HEAT.

The term Heat, in common language, is applied both , to the sensation excited in us by the approximation of a warm body, and to the cause of that sensation. To obviate this ambiguity, chemists and other cultivators of physical science have employed the word caloric to designate the cause of heat; but as there are few disquisitions in which this distinction is material to perspicuity, in this article we shall use either term to signify the cause of the sensation.

SECT. ï. NATURE OF HEAT.

One of the first inquiries that suggests itself is, what is caloric ? Two opinions on this subject have divided philosophers. The most generally received opinion is, that heat, or caloric, is a material agent of a peculiar nature, highly attenuated, and, from its affinity or attraction for all other matter, universally distributed amongst the particles of bodies, in quantities proportional to their mutual attractions, or, as it has been termed, the capacities of different substances for heat; whilst its tendency to diffuse itself amongst contiguous bodies has been explained on the supposition of its own particles being repellent of each other. The other opinion, which has been maintained by Bacon, Boyle, and several other philosophers, considers heat as a mere quality of matter, and ascribes it to a vibratory movement among the intimate particles of bodies; an idea which was adopted by Rumford, to explain his curious experiments on the excitation and communication of heat by friction. This opinion, however, seems vague and unsatisfactory. If we say that heat is motion amongst the particles of matter, still we have no explanation of the manner in which this motion is produced; for we cannot conceive any movement without an impulse, nor an impulse without a material agent. Heat pervades all sorts of matter: it remains in some circumstances dormant, or, as it is termed, latent, and may be again elicited from bodies by various means. Did it consist in vibrations or motions of the particles of other matter, it should pervade elastic bodies with the greatest celerity; which we know not to be the fact. It will, for instance, pervade a rod of lead, or of the softest copper, far more readily than an equal length of glass or of marble. If we mingle together equal quantities of water at different temperatures, the resulting temperature will be an exact mean between the extremes. But if heat consisted in such vibrations, there ought to have been a loss of heat, as in all other communicated motions. If we mix together equal quantities of different substances at different temperatures, the resultant temperature is not a mean: one body has lost more heat than the other has appeared to gain, or a part of the heat of the one has become latent in the other, and that in a constant ratio to the power of each substance of absorbing heat, as tried by comparing each with a third body in the same manner. It is very difficult to conceive this species of interchange, if heat merely consisted in vibrations amongst the particles of matter. Still more difficult is it to conceive how a permanent temperature could subsist among a great system of bodies, as the planets, if heat were nothing more than a vibration of the particles of bodies; for the original impulse ought to diminish with each communication.

It is possible, however, to modify this theory, by supposing that heat is produced not merely by the motions of the particles of the heated substance, but by the vibrations or undulations of a very subtile matter existing in all bodies. This will approximate the vibratory theory to that which has been generally considered as its antagonist, will accord 1 well with some recently discovered facts, and will assimilate the vibratory hypothesis of heat to the undulations ' now so generally received as explanatory of the phenomena of light, to which heat has so intimate a relation.

Caloric, like light, has been proved to be capable of radiation, of reflection, and refraction, whilst recent investigations have distinctly proved that refracted heat is susceptible of polarisation. But it is transmitted, reflected, refracted, and polarised by different substances in a different manner and degree from light; and hence some have inferred that light and heat are not the same agent, but are produced by different kinds of matter. Other philosophers have regarded them as modifications of the same matter, depending on the greater celerity or velocity of the undulations that produce them. Such speculations, however, are not yet susceptible of any direct proof; and, in the present state of our knowledge, it is safer to consider light and heat as produced by different but intimately connected agents.

These views lead us to the conclusion that the phenomena of caloric are owing to the movements of a subtile matter, universally diffused throughout other bodies, the particles of which are strongly repellent of each other, and have an affinity for those of all other bodies, differing in force according to each kind of matter. We may further conceive, that heat or caloric is the cause why the particles of the most solid bodies are not in absolute contact. If we diminish the temperature of a bar of iron, for instance, it shrinks in all its dimensions, z. e. its particles approximate; and the more we reduce the temperature, the nearer they approach. Each particle of matter would seem to be surrounded with an atmosphere of heat, which remains latent or quiescent, until disturbed by the approach of bodies of a different temperature, when the vibrations or undulations of the subtile matter of heat are induced, by the tendency of this matter to produce an equilibrium of temperature; and then we become sensible of the existence of heat.

The subtilty of the matter of heat is such, that we cannot ascertain its accumulation in any body by the nicest balance; its fluidity may be considered as proved by the ease with which it insinuates itself amongst the particles of matter; its affinity for other matter is shown by its being universally contained in all bodies, in proportions differing in each kind of substance; its repulsion amongst its own particles is proved by its tendency to exist in a state of equilibrium in contiguous bodies.

Our knowledge of the laws which regulate the distribution of caloric was very imperfect when we possessed no other measure of heat than our sensations. The susceptibility of the sense of touch varies in different individuals, and in the same individual at different times. We can even make the same object feel warm and cold to the same person, by previously cooling one hand, whilst the other is immersed in a warm fluid. Hence sensation could never afford any tolerable measure of varying degrees of heat; and we are indebted for more accurate notions on the subject to the invention of thermometers. The principle of these instruments depends on the expansion of solids, fluids, or gaseous bodies by heat, and their subsequent contraction on cooling. The construction of these useful instruments will be described under the article Thermometer. It will be here sufficient to state, that though the thermometer affords us indications of the changes in the sensible heat of bodies, it does not give us any information respecting their latent caloric, nor the absolute quantity of heat [7:11:181]they may contain. This mustbe sought for by other modes, which we shall shortly explain, after we have considered the modes in which caloric is diffused amongst bodies, and its general effects on different kinds of matter.

SECT. II. DIFFUSION OF HEAT.

The tendency of heat to diffuse itself equally amongst bodies is so great, that we are unable permanently to accumulate it in any substance. All that can be effected in this way by the most skilful contrivances, is to produce some retardation of this dissipation. The mode in which it is diffused through solids, liquids, and gases, is different, and demands a separate consideration.

I. Diffusion of Heat by Communication.

1. Diffusion in Solids. — When we place a heated solid in contact with a colder body, the superabundant caloric of the first immediately begins to flow into the latter. The nearest particles are first heated, and they communicate a portion of their caloric to the second series, and these last to a third, until both bodies acquire a common temperature; and this equilibrium will be established amongst all contiguous bodies. If a bar of iron twenty inches long be heated at one end, it will require four minutes for the smallest sensible increase of temperature to be perceived at the other. Biot has endeavoured to ascertain the rate of this transmission. He employed a bar of iron several feet in length, and bent into a right angle, at one end of which a steady heat of 216° Fahrenheit was applied. Thermometers were placed in holes drilled for the purpose, at intervals of four inches along the top of the bar. In four hours all the thermometers became stationary, the difference between the first and second = 21° ∙50; between the second and third, = 11° ∙25; between the third and fourth, = 7° ∙25; between the fourth and fifth, = 5°; between the fifth and sixth, = 4°; between the sixth and seventh (beyond which no sensible effect was perceptible), = 1° ∙75; which, allowing for the unavoidable errors in such investigations, would show, that, taking the distances in arithmetical progression, the decrease of temperature follows a geometrical ratio in penetrating solids. In such cases, the heat seems to be communicated from particle to particle, and is said to be conducted through the body.

All bodies do not conduct heat with equal celerity. If we place equal thermometers on equal cubes of metal, ivory, marble, and glass, heated by the same source, we shall find that the thermometer placed on the metal will rise soonest; next, that placed on the marble, then those on the ivory and the glass. The most dense bodies conduct heat, in general, more readily than rarer bodies; but experiment shows that their conducting power is not always in the ratio of their density, but probably depends also on their affinity for caloric. Spongy and light bodies are found to be extremely bad conductors of caloric. Silk, cotton, and wool, are especially so; and hence their utility in preserving our animal heat in cold climates. Count Rumford made a series of experiments on the conducting power of different substances of this nature, and found that raw silk, fur, and eider-down, were remarkably bad conductors of heat. They give to us the sensation of warmth, not by communicating heat to our surface, but because their bad conducting power prevents the waste of our animal heat by the ambient air. Their stopping the transmission of heat seems partly to depend on the air they entangle; for, by twisting them, i. e. by expelling a portion of the air contained in such bodies, their conducting power is increased,

The facility with which bodies conduct heat is not exactly in proportion to any of their sensible qualities, but is more nearly in the direct ratio of their density than any other quality. This may be ascribed to the greater intensity of the repulsive energy of the atmospheres of caloric surrounding each particle of dense bodies (by reason of their greater proximity) conveying each fresh addition of temperature with greater celerity through such substances. But if we conceive “heat to be a material agent,” this quickness of conducting power may also be modified by the different degrees of affinity between caloric and each kind of matter. However this may be, scarcely any two substances conduct heat with equal facility. Solids conduct much more readily than liquids. Of the former, the best conductors are the metals; and amongst these, the very best are gold, silver, platinum, and copper, whilst iron and lead are among the worst. The rapidity with which silver conducts away heat is well illustrated by wrapping a piece of muslin smoothly round a spoon of that metal, when the muslin may be held in the flame of a candle or a lamp, so as to boil water in the spoon, without burning the muslin.

The following table of the conducting power of different metals and other bodies is given by Despretz (Ann. de Chim. et Phus, xxxvi.) :—
[table]

2. Diffusion in Liquids. — The extreme slowness with which liquids conduct heat is shown by a beautiful experiment of Count Rumford. Freeze a little water in the bottom of a tube, and then pour water over the ice: by inclining the tube, the flame of a lamp may be applied to the surface of the liquid, so as to cause it to boil; and by slowly moving the flame towards the ice, we may raise the water to ebullition in successive portions; yet this ebullition will almost reach the ice before it shows any signs of melting. The same fact is exhibited by fixing an air thermometer in a vessel filled with water to one or two tenths of an inch above the ball of the thermometer, and pouring a little aether on the surface of the water. On kindling the aether, it will burn with a copious flame, without affecting in the slightest degree the submersed thermometer. This extreme slowness of liquids in conducting heat induced Count Rumford to suppose that they were absolute nonconductors of caloric; but this inference is not warranted by his own experiments, and was fully refuted by the investigations of Hope, Murray, and Traill, which proved, that though liquids conduct heat slowly downwards, they are not absolute non-conductors of caloric.

