Philosophy, Bacon's Works, vol. 12.

distinct from the sensation, or represent what may be deemed the matter of heat; but as no material confusion results from the use of the word heat, we shall continue to employ it, meaning "that cause, whatever it may be, which produces in us the peculiar sensation of heat or warmth." Respecting the nature of heat, many opinions have been held; the most common is, that heat is an imponderable substance and consists of material particles, which combine with all known bodies, and may, under certain circumstances, be separated from these and produce in us the sensation of heat;` of the other opinions—that heat is not a material substance, but merely a quality of matter, and that matter exhibits heat by a vibratory motion excited in its particles, or by the vibrations of a fluid which pervades all space-it is not our present purpose to enter on the consideration: each has had its powerful advocates, and like the theories relative to light, there are phenomena which are inexplicable by either the material or vibratory hypothesis se perately, and which have been triumphantly referred to as crucial experiments, by the supporters of each hypothesis; and as there seems to be at present no final decision on the merits of either, we shall, adopting the first as the most simple and most easily understood, proceed to consider a few of the principal facts given us in Dr. Lardner's most interesting volume.

If we place one hand on a piece of paper or woollen cloth, and the other on a piece of metal, we would be immediately induced to believe from the sensation of cold produced by touching the metal that the latter was much colder than the paper or wool; now that such is not the case must be evident if these substances have been exposed to the same temperature under the same circumstances, by which they will have acquired the same quantity of heat; whence then does this sensation of cold arise? why is it that our senses are deceived? The answer is at once afforded by the fact, that heat is conveyed or conducted with different velocities by different substances. When the warm hand is placed on a piece of metal or stone, these produce immediately in the hand the sense of cold by carrying off the heat of the hand immediately, while if we touch a piece

of paper or wool, these from their slow conducting power carry away little or none of the heat of the hand; hence it is that different articles of clothing seem to possess different quantities of heat, linen always feeling colder than silken cloaths, and these colder than woollen, when in fact under the same circumstances, they all have the same degree of heat.

Now if we wish to prove by experiment that this is the fact, and that heat passes more slowly through some bodies than through others, we may easily satisfy ourselves by very simple means; if we take a number of rods, of glass, wood, and different metals, and having coated these with wax, apply heat to the ends, we shall find that the wax is very quickly melted from the metallic rods, very slowly from the glass, and that a great length of time will elapse, before the wax is melted from the piece of wood, proving that heat passes through these different substances with different degrees of velocity. And we may remark, that in general, the conducting power is directly proportioned to the density of the substance; the metals being the best conductors of heat, next vitrified substances, then woods, and lastly wool, silk, and down, which are the worst conductors of heat of all solid substances. Of all the metals silver is found to be the best conductor of heat, consequently we find that when boiling water is poured into a silver tea-pot, it becomes immediately so hot that the hand can not be placed on the outside, and it is therefore necessary to have the handle made of wood, which being a bad conductor of heat, no inconvenience is felt from holding it in the hand; silver spoons also when immersed in a hot liquid become immediately heated from the same cause, while a piece of wood or glass may be held without difficulty after having been immersed in a hot fluid.

We extract the following interesting particulars relative to this subject from the 13th chapter of Dr. Lardner's treatise.

"The covering of wool and feathers, which nature has provided for the inferior classes of animals, has a property of conducting heat very imperfectly; and hence it has the effect of keeping the body cool in hot weather, and warm in cold weather. The heat which is

"When wine-coolers are formed of a double casing, the space between may be filled with some non-conducting substance, such as powdered charcoal, or wool; or it may be left merely filled with air."

produced by powers provided in the non-conducting substances, to prevent animal economy within the body has a the waste of heat. tendency, when in a cold atmosphere, to escape faster than it is generated; the covering, been a non-conductor, intercepts it, and keeps it confined. "Man is endowed with faculties which enable him to fabricate for himself covering similar to that with which nature has provided other animals. Clothes are, generally, composed of some light non-conducting substances, which protect the body from the inclement heat or cold of the external air. In summer, clothing keeps the body cool; and in winter, warm. Woollen substances are worse conductors than those composed of cotton or linen. A flannel shirt more effectually intercepts heat than a linen or a cotton one: and, whether in warm or in cold climates, attain the end of clothing more effectually.

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If we would preserve ice from melting, the most effectual means would be to wrap it in blankets, which would retard for a long time the approach of heat to it from any external source.

