"Of all the powers in nature," says Lord Bacon," heat is the chief, both in the frame of nature and in the works of art; heat and cold are the hands of nature." And this opinion, to any one who has given his attention to observing the constitution of nature and her various and continual alterations, will not appear extravagant, though to the casual or superficial observer such a rank may seem too exalted for this single agent; and at the present time, when the researches of modern science have laid open to our view so many of the secrets of the universe, it may seem strange that an aphorism of one who lived but during the infancy of natural science should remain, its truth unquestioned and its importance undenied. For of all the imponderable physical agents which are appointed for carrying on the course of nature there is none of such paramount importance; we witness the effects of heat in every process of nature; we see its genial influence exerted in the production of all those objects which serve for the convenience or luxury of man-the fruits of the earth-the flowers of the fields the sparkling rivulet--the mighty ocean are all dependant on this agent for their production or utility to man; without it existence would be impossible for beings organized as the present inhabitants of the world are; we see that where its influence is withdrawn, vegetation altogether ceases, animal life is extinct, and the mighty ocean itself is chained by eternal frost.

Nor is it less necessary to man, as an artificial, than as a natural agent-in all the conveniencies and luxuries of life we behold its importance; by its agency has man been enabled to add to his power and multiply his resources almost to an infinite degree. By means of the steam engine he is no longer

the sport of winds and waves-no longer dependant on animal strength for the performance of his wishes; he has acquired means of conveyance which enable him to vie in speed with the tenants of the air, and acquired powers which have increased the available population, as if there had been a new creation of human strength. "Fire, flood, and earth are the vassals of his will;" but it is to the first of these that he owes his mastery over the other elements. Such is the agent to which we would direct the attention of our readers, as there are very few who are in any degree acquainted with its theoretical applications, though its practical uses are continually before them, or who have any clear or definite notions relative to its nature and propertiesignorant alike of the investigations of some of the greatest philosophers of our times, and of the great practical results which have rewarded their labours ; this inattention to a most important branch of physical knowledge has chiefly arisen from its being generally considered merely as a branch of elementary chemistry, to which it no more properly belongs, than electricity and light do, but with which it has generally been studied, as its most important laws are disclosed to us by chemistry, and have been investigated by cultivators of that branch of science; and also from the fact, that the only treatises on heat were those given in works on chemistry, no separate general treatise having appeared on this subject until the present, with the exception of the admirable article on heat, by the Rev. F. Lunn, of Cambridge, in the Encyclopædia Metropolitana, which, from the form under which it was published, cannot be looked upon as calculated for general use; however, Dr. Lardner has in his present work, supplied the deficiency and

VOL. I.

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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

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 generally 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

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 seperately, 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

produced by powers provided in the animal economy within the body has a 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.

"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

non-conducting substances, to prevent the waste of heat.

"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."

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 be come 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.

"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.

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.

"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

starts out.

"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