If, however, we apply heat to the lower part of a vessel containing any liquid, it rapidly acquires a higher temperature. This, however, is in a different manner from conduction. The liquid is heated by the transportation of its particles in quick succession. In this case the particles nearest the heating cause become specifically lighter by receiving heat; they therefore ascend through the fluid, to which they impart part of their caloric, while their place is supplied by another series of particles, which become heated, and ascend in their turn; and this succession continues until, by these rapid changes, the whole body of the fluid attains its boiling point, if the heat be sufficient for that purpose. These motions may be rendered visible by throwing into the vessel a few particles of matter a little heavier than water, such as powdered amber.

It is by this transportation of their particles that liquids are principally heated; and the rapidity with which a piece of ice melts when it floats in ajar containing hot water, compared[7:11:182] to the extreme slowness of the melting of a similar mass of ice fixed in the bottom of a jar, and defended from the immediate contact of the hot water by a thin film of ice-cold water, exhibits, in a striking manner, the difference between the heating of fluids by transportation and by conduction.

3. Diffusion in Gases — The conducting power of gases is not so easily ascertained, because it is difficult to separate their conducting power from the effect of radiation of heat through them. The experiments of Professor Sir John Leslie and of Dr Dalton, however, decidedly show a difference in the conducting power of gases, which is also more nearly in the direct ratio of their specific gravity than of their other properties; hydrogen having the lowest conducting power, atmospheric air one considerably higher, and carbonic acid the greatest of all the gases subjected to this examination.

The difference of bodies in conducting heat is a most important subject, as on it depends not only many of our contrivances to concentrate artificial heat, and apply it to numerous purposes in the useful arts, as the obtaining of metals from their ores; but on it also depend the methods of defending our bodies against external cold. The living system has within itself the power of supporting a nearly equable temperature, notwithstanding the perpetual tendency of contiguous bodies to a common temperature; but if the naked surface be exposed to the elements in our climate, the heat of the body would soon be reduced below what is consistent with health or comfort. To preserve the animal heat, we surround our bodies with bad conductors of caloric, such as woollen, silk, or cotton; and the more imperfectly these defences conduct heat, the less will our temperature be reduced. Hence the worst conducting substances are the most suitable garments for a cold climate; and, in hot latitudes, the comfort of man requires that the coverings of his body should be of the kind that would most rapidly abstract his redundant animal heat.

Nature has beautifully adapted the covering of the lower animals to the climates they inhabit. The thick fur of the Greenland bear, the musk ox, and the arctic hare, adapts them to the rigours of their native climate; whilst the short and sleek hair of the antelope, the giraffe, the leopard, and the lion, proclaims them denizens of the warmer regions of the earth. Even in the same species inhabiting a changeable climate, nature adapts their covering to the season. The glossy sleekness of the horse, and of our domestic cattle, diminishes toward the close of autumn. The bear, the fox, and the weasels of northern regions, assume a longer and more shaggy coat on the approach of winter; and the sheep, which in Europe is covered with a thick fine wool, an extremely bad conductor of heat, in the burning plains of Africa is clothed only by a short and coarse hair, that presents comparatively a “mall obstacle to the evolution of animal heat.

In the vegetable kingdom a similar care is bestowed to defend plants from excessive cold. Plants of cold climates, which are perennial, are protected by a considerable thickness of bark; a substance which experiment proves to be a bad conductor of heat. In high latitudes they are further defended against the excessive cold of the climate by a spongy covering of snow, which, until it begin to melt, is found to be a very bad conductor of heat; and therefore tends to preserve the juices of plants from being frozen. Thus trees are more seldom killed by the freezing of their sap, when a fall of snow has preceded an intense frost; an accident not uncommon, even in the temperate climates of the earth, in a long continuance of what is termed a black frost. Recent voyages of discovery have also shown that the Esquimaux find the excessive rigour of their inhospitable climate very endurable in houses built of'frozen snow.

II. Diffusion of Heat by Radiation.

The diffusion of heat by the means already noticed is a v comparatively slow process, and is limited to bodies in contact with each other. But heat is capable of being diffused among bodies not in contact. A heated body suspended in vacuo emits its excess of heat in all directions; and in air, though much of its caloric apparently passes off with the ascending currents which it produces in the ambient air, the emanations of heat also pass off in directions contrary to these aerial currents. Thus a person standing before a fire perceives its warmth, though a light body like a feather will show that there is a current of air perpetually flowing toward the fire. This emission of heat is termed radiation, and is analogous to the emanations of light from a luminous object; each point of the heated surface emitting divergent rays, which are subject to the same modifications as those of light, by reflection from polished surfaces, and by refraction through transparent media.

When the rays of heat fall on a bright metallic surface, they are* reflected. As early as 1682, Mariotte showed that “the heat of a fire is reflected from a burning mirror, so as to be sensible in its focus; but that it is intercepted by a plate of glass interposed between the mirror and the fire.” The next important step was made by Lambert, who discovered that the heat might be so increased, by employing two concave mirrors, and a charcoal fire placed in the focus of the one, that a combustible might be kindled in the focus of the other. But the most successful cultivator of this branch of science during the last century was Scheele of Sweden, who proved that metallic surfaces are the most powerful reflectors of radiant caloric; that glass is far inferior in this respect; that if we cover the surface of the metallic mirror with a film of lamp-black, it does not reflect heat, but actually absorbs it; that radiant heat is separated from light by interposing screens of glass; and that it passes through air, without suffering any obstruction from the direction of the aerial currents through which it radiates.

Saussure and Pictet repeated the experiments of Lambert. They showed the instantaneous transmission of heat by radiation; that it was in such experiments material to place the heated body and the thermometer in the focus of each mirror; and that, a very little beyond the focus, the effect was trifling, although the thermometer was nearer the heated body. When the heated body was a red-hot cannon bullet, combustibles were speedily kindled in the focus of the other mirror at the distance of several feet. These researches were greatly extended by Pictet, who showed that a flask of hot water radiated heat which could be concentrated in the focus of a metallic mirror, and thus rendered sensible by a thermometer, showing that the invisible rays of heat might be reflected, as well as those emanated from a hot luminous body.

The experiments on radiant heat may be exhibited by means of a pair of concave mirrors of well-polished tinned iron, hammered into segments of spheres of about one foot in diameter; but still better with mirrors of thick brass plate, hammered, on Sir John Leslie’s plan, into a parabolic form. The writer of this possesses a pair twenty-two inches in diameter, hammered into a parabolic curve with surprising accuracy, by Mr Alexander Kilpatrick of Edinburgh, with which he has repeatedly melted lead by collecting the sun’s rays in one of them. This form of mirror is the best; because the rays which fall on the mirror parallel to its axis are reflected, not divergingly, but so as to meet in the focus of the parabola.

Pictet found the sensibility of the thermometer much increased by painting its ball black; and he showed that glass screens intercepted the rays of caloric from burning bodies or a heated bullet; but it was found that the ra[7:11:183]diant heat in the sun’s rays was not intercepted by a plate of glass, or even by a sheet of tin, which completely intercepts heat derived from other luminous bodies.

Such was the state of our knowledge of radiant heat, when our veteran astronomer, Sir William Herschel, discovered, toward the close of the last century, that rays of heat exist, independently of those of light, in the solar spectrum. When he received the solar rays through a prism of flint-glass, he found that a row of delicate thermometers placed in the coloured spectrum were differently affected at its two extremities. In the violet ray it only rose 2°, in the red ray it rose 7°; but his most interesting discovery was, that half an inch beyond the red ray it was still hotter. These very important results were fully confirmed by Sir Henry Englefield. In one of Englefield’s experiments, the following results were obtained :—In the blue rays, in 3' the thermometer rose 2°; in the green, in 3' it = 4°; in the yellow, in S' it = 6°; in the middle of the red, in 2'∙5 it = 16°; in the outer edge of the red, in 2'∙5 it = 17° ∙5; and beyond the spectrum, in 2'∙5 it = 18°. When the bulbs of the thermometers were previously blacked, the full red ray raised the thermometer in three minutes 22°; and jnst beyond the spectrum it rose 33°. Even half an inch beyond the spectrum altogether, the rise was 6° more than in the red ray.

These experiments show that the refrangibility of the rays of heat and light are different, and that the former are less refrangible than the latter.

The experiments of Berard and of Leslie confirmed the fact, that the point of greatest heat in the solar spectrum is in the red rays; and Leslie states the result of his experiments to have made the difference between the violet and red as one to sixteen; but neither of these philosophers detected any heat altogether beyond the spectrum. The conclusions of Herschel have, however, been confirmed by more recent investigations; and Śeebeck has shown that prisms of different substances produce a different refraction of the rays of heat. With a hollow prism filled with water, the greatest heat is in the full yellow light; with sulphuric acid it is in the orange; with crown glass it is in the dark limit of the red.

We may here state, that not long after Herschel’s discovery, Ritter, Wollaston, and Beckmann, simultaneously discovered the existence of other invisible rays in the solar spectrum, which are only known by their chemical effects in decomposing some metallic saline compounds, aš the nitrate of silver. These chemical rays are the most refrangible of all, and exist in greatest abundance toward the violet end of the spectrum, and even entirely beyond it. Thus the solar spectrum would seem to consist of three species of rays, the luminous, the calorific, and the chemical; all differing in their refrangibility, and in their apparent effects: and if we consider white light as composed of red, yellow, and blue rays, we have five kinds of rays in the solar beam, three of which are visible, and two invisible. In the solar beams these are intimately blended, but may be in some degree separated by refraction through diaphanous prisms. The separation of the luminous and calorific rays may be made by black opake bodies, through which the sun’s heat will penetrate without admitting a single ray of light. The sun’s rays, however, pass through all transparent media, without a separation of light and heat. Glass and ice intercept the rays of terrestrial heat, the first partially, the latter wholly; yet the sun’s rays passing through and collected in the focus of a lens of glass, produce the most intense heat; and Scoresby and others have shown, that a lens of ice will concentrate the sun’s rays, so as to ignite inflammable substances.

The publication of Sir John Leslie’s Inquiry into the Nature of Heat, in 1804, forms an important era in the history of the radiation of caloric. This very original and able philosopher, by the simplicity and delicacy of his apparatus, and the ingenuity of ĥis well-devised experiments, did more than has been accomplished by any other individual to develop the laws which regulate the transmission and reception of this mysterious agent; and his work will remain a land-mark in the history of this branch of physical science. In his experiments, a single mirror only was employed; and the source of heat generally used was a cube or square canister of tinned iron placed before the mirror, at the distance of three or four feet, whilst the ball of an air thermometer was placed in the focus of the mirror. The air thermometer employed by him was his own modification of that figured by Sturmius. {Colleg. Curios, p. 53, 1676.)