"Glass and porcelain are slow conductors of heat; and hence my be explained the fact, that vessels formed of this material are frequently broken by suddenly introducing boiling water into them. If a small quantity of boiling water be poured into a thick glass tumbler, the bottom, with which the water first comes into contact, is suddenly heated, and it expands; but the heat, passing very slowly through it, fails to affect the upper part of the vessel, which, therefore, undergoes no corresponding expansion: the lower parts enlarging, while the upper part remains unaltered, a crack is produced, which detaches the bottom of the tumbler from the upper part of it.

"In the construction of an icehouse, the walls, roof, and floor should be surrounded with some substance which conducts heat imperfectly. A lining of straw matting, or of woollen blankets, will answer this purpose. Air being a bad conductor of heat, the building is sometimes constructed with double walls, having a space between them. The ice is thus surrounded by a wall of air as it were, which is, in a great degree, impenetrable by heat, previded no other source of radiation be present. Furnaces intended to heat apartments should be surrounded with

We see, therefore, the reason why mankind, without any knowledge of the theory of heat, have adopted particular kinds of clothing to protect themselves against the inclemency of the weather, and why the Allwise Providence has varied the coverings of animals in different parts of the globe, giving to those of the arctic regions the close and soft fur, while the inhabitants of the torrid districts of the globe have no covering but of loose straggling hair; the covering of each being accurately adapted to the circumstances under which they are placed.

Now

We have now seen that heat enters into different substances, and passes through them with different degrees of celerity, that metals are the best conductors or carriers of heat, and that wood, wool, and down are the worst. there is a circumstance attendant on the entrance of heat into different bodies which next deserves our attention, and is of great practical importance, as will appear from a consideration of its effects, and this is, that the shape of those bodies into which heat enters is altered, or in other words, that heat in general expands those bodies with which it combines. If we fill a vessel completely with water and place it on the fire, we find that as the water gets heated it expands and overflows the vessel in which it is placed; if we half fill a tight bag with air, a bladder for example, and hold it before the fire, the air will, on being heated, expand so much as to completely fill the bag; if a bar of metal be fixed between two points and heated, it will either be bent out of shape or force away the obstacles by which it is confined. Now all these effects arise from the expansion of these bodies by heat; which do not all expand in an equal degree, airs expanding more than fluids, and fluids more than solids; all of which have different degrees of expansibility, metals being expanded more by heat than wood or glass; or in general those solids which are good conductors being more altered in their dimensions than

those which are bad conductors of heat. We cannot enter minutely into the various phenomena attendant on the dilation of solids, nor consider the various apparent exceptions to the laws of expansion by heat and contraction by cold. We will content ourselves, therefore, with giving the following quotations from Dr. Lardner's book, relative to some of the consequences of this law:

"The result of the reasoning and experiments explained in the present chapter, shows that the solid bodies by which we are surrounded are continually undergoing changes of bulk with all the vicissitudes of temperature to which they are exposed. When the weather is cool, they shrink and contract their dimensions. On the other hand, when the temperature of the weather increases, their dimensions become enlarged; and these effects take place in different degrees in bodies composed of different materials. Thus, one metal will expand and contract more than another, and metals in general will expand and contract more than other solids.

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If hot water be poured into a glass with a round bottom, the expansion produced by the heat of the water will cause the bottom of the glass to enlarge, while the sides, which are not heated, retain their former dimensions; and, consequently, if the heat be sufficiently intense, the bottom will be forced from the sides, and a crack or flaw will suround that part of the glass by which the sides are united with the bottom. If, however, the glass be previously washed with a little warm water, so that the whole is gradually heated, and, therefore gradually expanded, then the hot water may be poured in without danger; because, although the bottom will expand as before, yet the sides also enlarge, and the whole vessel undergoes a similar change of bulk.

"When the stopper of a decanter becomes fixed in so tight that it cannot be removed without danger of fracture, it may be removed by a method derived from the property of expansion here explained. Let a cloth dipped in hot water be wrapped round the neck of the decanter so as to heat the glass of the neck; it will expand, and increase its dimensions; meanwhile, the heat

not having reached the stopper, it will retain its former dimensions, and, consequently, will become loose in the decanter, and may be easily withdrawn. If the neck of the decanter be thick it will be necessary to maintain the application of heat to it for a considerable time to accomplish this, because heat penetrates glass very slowly, it being one of the worst conductors.

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Vats, tubs, barrels, and similar vessels, formed of staves of wood, are bound together by iron hoops which surround them. If these hoops be put upon the vessel when highly heated, and then be cooled, they will contract so as to draw together the staves with irresistible force.

"The same method is used to fasten the tires of the wheels of carriages. The hoop of Iron by which the wheel is surrounded, is constructed so as exactly to fit the wheel when it is nearly redhot. It this state it is placed on the wheel, and then cooled; it undergoes a sudden contraction, and thus strongly binds the fellies upon the spokes.

"When ornamental furniture is inlaid with metal, care should be taken to provide some means for allowing the metal to expand, since its dilatability is considerably greater than that of the wood in which it is inlaid. Inattention to this circumstance frequently causes the inlaid metal to start from its seat, and this is particularly the case when it is inlaid upon a curved surface, such as the back of a chair. The metal, being more dilatable than the wood, becomes, in a warm room, too large for the seat in which it is inserted, and therefore

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In the systems of metallic pipes by which water is conducted to great distances for the supply of towns, and and other similar purposes, the changes of temperature at different seasons of year cause the lengths of the pipes to undergo such a change, that it is necessary to place, at certain points along the line, pipes so constructed that they are capable of sliding one within another, in a manner similar to the joints of a telescope, in order to yield to the effects of these alternate contractions and delatations. If this provision were not made, the series of pipes would necessarily break by the force with which it would contract or expand. Similar means are used for the same

purpose in all great structures of iron, such as bridges, and are called compensators."

"The enormous power which solid bodies exert in dilating and contracting their dimensions by change of temperature, will be understood if we consider, that it must be equal to the mechanical force necessary to produce similar effects in stretching or compressing them. Thus a bar of iron heated so as to increase its length by a quarter of an inch, would require a force to resist its increase of length equal to that which would be necessary, supposing it be maintained at the increased temperature, to reduce its length by compression a quarter of an inch. In like manner, a body in contracting by diminished temperature, exerts a force exactly equal to that which would be necessary to stretch it through the same space.

This principle was beautifully applied by M. Molard, some years ago, in Paris. The weight of the roof of the large gallery of the Conservatoire des Arts et Métiers pressed the sides outwards so as to endanger the building; and it was requisite to find means by which the wall should be propped so as to sustain the roof. M. Molard contrived the following ingenious plan for the purpose. A series of strong iron bars were carried across the building from wall to wall, passing through holes in the walls, and were secured by nuts on the outside. In this state they would have been sufficient to have prevented the further separation of the walls by the weight of the roof, but it was desirable to restore the walls to their original state by drawing them together. This was effected in the following manner :- -Alternate bars were heated by lamps fixed beneath them. They expanded; and consequently the nuts, which were previously in contact with the walls, were no longer so. These nuts were then screwed up so as to be again in close contact with the walls. The lamps were withdrawn, and the bars now allowed to cool. In cooling they gradually contracted, and resumed their former dimensions; consequently the nuts, pressing against the walls, drew them together through a space equal to that through which they had been screwed up. Meanwhile the intermediate bars were heated and ex

panded, and the nuts screwed up as before. The lamps being again withdrawn, they contracted in cooling, and the walls were further drawn together. This process was continually repeated, until at length the walls were restored to their perpendicular position. The gallery may still be seen with the bars extending across it, and binding together its walls.

There is, however, an exception to this law of expansion by heat, and contraction by cold, to which we would wish to advert, from its great importance in nature, and from the frequency with which the phenomena are presented to us, namely, that afforded by the freezing of water. This fluid does not obey the law of contraction by cold, after a certain limit has been arrived at; in other words, when it has been cooled to a certain point, it ceases to contract, and on being further cooled it expands, and continues to do so until it has been converted into ice. This curious fact was first ascertained by the celebrated Florentine Academi cians, and an account of it was published in the Transactions of the Royal Society, in the year 1670; they observed, that when a vessel, containing water, was placed in a mixture of snow and salt (by which a great degree of cold is produced,) that the fluid expanded and rose in the neck of the vessel; these experiments were repeated by De Luc, who ascertained that at about the temperature of 40o water attains its greatest density, and that any further degree of cold expands it. The importance of this circumstance will be apparent when we consider, that if ice were heavier than water, seas and lakes would be rendered solid by cold, and therefore unfit for animal life; but by this beautiful adaptation of nature to the wants of all her creatures, neither the extremes of heat, or cold of the external air can ever penetrate farther than the surface of masses of water; and the truth of this law of nature, is also fully established by the fact, that during the most intense cold the lower parts of the sea or of lakes is never more than 40°. water expands during the process of congelation has, probably, been frequently observed by most of our rea ders; water is frequently, during severe frosts, congealed in the water

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