In the instrument of Leslie, termed by him a differential thermometer, both limbs of the instrument, as well as both bails, are equal; and instead of being joined with cement, the recipient ball is united by the blowpipe to the same piece with the sentient ball. These changes give additional delicacy and accuracy to the instrument; in which the coloured fluid is sulphuric acid tinged with carmine.

Leslie’s principal object was the relation of different surfaces in emitting and receiving calorific rays. Of his cubical canister, one side was polished, or, as it is termed by workmen, planished ; the second was covered by a plate of glass; a third with white paper smoothly pasted on; the fourth was painted with lamp-black mixed with size. The cubes he used were from four to ten inches, and were filled with boiling water. When the polished side was turned toward the thermometer, placed at four feet from the mirror, the increase of temperature was no more than 12°; when the glass side was presented, the differential thermometer under the same circumstances rose to 90°; when the papered side was the radiating surface, the temperature was 98°, and the painted side indicated 100°; when the polish of the planished side was destroyed, by ploughing it in one direction with a finetoothed plane, its propelling or radiating power rose to 19°; and when scratched in one direction with a fine file, its effect was as much as 26°. On covering one of the surfaces smoothly with gold and silver leaf, the effect was about equal to the surface of polished tin; a plate of polished iron gave 15°; a surface of fresh lead 19°; but when the same became tarnished, its effect was equal to 45°; and painting it with red oxide of lead raised it to 80° w An amalgam of mercury and tin, when fresh, gave no more than 20°.

Leslie then investigated the relative receiving power of different surfaces, by coating the sentient ball of the differential thermometer with different substances. When that ball was smoothly coated with tinfoil, the effect of the blackened side of the canister was only 2°∙5, or about one fifth of what it produced on the naked ball; and he found, that of either side of the canister the effect was now just one fifth of that observed with the naked ball. On the other hand, when the ball was covered with a coat of china ink, or formed of a black enamel, the effect of either side of the canister was greatly increased.

The power of surfaces in reflecting heat was also investigated. In fact, it was shown by the last series of experiments with the coated ball; but he proved it also by varying the reflecting surface. When a glass concave mirror, two feet in diameter, was substituted for the metal reflector, the effect of the blackened side of the canister on the naked ball was but just perceptible; and if a film of china ink be spread over the surface of the mirror, even this slight effect totally disappears. If, however, the concave surface of the glass mirror be smoothly coated with tinfoil, the effect of the black side of the canister [7:11:184] will be ten times more than with the naked glass surface. Removing the silvering from the back of the mirror produced no effect on its reflecting the calorific rays, neither was this affected by roughening the back of the mirror. Hence Leslie infers, that reflection of heat takes place at the surface of the glass mirror, or principally so.

A polished tin reflector had its power diminished one third by being coated as smoothly as possible with tinfoil, evidently by the imperfection of the smoothness of its surface. Scratching its surface with sand-paper diminishes its effect one tenth; and he found that the mirror seemed to have its reflecting power more impaired when the scratches were all in one direction, than when they crossed. A film of tallow on the surface of the mirror reduced the effect of the blackened side from 100° to 8°; but if held before the fire until all that could be thus removed had run off, the effect of that side rose to 37°. When the surface of the mirror was covered with a very thin iridescent film of isinglass, the blackened surface gave an effect of 80°; but when that film was only ru ⅛ σ th of an inch, the effect was reduced to 15°.

The results of his experiments with other reflecting surfaces gave the following proportions :—A reflector of polished brass = 100°, of the same coated with tinfoil = 85°, of steel = 70°, of fresh lead = 60°, of glass = 10° ∕

The inference from these investigations is, that the reflecting power of various surfaces bears some inverse proportion to their propelling and absorbing powers. The numerical results of Leslie’s experiments would give the ratio between metallic surfaces and glass, in reflecting power as ten to one; in propelling power as one to eight; in absorbing power as one to five. It was, however, supposed that some minute circumstances, of which it is difficult to estimate the effect, interfered in the two last processes, and that the propelling and absorbing powers are equal in all bodies: we shall find this to be incorrect.

One of the most interesting parts of Leslie’s investigations was the effect of screens of different kinds, interposed between the sources of heat and the thermometer. When he interposed a screen of tinfoil, the effect of the blackened side of the canister was 0°; a thin sheet of crown-glass was = 20°; a sheet of common writing paper, placed about two inches from the cube, was = 23°. If the screen of any material was placed one foot from the cube, the effect was only one thirtieth of what it was at the distance of two inches. From this he inferred, that the screen prevents all transmission of radiant heat until it becomes itself heated; and then it radiates from its other surface toward the thermometer. This was confirmed by substituting a plate of ice (a substance the temperature of which cannot rise above 32° F.) for the screen, when the effect was 0°. This view he considered as confirmed by his beautiful contrivance of the double or compound screen. He coated one side of two plates of glass with tinfoil; when the coated sides were outermost, the thermometer did not rise; when the glass surfaces were outwards, the thermometer rose to 18°. He blackened one surface of two plates of tinned iron; when the blackened surfaces were outwards, the effect was 23°; but if the plates were separated from each other, the thermometer fell back to its former station. When the tinned surfaces were outermost, the thermometer was not at all affected.

Leslie included his whole apparatus in a trough of water, in such a way as to be able to fill the canister with hot water after the whole was adjusted; but there was no radiation of caloric.

The inference which this philosopher drew from his investigations is, that heat is an elastic substance, extremely fluid and active; and he advanced strong arguments against the theory which ascribes all the phenomena of heat to vibrations in the particles of matter. (Vide Inquiry, p. 139 to 150.) Yet he is disposed to consider the phenomena of radiation as depending on certain undulations, produced by radiating surfaces in the ambient air. This view has been ably combated by the late Dr Murray, with the sagacity which distinguished that philosopher. But the limits of the present article will not allow us to enter into this part of the subject, for which we must refer the reader to Leslie’s Inquiry, and Murray’s Chemistry.

The more usually-received theory of radiation is, that from heated bodies emanate rays of caloric in all directions, which proceed through gaseous bodies with little or no sensible interruption, and with amazing velocity; that these rays are absorbed by dark and rough surfaces, and are reflected by polished bright surfaces.

There is, however, one curious experiment, which is rather difficult of explanation, namely, the seeming radiation of cold. The Florentine philosophers of the Academia del Cimento found, that when a mass of snow was placed in the focus of one mirror, the thermometer placed in the focus of the other sunk, or indicated cold. This subject has been investigated by Pictet and by Leslie. The latter observed that his canister, filled with snow, produced the greatest effect when its blackened side was towards the thermometer and the mirror, and the least when its polished side was in that direction. The effect of screens, in retarding the influence of the cold body, he found analogous to their effect on the radiation from the hot water.

These facts were considered by Leslie as proving the existence of what he denominated cold pulses from the snow towards the mirror, “on the wings of the ambient air;” but the explanation of Pictet appears to account for it well, without the necessity of inferring the existence of frigorifie particles, which is a highly improbable supposition. On this view, radiation is considered as only taking place amongst bodies unequally heated. He conceived that bodies at the same temperature do not radiate heat to each other, because in this state caloric exists in them all in an equality of tension; but when a cool body is introduced, all radiate heat towards it, and consequently their temperature falls. Hence radiation is nothing more than the tendency of caloric to establish an equilibrium of temperature. The rays of heat enter into the snow from the surrounding matter, and, amongst others, from the thermometer, which is now a radiating body; and these collected in the mirror pass in right lines to the snow, with a celerity in proportion to their absorption by the cold body. Hence the caloric of the thermometer will more rapidly leave it when the blackened side of the cold canister, that is, its most absorbent side, is turned to the thermometer.

Leslie explains this phenomenon by his theory of aerial pulsations. He considers the cold surface as abstracting part of the caloric of the contiguous stratum of air, which induces a momentary contraction of'that portion; and this contraction produces pulsations, accompanied by a discharge of heat, in a continued chain from the thermometer and the mirror to the snow.

The effect of surface on the refrigeration of bodies, an important part of the consequences of radiation, has been ably examined both by Sir John Leslie and Count Rumford. The experiments of both show, that to preserve the heat of any liquid, a bright metallic vessel is the best; and Rumford has pointed out many important economical purposes to which these principles may be applied. Thus, where it is of consequence to preserve the heat of liquids, of steam, or of hot air, they should be conveyed in vessels and tubes of polished metal. On the other hand, if we wish to have the greatest radiant heat from a stove or grate, its surface next the room should be dark and rough, as these are the most favourable for radiating heat into the apartment. The same principles show why a silver [7:11:185]I tea-pot makes better tea, and keeps it longer warm, than a china one.

' One of the most beautiful applications of the principle of the radiation of heat, is Dr Wells’ explanation of the phenomena of dew and hoar-frost. Dr Wilson of Glasgow had observed, that bodies upon which dew and hoarfrost formed, were always colder than the surrounding air. This cold he ascribed to these depositions; but an attentive examination of facts led Dr Wells to draw an opposite conclusion, and to infer that the coldness of the bodies was the cause of the deposition of dew and hoar-frost. This he successfully established, by proving that, before any dew formed, the surface on which it condensed was uniformly cooler than the ambient air. And it was reserved for this accomplished man to offer a theory of those meteors, complete in almost all its parts, and perfectly satisfactory. He ascribed it to the radiation of heat, without any return from the air to the surface of the earth. He observed, that it was chiefly in serene, clear nights, that dew was formed; that exposure to the open clear sky favoured the formation of dew; and that cloudy skies were unfavourable to its formation. These phenomena he beautifully. explained on the theory of radiation. The upper regions of the atmosphere are well known to be the abodes of perpetual congelation, as is seen whenever mountains reach a certain altitude, differing, it is true, in different climates, but yet invariable over the earth. When we have a clear atmosphere at night, the surface of the earth rapidly parts with the heat it had acquired during the day, by radiation to the superior regions, whence it can receive no heat in return. In this case, the empyrean regions act the part of the snow in the Florentine experiment; and the earth’s surface may represent the thermometer. But if fleecy clouds intervene, they act the part of screens, intercepting the passage of radiant caloric from the earth, and consequently retarding the nocturnal cooling of its surface. Air at an increased temperature contains more water than cool air, and on the reduction of its temperature deposits its surplus water. Now, as the radiation from the earth’s surface cools it more rapidly than the air during serene nights, its temperature rapidly falls, as the thermometer shows; and the consequence is the cooling of the stratum of air in immediate contact with the ground, and the deposition of its superabundant moisture, in the form of dew or hoarfrost, according to the celerity and intensity of the refrigeration.

This theory is experimentally proved by placing substances absorbent of moisture, along with thermometers, below and above screens, and then noting the temperature and the increase of weight. If, for instance, a light table, about three feet high, be placed in a garden on a clear night, and a few grains of wool, previously weighed, be laid under the table, and as much on its upper surface, with a thermometer by each parcel of wool, it will be found that the upper thermometer will indicate the greatest degree of cold, and the wool on the table will have imbibed much more moisture than that below. The table, in such experiments, acts the part of clouds in intercepting the discharge of radiant heat, and preventing the cooling of the earth’s surface. The theory agrees with the fact, that dew is heaviest in our climate in serene nights, after a hot day; and that the dews of hot climates are far heavier than with us, so as, in clear weather, in the south of Europe, to drench the clothes of persons exposed to the air about sunset. The slight anomalies which sometimes occur in such experiments are easily explicable by the different conducting power of substances in regard to heat, by which the influence of radiation may be in some degree modified; but undoubtedly the principal effect is due to radiation.

The influence of a clear sky in reducing the temperature of the earth’s surface, and the effect of clouds in preventing this change, are beautifully illustrated by Leslie’s elegant invention, the Aethrioscope. (See, for the description of the aethrioscope, the article Climate.) This instrument is so delicate, that it instantly indicates cold on presenting its uncovered ball to the clear sky; but if a passing cloud cross the zenith, even momentarily, the movement of the fluid in its stem immediately shows an increase of temperature. If one walk in a clear night, with this instrument in one hand and a parasol in the other, it may be kept in a perpetual state of fluctuation, by alternately projecting it beyond and drawing it under the parasol.

The radiant heat afforded by the sun’s rays is the most important phenomenon of this class. Light and heat are in these rays so united, that experiment would seem to prove the one to be always in proportion to the other. This is by no means the case with the light and heat of common combustibles, or what we may term terrestrial, in contradistinction to solar emanations of light. Phosphorus gives an intense light during combustion, but a feeble heat; whilst hydrogen, which has a very feeble light, excites a high temperature by its combustion. Solar light and heat, on the other hand, are uniformly proportional. There are more marked differences between solar and terrestrial radiant heat. Screens of glass greatly interrupt the passage of the latter, but do not sensibly intercept that of the sun. A plate of the most diaphanous ice totally intercepts terrestrial radiant caloric, but does not impede the sun’s heating rays. This has, with considerable reason, been supposed to depend on the different velocities of the two species of calorific emanations. Sir John Leslie considered “that the phenomena of solar radiation proved heat to be only light in a state of combination. ” (Essay, 162.)

For thirty years after the publication of Leslie’s Experimental Inquiry, little appears to have been attempted on this subject, until within a recent period, when the experimental researches of Melloni and of Nobili, particularly of the former, opened a beautiful field of investigation, which has already been cultivated with success by Professor James Forbes of Edinburgh. Melloni has, by means of a thermo-magnetic combination, invented a very delicate test of minute degrees of heat, wholly inappreciable by any thermometer, and has successfully applied it to investigate the laws of radiant heat. By uniting fifty small bars of antimony and bismuth into one bundle, about three fourths of an inch square, and about 1∙17 inch in length, and connecting this with a galvanometer, he obtained an apparatus so sensible to heat, that the warmth radiating from the human hand, at the distance of several inches from the. end of the bars, is indicated by the deviation of the needle of the galvanometer.

Melloni’s instrument is represented in the adjoining figure, where a firm sole of wood is seen, provided with a groove m which the different parts of the apparatus slide to adjust [7:11:186]their relative distances. A is the bundle of metallic bars, enclosed in a square case of brass; B is the source of the heat; C, D are the wires proceeding from the bars, to convey their thermo-magnetism to the nearly neutralized needle or galvanometer, which is not here represented; G is the stage for occasionally supporting various substances, the effect of which on the calorific rays it is intended to ascertain; FF are screens of brass, moveable on joints, for cutting off at pleasure the radiant heat, or for obviating the influence of extraneous sources of heat. In F is a hole through which the heat radiates to A when the screen is removed.

This apparatus has been employed by Melloni to investigate the laws of radiant heat; and he has not only confirmed the general results of Leslie, but extended greatly our knowledge of this mysterious agent.

Melloni found, that the radiant and absorbent power of surfaces were not always proportional, as the following tables show.
[table]

Melloni also found, that the absorbent powers of the surfaces varied considerably, according to the source of the radiation, and the temperature of that body. Thus, radiation from incandescent platinum wire, from copper at 400° and copper at 100° centigrade, gave the following results.
[table]

This experiment proves,

1. That bodies do not always agree in their emitting and absorbent powers, though generally nearly so.

2. That their absorbent power varies very remarkably with the origin and intensity of the calorific rays.

3. That they approach each other more and more in their power of emitting and absorbing rays of heat, when the temperature approaches that of boiling water; and that, when exactly at that temperature, the emitting and absorbing powers coincide.

With respect to the reflection of radiant heat, he has shown, that it is equally reflected by metallic surfaces, from whatever source it emanates.

But Melloni’s most original experiments are those on the transmission of radiant heat through various transparent media.

1. He showed that radiant heat is intercepted in a greater or less degree by all diaphanous bodies, in proportion to the lowness of the temperature of the radiating body.

2. That of two bodies unequally diaphanous, it may happen that the thickest and least diaphanous may transmit most radiant heat. Thus he showed that a thin plate of very transparent alum, placed on the stage G, transmitted four times less heat than a plate of almost opake quartz, about 100 tiŋaes as thick; but he found that in the same substance the transmission of radiant heat is diminished by ' the thickness of the plate interposed, and this diminution is proportional to· the lowness of the temperature of the radiant body.

3. That there are combinations of two media, which allow a notable quantity of light to pass, but totally intercept radiant heat; whilst others transmit heat, but wholly intercept light.

4. That in traversing a transparent plate, radiant heat undergoes certain modifications, variable with the nature of the plate; a change which renders it more or less susceptible ultimately of being transmitted through other diaphanous substances. Melloni instances this last property mi glass, in crystallized citric acid, and in alum.

Delaroche had inferred, from his experiments, that it was a general law of radiant heat, that the permeability of plates to this agent depended upon the intensity of the source of the caloric; and in this way he explained the instant permeability of glass and ice to the calorific rays of the sun, whilst they retarded those from terrestrial sources of heat; butMelloni has discovered one substance which he found to be equally pervious to heat, from whatever terrestrial source, whether proceeding from the brightest flame, or from water far below the boiling point.

The power of penetrating glass and other media increasing in proportion as the radiating heat approaches the state of light, had been used by Delaroche as au argument for their identity; but the anomaly of rock-salt destroys the universality of the supposed law on which the argument is founded. Yet Mrs Somerville has ingeniously employed the unlooked-for analogy between light and heat, in the equal transmission of the latter, however eliminated, through rock-salt, as an argument for their being modifications of the same principle. The condition of visibility or invisibility, she contends, may depend on the construction of our eyes, not on the nature of the agent producing the sensations of vision and of heat.

“The sense of seeing, like that of hearing, may be confined within certain limits; the chemical rays beyond the violent end of the spectrum may be too rapid, or not sufficiently excursive in their vibrations to be visible to the human eye; and the calorific rays beyond the other end of the spectrum may not be sufficiently rapid, or too extensive, in their undulations, to affect our optic nerves, though both may be visible to certain animals or insects.”

She has traced the analogies between light and heat in their reflection by polished surfaces, their refraction through transparent media, with their concentration by concave and dispersion by convex mirrors; and since the publication of her beautiful essay on the connection of the physical sciences, Professor Forbes has drawn the analogy closer, as we shall presently see.

But to return to Melloni. This able philosopher has shown that radiant caloric is susceptible of refraction; and when it arrives at the second surface of the refracting angle, with a certain obliquity, it is, like light, reflected toward the interior of the prism, and issues at the opposite face.

By interposing the same plate of glass, he ascertained the influence of transmission on the absolute power of different radiating surfaces thus:
[table]

Melloni, however, failed to detect the polarisation of radiant heat: indeed, he states that the direction in [7:11:187]which we slice crystallized bodies does not exert any influence upon the quantity of radiant heat immediately transmitted by them; and adds, that radiant heat is not polarised by transmission through tourmaline. In this, however, Melloni was deceived; and it was reserved for our countryman Professor Forbes of Edinburgh to complete the analogy between light and heat, by demonstrating the polarisation of the latter.

Since the characteristic phenomenon which marks the polarisation of light is its variable susceptibility as to reflection or transmission, under circumstances in which common light would be reflected or transmitted, it will appear that the correlative fact in the case of heat would be indicated by a diminished effect on the thermometer, where the intensity of light, under similar circumstances, would be a minimum, and vice versa. The importance of establishing this effect with regard to heat is far greater than the mere addition of such facts to our knowledge; for, as the corresponding facts in the instance of light have been completely brought into the domain of analysis by Fresnel, the polarisation of heat must be considered as almost decisive of its nature.

Mr Forbes employed Melloni’s apparatus; and by interposing two plates of tourmaline, cut parallel to the axis of the crystal, and mounted on two slips of thin glass, he made a series of successive observations under the two conditions of the axes parallel and perpendicular to each other. Two measures of intensity in the position in which least light is transmitted were noted, and in the following table this position is indicated by dark ; their mean is given, which is then compared with the intervening observation, in the position of greatest illumination, which is marked light.

The source of heat was á small oil lamp placed on the stage, six inches from the centre of the pile of Melloni’s apparatus; the numbers indicate the degrees of the galvanometer.
[table]

He afterwards obtained the polarisation of heat from various luminous and non-luminous sources, such as brass heated by a spirit lamp to 390° centigrade. The quantity of heat from different sources, polarised by the tourmalines, was as follows :—
[table]

The most convenient way of polarising heat is by transmitting it through a bundle of extremely thin laminae of mica, inclined to the incident ray at the polarising angle; mica having the property of transmitting heat very readily. The amount of polarisation is indicated by the relative quantities of heat reaching the pile, or thermo-magnetic combination of the instrument, through a second bundle of thin plates of mica, placed alternately in a parallel or perpendicular position to the first. With such an apparatus Mr Forbes demonstrated, in the most decisive manner, the polarisation of heat; and obtained this effect, even with water below 200° F. as the source of heat. The quantity polarised, however, always bears a proportion to the temperature of the source of the radiant heat, as is seen by the following tabular results.
[table]

Mr Forbes next proceeded to attempt the polarisation by reflection ; and in this also he succeeded by the use of reflecting surfaces of mica, as in the corresponding case of light.

Ί he success of these investigations, and the analogy of light, led him to the more delicate problem of the depolarisation of heat by plates of mica. By interposing a film of mica between the two bundles of mica plates already mentioned, having their planes of incidence at right angles to each other, and marking the difference of the heat transmitted to the galvanometer, when the principal section of the film of mica was parallel to the plane of primitive polarisation, or inclined to it at an angle of 45°, he succeeded in demonstrating the polarisation of the rays of heat, even when heat without light was employed. In these experiments, when the principal section coincided with the plane of polarisation, the depolarising effect was nil; but when it was inclined at the angle of 45°, he obtained the following proportions in one series of experiments.

100: 118:: — 100: 120 — 100: 120 — 100:113 —

The depolarisation is still more marked with incandescent platinum; as the results were

100: 126 — 100: 138 — 100: 138.

One of the most striking proofs of the depolarising power of mica is obtained, when the two bundles of mica plates are crossed, so as to intercept most heat, and we interpose a very thin plate or film of mica as above mentioned; then the galvanometer moves towards zero, or the thin plate evidently stops more heat than it depolarises; but if we substitute a much thicker plate of mica for the film, the instrument will indicate a higher temperature than when no mica at all is interposed, or the thick plate depolarises more heat than it intercepts.

These experiments were varied in a great variety of ways, so as to establish the fact of the depolarisation of heat; and if we admit that it depends on a similar cause to the analogous phenomena of light, it follows that the rays of caloric are susceptible also of double refraction; that the two pencils are polarised in opposite planes, and that they become capable of interference by the action of the analysing plate.

These curious facts would indicate at least a great similarity between light and heat; and the concluding observations of Professor. Forbes’s paper (Edin. Phil. Trans. xiii.) tend to confirm their identity..

SECT. III.—GENERAL EFFECTS OF CALORIC.

The general effects of heat applied to other matter are, expansion, fluidity, vaporization, and incandescence. The most general effect of heat, however, is,

1. Expansion.

When a body is heated, it expands in all its dimensions; but when the heat is withdrawn, the body returns to its original size. This is well shown by having a turned rod of metal, loosely fitted to a gage, to ascertain its length, and provided with a hole which first allows it, when cold, to pass through. This expansion is small in solids, but has been most accurately measured by philosophers, for [7:11:188]the important purposes of ascertaining with precision the true length of the pendulum vibrating seconds in any latitude, and for obtaining a perfect standard of length. This difficult subject engaged the attention of Ellicot, Smeaton, Roy, Troughton, Lavoisier, and Laplace. The latest experiments were made by the two last-mentioned philosophers, and were first published by Biot in the following table, which shows the expansion which different solids sustain, in passing from the freezing to the boiling point of water, in fractions of their own lengths.
[table]

A very elaborate and interesting set of experiments on the expansion of building materials by heat, with a view to determine how far the changes produced by temperature may affect the stability of edifices into which these different materials enter, were lately laid before the Royal Society, by Alexander J. Adie, Esq. civil engineer. He exposed square rods of these substances, twenty-three inches in length, and either half an inch or an inch in diameter, in a pyrometer of his own invention, to a heat of 212°. The increments in length were accurately determined by a microscope-micrometer, attached to the instrument; and the following table gives the increase of the whole length produced by 180° of heat, in decimals of an inch :—
[table]

The expansion of solids at different temperatures appears to be nearly equable, as far as we can ascertain. The ratio of their expansion really increases with their temperature; but their whole expansion is so inconsiderable, that the increasing rate is inapplicable, except in the nicest investigations.

The expansion of liquids is much more considerable. In passing from the freezing to the boiling point of water, the expansion of several is as follows:
[table]
[table]

The inequality of the expansion of fluids by heat has long been known to form an obstacle to the accuracy of thermometers. Deluc found, that of all fluids mercury was the most equable in its expansions, which is probably owing to the great distance between its freezing and boiling points; for it is found that those liquids which have a low boiling point are the most irregular in their expansion at ordinary temperatures; and hence alcohol is not a suitable fluid for thermometers.

The expansibility of liquids is not in proportion to their density; but it is more nearly in the inverse ratio of their density than of any other known property.

The expansion of gases by heat is much more considerable than that of the other two forms of matter.

The higher increasing ratio in the expansibility of liquids with their augmenting temperature, than in solids, led philosophers at first to infer, that the inequalities of the gaseous bodies, in this respect, would be still more considerable; but the elaborate researches of General Roy have disproved this idea; and some experiments of the late Dr Murray of Edinburgh, and of Dr Dalton, would rather indicate a decrease in the ratio with the increased temperature. This, however, may probably be owing to the imperfection of our best thermometers, and to the difficulty of estimating the expansions of the glass containing the air examined.

Dr Dalton has ascertained that all gases expand equally by equal increments of heat; a conclusion which has been confirmed by Gay-Lussac. They have shown that, between the freezing and boiling point of water, 100 cubic inches of atmospheric air expand to 137∙50, that with hydrogen the result was 137∙52, with oxygen gas 137∙48, and with nitrogen 137∙49; differences so slight as to be within the probable limits of experimental error.

The force opposed to expansion would appear to be cohesion. Expansion is least in solids where the cohesion is strongest; it is more considerable in liquids where the cohesion is greatly weakened; and it is greatest in gases, in which cohesion is wholly overcome.

The expansion of bodies by heat, and their contraction on the reduction of the temperature, would show that the atoms of bodies are not in absolute contact. In fact, we may suppose them surrounded by atmospheres of heat, which prevent, by the repulsive energy of caloric, their absolute contact; whilst the force of cohesion limits the diffusive influence of the contained caloric. In some, the superior force of cohesion gives rise to solidity. When more heat is introduced, the cohesion is weakened, and the body becomes a liquid; and a further addition of caloric destroys cohesion altogether, separates the atoms, and the body becomes a gas.

The expansion of bodies by heat proves the mutual repulsion of their particles; but this limits the repulsive energy of heat to insensible distances. In 1824, Libri endeavoured to prove, from the movements of globules of water along fine wires, that the repulsive power of heat was exerted also at sensible distances. But his experiments are not conclusive; for the motions may have arisen from the formation of vapour at one end of the globule. Fresnel attempted to prove this point by bringing discs of foil or of mica, on the end of a delicately suspended magnet, into contact with fixed discs in vacuo, and marking the effect of heat collected from the sun’s rays. The moveable discs sensibly receded; but this may have arisen from some change produced by the heat in the form of the discs. Professor Forbes has happily applied[7:11:189] the repulsive energy of heat acting at sensible distances, to explain the curious vibrations of the metal bars in Trevelyan’s experiments. Still more lately Professor Powell of Cambridge has demonstrated the repulsion of heat at sensible distances (Phil. Trans, for 1834), by the changes produced, on the approximation of a heated body, in the Newtonian coloured rings of plates of glass which are pressed together. When, for instance, a flat plate is laid on a slightly convex surface of glass, the rings appear. On bringing a heated body near the upper surface of the plates, the rings instantly contract, and again enlarge on withdrawing the heated body. On repeating the experiment with the colours formed under the base of a prism placed on a lens of small convexity, he found the repulsion of heat acting at distances, which Sir J. Herschel has calculated at τ⅛θth of an inch.

Expansion is so general an effect of heat, that there are only two known exceptions, viz. in clay, and in water -at a certain limited range of temperature. It is well known that porcelain clay contracts in baking, and ever afterwards retains its contracted dimensions. It was this quality which induced Wedgwood to employ its contractions as a pyrometer. (See Thermometer and Pyrometer.) -This property, however, seemingly depends on heat producing in the heterogeneous substance clay, a more intimate union of its parts, or a partial conversion of this mechanical mixture into a chemical compound.

The exception of water between 42° and 32° Fahrenheit, is, however, real. When water is cooled down from the ordinary temperature, say 60°, it regularly contracts by the cooling, until it has attained 42°; but whilst passing from that point down to its freezing point, it continues to expand gradually^ until converted into ice.

The important purposes which this constitution of water serves in the economy of nature, the immense quantity of heat which is by this contrivance saved to our lakes and other collections of water, are strikingly pointed out by Count Rumford in his Essays; and this peculiarity in water has been confirmed by the well-devised experiments of Professor Hope and Dr Murray.

Many liquids at the moment of congelation expand, from the crystalline arrangement of their parts. This is familiarly known in the floating of ice in water, in the bursting of water pipes by frost, and in the splitting of masses of rock by the congelation of the water which has insinuated itself into their fissures. This force is well known to be enormous, as was shown by the experiments of the Florentine academicians, of Huygens, and of Major Williams at Quebec (see Edin. Phil. Trans.). The same cause produces the expansion of cast iron at the moment of becoming solid; and it is to this property that we are indebted for the sharpness of the casts obtained from this metal. In these instances we do not find an exception to the law of contraction by diminished heat. It is wholly owing to a crystalline arrangement of the particles, by which interstices are left between them, and consequently the solid occupies more space than if solidification took place without crystallization. Hence lead expands not in cooling, though cast iron does.

Many operations in the arts depend on the contraction of metals as they cool. It is in this way that the tyers of coach wheels are fitted tightly to the fellies. The expansion of metals by heat seemed at one time to form an insurmountable bar to the perfection of the going of a pendulum clock; but the ingenuity of an English artist showed how, by a combination of bars of metals of unequal expansibility, this property might be applied to keep the pendulum, at all temperatures, of precisely the same length. This first produced the gridiron pendulum; and more lately the compensation pendulum, with a mercurial cistern at the end of a metallic rod. Similar principles have been successfully applied to the balance-wheels of pocket chronometers.

2. Liquefaction, or Fluidity.

When solids are heated, they first expand, and then melt. This is a very general effect of caloric, the exceptions to which are disappearing, as we discover new sources of intense temperature. The oxyhydrogen blowpipe has fused almost all the most refractory substances; and the sun’s rays collected by immense lenses, or metallic mirrors, have melted, or even dissipated in vapour, many bodies, which were long regarded as incapable of fusion. We are therefore entitled to regard solidity as the natural state of all matter, and its two other modifications as resulting from its union with caloric.

Those bodies always fused at the ordinary temperatures are no exception to this law, since, by artificial cold, we have reduced most of them to a solid state. Thus mercury, at about — 40° of Fahrenheit, becomes a solid metal with the lustre of silver; and alcohol, which has not yet been frozen, may be considered as having its melting point lower than any yet discovered artificial cold. The point at which bodies become fluid differs widely in different substances, but remains uniformly the same in the same fluid under similar circumstances. Thus ice always melts at 32°.

It was long believed, that when solids began to melt, they were converted into liquids by a small increase of heat; and that they might again return to the solid state by a small diminution of their temperature. An attentive consideration of the process of liquefaction convinced the celebrated Dr Joseph Black of the insufficiency of the commonly-received opinion; and he promulgated in 1757 more philosophic views of this subject, which he illustrated by a beautifully simple experiment.

He introduced equal quantities of water into two thin glass flasks of the same form and weight. One of them he froze, by placing it in a freezing mixture; the other he reduced by similar means to the temperature of 32°, or its freezing point, but without allowing it to become solid. When removed from the freezing mixture, the flask of ice soon acquired the same temperature as the ice-cold water, and both were suspended in a room at 47°. In half an hour the thermometer in the ice-cold water had risen to 40°, but it required twenty-one half-hours to raise the temperature of the flask which had been frozen to the same point. As both were exposed to the same medium, equal quantities of heat must have been imparted to each in equal times; but it required twenty-one times as long to raise the frozen flask 8°, as sufficed to impart 8° to the ice-cold water. Dr Black inquired what had become of this quantity of caloric, which was not indicated by any rise in the thermometer? He inferred that it had entered into the ice during its liquefaction; and as the quantity so absorbed was not indicated by the thermometer, he denominated it Latent Heat. — In repeating the experiment with much care, he found that a pound of ice required twenty times as long to rise through 7°, as did as much ice-cold water; and therefore inferred, that during the conversion of that ice into water, as much heat disappears, or is absorbed, as would have elevated a pound of ice-cold water 140°. This absorption of heat during liquefaction is easily shown. If we add a pound of boiling water to a pound of ice, the temperature of the mixture will still remain at 32°; but if to a pound of water at 32°, we add a pound of boiling water at 212°, the temperature of the mixture will be found about J 22°, or a mean between the extremes of temperature.

Similar absorptions of heat take place during the melt» ing of tallow, bees-wax, and of the metals.
[7:11:190]

When liquid bodies congeal, or become solid, their latent heat is again given out.

It is possible, by nice management, to cool down water considerably below its freezing point. The principal circumstance necessary for this experiment is to leave it at perfect rest, in an atmosphere from 10° to 15° below 32° (Dalton succeeded in this way in cooling water as low as 5° without freezing); but on slightly agitating it, the water suddenly freezes; and if a thermometer has been suspended in it, the instrument suddenly rises to 32°, owing to the conversion of latent into sensible heat.

Another experiment shows this fact in a striking point of view. Into a glass flask introduce a mixture of sulphate of soda and water, in such proportions that it will form a saturated solution about the point of ebullition. When this is heated to that point, pour a little oil on its surface, introduce a thermometer, and remove it from the fire. When quite cold, drop into it a small crystal of sulphate of soda, and the solution will speedily crystallize into a solid mass, during the formation of which the thermometer will be seen to rise, indicating the evolution of sensible heat, during the conversion of the liquid into a solid.

The absorption of sensible caloric on the liquefaction of bodies forms the basis of most of the processes by which we obtain artificial cold. When to some salts, such as sulphate of soda, we add nitrous acid diluted with an equal part of water, the salt rapidly melts, and the temperature is reduced to the beginning of Fahrenheit’s scale. Diluted acid added to snow rapidly melts it, and the temperature is greatly reduced. A mixture of common salt and snow, which is the mixture generally employed to procure ice-cream, will sink the temperature to the beginning of Fahrenheit’s scale. Dry muriate of lime added to dry snow will reduce the temperature, during their liquefaction, so low as to freeze mercury. In all these instances it is the absorption of heat caused by the liquefaction, or the conversion of sensible into latent caloric, that produces the cold.

Dr Black applied his theory of latent heat to explain many phenomena. The ductility of a body appears to be owing to the presence of latent caloric; for if we hammer a piece of iron smartly, it becomes intensely hot, by parting with its latent caloric, and at the same time has its ductility greatly impaired. This ductility is only restored by again heating the metal in the fire, by which it re-acquires latent heat, that may again be forced out by a repetition of the hammering.

The absorption of heat by bodies whilst melting is an important law in the economy of nature. Had it merely been necessary, for the immediate conversion of ice or snow into water, to raise the atmospheric temperature a few degrees, the sudden formation of water would have deluged the earth on every occurrence of a thaw. On the other hand, had the slightest lowering of the temperature of the air below 32° been all that was requisite to convert water into ice, the sudden expansion of the congealing juices of vegetables must have burst their sap-vessels, and rent asunder the strongest ornaments of the forest. But the law of the gradual absorption and emanation of caloric during these transitions from the solid to the liquid, and from the liquid to the solid state, produces those changes tranquilly and beneficially. The melting snow gradually augments the sources which fertilize the valleys; whilst the soil, loosened by the expansion produced by the previous frost, when softened by the succeeding thaw, is fitted for the reception of the roots of plants.

The influence of these processes on climate is not inconsiderable. The absorption of heat during the liquefaction of ice on tropical mountains, sends down into the heated valleys copious sources of cool water, which by its immediate contact, and still more by its evaporation, assuages the fervour of a broiling climate; and in high la- ' titudes the caloric, eliminated on the freezing of water,, tends to mitigate the rigours of an arctic winter.

3. Vaporization.

When liquids are heated, the first effect is expansion; but if the application of heat be continued, they assume the aeriform state, or pass into vapour ; and when the caloric iŝ abstracted, they again assume the liquid form. When water is heated to 212° Fahrenheit, it boils, and is converted into an invisible aeriform fluid, which remains perfectly transparent and colourless as longas its temperature is not below 212°; but what in common language is called steam, is this elastic fluid partially recondensed into water, by the loss of a portion of its heat. The invisible elastic vapour is capable of occupying space and expelling atmospheric air, as is shown by corking a flask when boiling, and opening it underwater; when the flask will be suddenly entirely filled with the water, which condenses the steam.

Liquids, however, pass also into vapour by a more gradual process. If exposed to the air, water, for instance, gradually disappears; and if the process be carried on under a glass vessel, the included air becomes charged with moisture, which may be again abstracted from it by dry quicklime, or other substance having a strong affinity for water. The process by which liquids are thus converted into vapour is termed Spontaneous Evaporation ; an important operation in nature, as on it depends the charging of the atmosphere with water, for the formation of clouds, mist, rain, and dew; all elastic fluids, however, are not capable of being condensed into liquids by any decrease of temperature we can command. Thus, no artificial cold has hitherto been discovered capable of converting atmospheric air into a liquid.

The common property of all aeriform fluids is elasticity, or^the tendency, when forcibly compressed, to resume their former bulk. Thus, if we throw air, by means of a forcing pump, into an air-tight cistern, provided with a small orifice commanded by a stop-cock; on opening the latter, the air will issue out with great force, until the air has regained its former volume.

But vapours, or those aeriform bodies which are not permanently elastic, may, by strong pressure, even whilst their temperature is above their vaporific point, be condensed into liquids.

The elasticity of all aeriform bodies is increased by augmentation of temperature. In atmospheric air this increase has been found equal to ψ⅛θth of its volume for every 1° Fahrenheit; and the elasticity of steam, or the vapour of water, is nearly doubled by 30° of increased temperature above 212.

We are indebted to the celebrated Dr Dalton for accurate ideas as to the elasticity of aeriform bodies at different temperatures. He showed that the vapour of water, under a barometrical pressure of thirty inches at the boiling point, is just equal to the elasticity of atmospheric air under the same pressure; that the ratio of increase is rather less than a geometrical series, when the temperature is taken in an arithmetical progression; and, what was less obvious; that the elasticity of all vapours is precisely the same with the elasticity of the vapour of water, at the same number of degrees above the boiling point of each liquid. Thus water, under a mean barometrical pressure, boils at 212°; and the elasticity of its steam at 220°, or 8° above its boiling point, was found by Dalton to be = 34∙99 inches: alcohol boils at 175°, and the elasticity of its vapour at 183°, or 8° above its boiling point, is just = 34>∙99.
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The bulk of a body is very much increased by its conversion into vapour. Dr Black and Mr Watt made experiments to ascertain this increase. They boiled water in a flask, and, as the last drop was converted into steam, accurately closed the flask, which was then carefully weighed; on opening the flask below the surface of water, the quantity of water which rushed in was easily ascertained by a second weighing of the flask. The mean of several experiments showed that water, in the state of vapour, occupied 1800 times the space it filled as water. When heat is applied to solids, its first effect is expansion, next liquefaction, and, lastly, conversion into vapour. A few solids pass at once into the state of vapour, as carbonate of ammonia.

Different liquids acquire different degrees of heat for their vaporization. Thus aether becomes vapour at 104°, alcohol at 175°, water at 212°, and mercury requires a temperature about 692°. The vaporific point, however, remains constantly the same, in the same liquid, under the same barometric pressure. If, however, we diminish the pressure, the liquid will boil at a lower temperature. This is easily shown by the air-pump, in the exhausted receiver of which aether will boil at a temperature considerably below the freezing point of water. It is also strikingly exhibited by the following experiment: If a portion of water, say two.ounces, be boiled in a flask capable of holding eight or ten, and if it be corked whilst briskly boiling, a vacuum will be formed on its surface, by the condensation of its vapour, on removing it from the lamp. As the steam condenses, the liquid in the flask will begin to boil more briskly as the flask cools; and if ŵe pour cold water on this flask, the more will the pressure of the vapour in the flask be removed', and the more violently will the contained water boil. If now we pour boiling water on the flask, more steam will be formed, and the boiling will cease, but will be again renewed on a second application of the cold water. This may be alternated for several times if the flask be well corked.

As the boiling point of liquids varies exactly in the ratio of the barometrical pressure, it is obvious that the height of mountains may be ascertained by noting the thermometrical degree at which liquids boil on them. A portable instrument, constructed for this purpose, was devised by the Reverend F. Wollaston.

We cannot heat any liquid beyond its boiling point in an open vessel. Water placed on the fire soon rises to 212°, but a thermometer plunged in it remains at this point, however long it boils; but if the vessel be provided with a steam-tight cover, the temperature of the liquid may be much increased, according to the strength of the vessel. The elasticity of the steam in such cases is enormous; and experiments with steam under high pressure are hazardous, unless the vessel be of great thickness. The Marquis of Worcester seems to have burst a cannon by this means; and the frequent explosions of steam-engine boilers is a familiar instance of the same. Dr Black and Mr Watt heated water in a strong copper vessel to 400°; and in some of Perkins’ experiments lead was melted, it is said, in water subjected to strong pressure; yet in an open vessel we cannot heat water to more than 212°.

Dr Black sagaciously and happily applied his doctrine of latent heat to explain the conversion of liquids into vapour. He remarked, that when a kettle was placed on the fire it soon rose to 212°; but though the same heat continued to be applied, it rose no higher. In one of his beautifully simple and conclusive experiments, a vessel containing some water, at temperature 50°, was placed on a red-hot iron plate; in four minutes it began to boil, but it required twenty minutes to convert the whole into vapour. In the first four minutes it had acquired an increase of 162° of tem perature; and as the heat was uniform during the whole time of the experiment, it must have received an equal quantity of heat during the whole interval; or, during the other sixteen minutes, 810° must have flowed into it, yet during the whole time a thermometer in it rose no higher than 212°. Black naturally inferred that this large quan tity of heat, which disappeared, had entered into the vapour in a latent form.

A series of experiments were undertaken by him, and by his friend Mr Watt, from which they inferred, that when water is converted into steam, it unites with 940° of heat, which the thermometer does not indicate; or, in Black’s phraseology, that quantity becomes latent in the steam. This determination nearly coincides with the experiments of Lavoisier, who estimated the quantity which thus disappears at 1000° Fahr.

The absorption of heat during the formation of vapour is easily demonstrated. A piece of muslin moistened with any liquid, laid on the bulb of a thermometer, sinks the temperature; and if that liquid be very evaporable, the temperature thus produced will be low in proportion. The evaporation of aether will freeze water under the receiver of the air-pump; and the evaporation of the fluid called sulphuret of carbon is so rapid, that, in a well-exhausted receiver, it will freeze the mercury in the bulb of the thermometer.

A liquid may even, by particular management, be frozen by its own evaporation. This is the principle of Wollaston’s philosophic toy, called the cryophorus ; and it was ingeniously applied to an important practical purpose by the late Sir John Leslie, viz. the production of ice at a cheap rate in all climates. The apparatus employed by this philosopher is a powerful air-pump, which can at once exhaust from three to six flat receivers about twelve inches in diameter. These are fitted to different plates, each connected with the pump, and each provided with its own stop-cock. A shallow glass dish, nearly the width of the receiver, intended to hold a thin stratum of sulphuric acid, is introduced under each receiver, and a cup of porous earthenware, supported on a glass tripod about an inch above the surface of the acid, is under each receiver. Water is to be poured into this cup, which is to be placed on its tripod, and the whole covered by the receiver. By working the air-pump, each receiver may be exhausted in succession. The withdrawing of the atmospheric pressure causes the rapid evaporation of the water, the vapour of which is immediately absorbed by the sulphuric acid; and thus the vacuum is sustained. The latent heat necessary for the conversion of the water into steam or vapour is derived from the water itself; its temperature therefore falls; and the absorption of the vapour by the acid, as quickly as it is formed, keeps up the vacuum, and speedily reduces the whole water to the freezing point, when it soon forms a cake of ice. By a full-sized machine of this kind, about a quarter of an hour’s labour will set the process in full operation; and within the period of an hour afterwards six pounds of solid ice may be obtained. During this process, the water loses only about one fiftieth of its bulk; and the acid will be sufficiently strong for repeated operations of the same kind.

Leslie showed, that any substance which is powerfully absorbent of watery vapour, may be substituted for the acid; and he found that highly toasted oat meal, or well-dried powder of greenstone, had very considerable power to produce ice, when employed instead of sulphuric acid. He even showed, that by enclosing a globule of mercury in a small pyriform mass of ice, suspended over the acid in a good vacuum, the mercury might easily be frozen.

The latent heat of steam may be shown by the large quantity of water which may be rapidly heated by a small [7:11:192]quantity of steam. The elegant distillatory apparatus figured (No. 46) in Henry’s Chemistry readily proves this. In an experiment with a similar apparatus, the condensation ol one ounce of steam heated eight ounces of water from 60° to 180°; that is, the whole water gained 120° of temperature; consequently that ounce of steam had lost as much caloric as would have elevated an ounce of any fluid, capable of being so heated, to 960°.

The same phenomena attend the condensation of all other vapours; and we are to regard the discovery of Black, that during the conversion of solids into liquids, or of liquids into vapours, heat is absorbed, which is again given out on their recondensation, as a general law, and one of the highest importance, both in its practical application, and in explaining the phenomena of nature.

With regard to those aeriform bodies which we cannot condense, or, as they are called in chemical language, permanently elastic fluids or gases, we have every reason to consider them but as vapours, of which the po⅛t of generation is so low, that we have not yet found any means of exhibiting them in their unelastic state. This view is supported by analogy, and by recent discoveries. Some of the gases which a few years ago were reckoned permanently elastic, have been, by a great pressure, reduced to the state of liquids. This has been shown in the case of chlorine, muriatic acid, ammonia, and carbonic acid. It can also be shown that gases contain latent caloric. Thus, if we suddenly compress atmospheric air in a small tube fitted with a piston, so much heat is given out as to ignite touchwood. The sudden expansion of air, too, is always attended with the absorption of heat. Thus, on discharging an air-gun, the sudden expansion of the air produces a sensible degree of coolness in the condenser.

The same facts are shown when chemical action of the gases with each other produces condensation, or when they are evolved from their combinations.

The qualities of vapour are applied to many important purposes in the useful arts; and, on account of its economical fitness, the vapour of water, or steam, is that almost always employed.

The application of steam to domestic purposes is familiar to all; and it is frequently employed to communicate an equable heat, when a temperature above 212° would be injurious or dangerous. Thus steam, confined in metallic tubes, is used to dry some delicate articles of manufacture; and in some instances, where there is risk of explosion from even a moderate increase of temperature, the same contrivance is adopted. Steam has also been employed to warm apartments. It is employed to heat the dye-vats in calico-printing, and other species of dyeing, and has likewise been used for h¢nting warm baths. For these last purposes, the steam is usually allowed to escape into the fluid to be heated by numerous small apertures in pipes coiled in the bottom of the vessel; which may thus be made of less costly materials than if it were necessary to subject it to the fire.

The most important application of steam, however, is as a moving power in the most stupendous of human inventions the steam-engine, the perfecting of which has conferred an enviable immortality on the name of Watt. The application of this noble discovery to the moving of machinery of every sort, from the ponderous hammers of the forge to the slender needles of the tambouring frame, to the drawing out and twisting the gossamer filaments of the cotton factory, to the weaving of cloth with a celerity that gives the process the air of enchantment, the winged velocity of the locomotive engine on the railway, and the stately mechanism which renders navigation independent of the winds, belongs to different branches of practical mechanics.

The conversion of liquids into vapour is the foundation of the important arts of evaporation and distillation. By the first we obtain salts from their solutions; by the latter, the spirituous or more volatile particles of compounds. See Evaporation and Distillation.

4. Incandescence.

When the temperature of a body is raised to a certain pitch, it begins to emit light as well as heat; and this is termed incandescence, or glowing heat; a designation preferable to ignition, which may be confounded with combustion ; a process totally different from that treated of in this section, inasmuch as it is accompanied by important chemical changes in the body acted on; whereas incandescence may be repeated innumerable times with the same body, merely by raising its temperature. The point at which this takes place would seem to be the same in all bodies, and has been approximated by Newton; but as the determination depends on the acuteness of the eye, and the degree of obscurity in the apartment where the experiment is made, it is not easily fixed. According to his calculations, a good eye can perceive a body faintly luminous about 635° Fahrenheit; it shines with a full red in the dark about 752°, it is luminous in twilight at 884°, and glows in broad day-light when its temperature has reached 1000°. The experiments of Irvine, who found that mercury, which boils at 660°, is not in the slightest degree luminous at that point, prove that Newton fixed the point of incandescence too low. On the other hand, the determination of Wedgwood appears too high. In general the point of incandescence may be stated at about 800° Fahr. Its lowest pitch is at first a dull red, which becomes a full red with an increased temperature, or the least refrangible rays first meet the eye; if the heat be increased, these rays become mingled with the yellow; and, when the temperature is raised to the utmost, all the other rays of the spectrum are evolved in the proportions which constitute white light.

All solids which are not volatilized by heat may be rendered incandescent; and all liquids may be heated to redness, provided we can repress their volatility. But it has been doubted whether gases be capable of incandescence. Dr Fordyce had remarked that the extremity of the flame of a blowpipe, which was itself invisible, heated a thin rod of glass to a white heat; and some experiments of Mr T. Wedgwood prove that air, heated in a bent tube passing through a crucible filled with red-hot sand, was not luminous, although a gold wire suspended in the heated air became red. The speculations of Sir Humphry Davy on flame show that his opinion inclined to consider flame as luminous gases; but in all such cases there is reason to consider the light as derived from the combustible. Incandescence can be excited by the mere percussion of hard bodies against each other. Thus, two pieces of quartz struck together will produce light; the same will take place with two fragments of porcelain; and the light produced by the collision of flint and steel is partly incandescence, partly a species of combustion of the steel.

It has been supposed that incandescence affords a probable evidence of the identity or convertibility of heat into light. But this is not a legitimate deduction; for we may conceive that light and heat, though two distinct fluids, may have an intimate affinity for each other, or a tendency to unite, that both may exist united in all matter, that heat is most easily separated by percussion and friction, but that, when the percussion is violent, both are given out.

SECT. IV— quantity of heat in bodies.

Equal weights of the same body, at the same temperature, contain equal quantities of heat; and when their tern·
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peratures are unequal, their heat is in proportion to their temperature. But it is very different with dissimilar bodies, which can be shown to contain unequal quantities of heat at the same temperature. The following is the method of proving these positions. If we mix a pound of water at 40° with a pound of water at 112°, the resultant is 76°, the mean between the extremes of temperature.

But if we mingle a pound of water at temperature 112° with a pound of mercury at 40°, the resultant temperature will not be 76°, the mean, but 109°. Here the temperature of the water has only been diminished 3°, yet that of the mercury has risen 69°. If we reverse the experiment, and take water at 40° and mercury at 112°, the product will be 43°; the mercury losing 69°, whilst the water only acquires 3°; or the same quantity of heat which can elevate the temperature of mercury 23° will only augment that of water l o . If a similar experiment be made with spermaceti oil, Dr Thomson has shown {System of Chemistry) that the quantity of caloric which will elevate the temperature of the oil 2° can only raise that of water 1°.

Dr Black was undoubtedly the first who promulgated the idea of the absorption of heat during liquefaction and the formation of vapour; and this doctrine was publicly taught in his lectures in Glasgow university from 1757 to 1764. His pupil, Dr Irvine, continued these experiments at his suggestion, and ascribed the absorption to a change in the capacity for heat. These experiments were made between 1765 and 1770; and in 1779 Dr Adair Crawford published his treatise on Animal Heat, in which the capacities of numerous substances are given, which were corrected in his edition of 1788, from the results of most elaborate experiments. Professor Wilcke of Stockholm published some valuable experiments on the same subject, in the Stockholm Transactions for 1781. This philosopher introduced the term specific heat for what Irvine named capacity for heat. Various experiments on the same subject were made by Lavoisier and Laplace, with the instrument called a calorimeter, in which the specific caloric was estimated by the quantity of snow melted by different substances heated to the same pitch. The subject of experiment in their investigations was introduced into a wire cage suspended within a vessel of tinned iron. This vessel was filled with snow or ice; and to secure that snow from being melted by the external air, the vessel containing it was placed within another exterior case, and the space between them also filled with snow; and the whole covered by a lid, likewise covered with snow. Thus, if water in passing from 2l2° to 32° melted one pound of ice, and the same weight of oil melted a half pound; if the specific caloric of water be termed 1∙0, that of oil will be 0∙5.

A considerable series of experiments on this· curious subject were made by Dr Dalton {New System of Chemical Philosophy). In these investigations Dalton pursued a method, also employed by Leslie, namely, to observe the comparative rates of cooling, as was proposed by Meyer. This would be preferable to the other modes of ascertaining the specific caloric, were we sure that the cooling of bodies is not influenced by other circumstances than their capacity; but in this method it is difficult to obviate the effects of radiation and conducting power on refrigeration. In all these determinations there are to be foynd discrepancies inseparable from the difficulties attending such delicate investigations; but they have sufficient accordance when we attempt to estimate the capacities of solids and of liquids. It is a far more difficult and delicate problem to determine the specific caloric of gaseous bodies, from the minute quantities of matter to be operated on in such experiments, and the difficulty of obviating the chance of accidental error where the changes of temperature are necessarily very minute. Crawford found the difference in the specific heat of the gases rarely to exceed 1 1 θth of a degree of Fahrenheit. He employed two equal hollow spheres of brass, united by a bar of the same metal, and furnished with stop-cocks, and an adapting piece, to be screwed to an air-pump. One of these was filled with the gas to be tried, and the other was exhausted of air. Each ball had cemented into it a very delicate thermometer; both were heated to the same pitch by exposure to the same source of heat, conveyed by cylinders surrounded with warm water. They were then simultaneously plunged into separate vessels containing each equal quantities of cold water, and the elevation of temperature of the water in both vessels ascertained by delicate thermometers, indicating as small a change as Ath of a degree of Fahrenheit. The temperature communicated by the vacuum being subtracted from that given out by the other ball, the difference exhibited the heat communicated by the included air alone; and the accuracy of the method was afterwards tested by again comparing them when the exhausted vessel was filled with atmospheric air.

The specific caloric of gases has been again investigated by Berard and Delaroche in 1813. They passed a current of each gas, at a given temperature, through a spiral tube, fixed in a cylinder of thin copper filled with water and then closed. The temperature communicated, by the passage of the gases from a gasometer through the spiral, to the surrounding medium in a given time, being proportional to the excess of temperature each gas acquires from the source of heat above that of the medium in the cylinder, the comparative specific heat of each gas may be ascertained.

For the success of such experiments the current of gas must be uniform, and the temperature of the gas, when entering and escaping from the cylinder, must be accurately ascertained, as well as that of the water in the copper vessel.

The conclusions of these philosophers are directly at variance with those of Crawford, and, indeed, would tend to overturn some of the most important points of the philosophy of heat now generally received. But these experiments, though highly ingenious, are not more satisfactory than those of Crawford. The only objection to his conclusions is derived from the smallness of the quantities operated upon in his experiments; but the simplicity of his apparatus, the delicacy of his instruments, and the apparent care of his manipulations, more than fully counterbalance that objection; whilst the complexity of the apparatus of Berard and Delaroche, the acknowledged difficulty of keeping up an equable current through the spiral tube, the impossibility of obviating in such investigations the influence of changes in the medium during the experiments, and the nicety requisite to ascertain the temperature of the entering and escaping currents, present sources of error of which it is almost impossible to estimate the amount. On these grounds we may consider the conclusions of Crawford as not yet overturned.

The deductions of these gentlemen would lead to the conclusion that all the gases, with the exception of hydrogen, have an inferior specific gravity to water; and they even make steam inferior in capacity to water in the ratio of 847 to 1000. If this last were true, instead of an absorption of heat when steam is generated, we should have an extrication of caloric from it; and water in the worm-tub of a still, or in the condensing-bach of a steam-engine, ought not to increase in temperature. It may be added, that more recently the conclusions of Berard and Delaroche have been controverted by Delarive; and his experiments, as well as those of Clement, support the opinions of Crawford.

Mr Haycraft (in a naper in Trans. Royal Soc. Edin. x.) [7:11:194] has endeavoured to show that all gases at the same temperature, when perfectly freed from moisture, have the same specific heat; and that when they are saturated with water, their specific caloric is a certain ascending arithmetical ratio, in proportion to the quantity of moisture they contain. These views are rendered not improbable by the well-ascertained fact, that the elasticities of all the gases are the same at the same temperature.

The capacities of bodies are more nearly in the inverse ratio of their density, than of any other sensible property. Thus solids in general have less capacity for caloric than liquids, and liquids less than vapours or gaseous bodies. In the same body a change of capacity accompanies a change of volume. Thus gases compressed have their capacity diminished, and heat is extricated; and when they expand, their capacity is increased, which is the cause of the coldness felt on a sudden expansion of the air. Crawford endeavoured to show that this was also the case with liquids; but his experiments are scarcely to be relied on as establishing that point. The contraction of Wedgwood’s pyrometrical pieces would seem to diminish sensibly their capacity for heat. The capacity of bodies is not, however, exactly in the inverse ratio of their density, which probably arises from the effect of density on capacity for heat being modified by a difference in the force of affinity between caloric and various substances. There seems also to be some relation between capacity and power for conducting heat, as the former is nearly in the inverse ratio of the latter. If these views be correct, we may assume that the capacity of all bodies for caloric is directly as their volume and their affinity for heat, and inversely as their conducting power and their density.

When a body changes its form of existence, its capacity for heat is also changed. When a solid is melted, its capacity is increased, and the specific heat of the same substance is still further increased when it is converted into vapour. Thus, according to the best experiments, the capacity of ice is 0∙9000, that of water being 1∙000, and that of steam 1∙500.

This important law was applied by Dr Irvine to explain the liquefaction of solids. Dr Black regarded the liquefaction as owing to the absorption of heat; Dr Irvine ascribed this absorption to a change in the capacity of the body. The first ascribed the melting of the solid to the absorption of the heat, whilst the other attributed the absorption to the change of form. As the change of form and the absorption or extrication of caloric are in such cases simultaneous, it is obvious that the question cannot be decided by direct experiment. It has been objected to Irvine’s theory that it assigns no cause for the change of form, whilst Black’s ascribes the change to the ingress of caloric. On the other hand, Black’s theory does not explain why the heat is absorbed. When we heat a solid, the first effect is expansion, and this expansion keeping pace with the increasing temperature, a point will be attained when the expansion has so far overcome the cohesion of the solid that its particles move freely among each other, that is, when the body will become liquid. Thus far the change may be attributed to sensible heat; but the capacity of the body for heat has all the time been increasing, and, to satisfy this increased capacity, sensible heat has become latent. This appears the simplest view of the subject, ascribing the change of capacity to the expansion by the sensible heat; and the difference between the solid and fluid states may be conceived to depend on the prevalence of one of two opposing forms, the cohesive attraction of the particles of matter for each other, and the repulsive energy of caloric.

Dr Black has supposed that latent heat is retained in bodies by an affinity superior to that between sensible caloric and the particles of matter, and liquefaction is ascribed to this more intimate union. This opinion is scarcely perhaps reconcilable with the immediate effect of mixing ice cooled to 20° and water a little above the freezing point, when the water parts with its latent heat to raise the temperature of the ice; or with the effect of mechanical pressure in causing gases to part with their latent caloric.

Absolute Quantity of Heat in Bodies.

It will be sufficiently obvious, that neither by the thermometer nor by the capacity of bodies do we determine the whole heat which they contain at any temperature. The first is evidently nothing more than an indication of changes in a scale, of which the two extremes are unknown; the last mode affords us but the relative quantity of caloric required to elevate the temperature of other bodies compared to water, but it does not point out how many degrees any given temperature is above that point at which a body is deprived of all its heat. Irvine appears to have first conceived the idea of ascertaining by calculation the absolute zero, or deprivation of all heat, on the supposition that the whole heat in any body is proportional to its capacity. If this be granted, the whole caloric it contains at a given temperature may be found by ascertaining the quantity of heat it absorbs when passing from the solid to the liquid state. Thus ice has the capacity of 9 to water as 10, and, when both are at temperature 32°, water will contain one tenth more heat than the same weight of ice; but this excess is given out when water freezes, and as much is again absorbed when it melts. According to Black’s experiment, ice absorbs as much caloric, whilst passing into water, as would elevate an equal quantity of ice-cold water 140° Fahrenheit. Therefore 10 × 140 = 1400°, will give the natural zero, or the point of the absolute deprivation of heat Almost the same result is obtained by comparing the capacity of steam and water, viz. 1∙0 and 1 ’ 5. Water, in passing into steam, absorbs 940° of sensible heat, and 940 × 1∙5 = 1410.

The following general formula, as applicable to this investigation, is given by Professor Robison in his notes on Black’s Lectures. Let the capacity of water be 1. Let the quantity of water be W, and its temperature be w. Let the quantity of the body whose capacity is tried be B, and its temperature be b, and the temperature after mixture be m. Then the

W X m — w λ , capacity of B = ——- —-~ . Or if the water be the 1 j B X m — b hottest of the two bodies mixed, the formula is

ġ W X w — m

B X wi — b

The accuracy of this conclusion, however, depends on three points: first, the perfect determination of the specific heat of water, and of its two other forms of existence, to which it is probably impossible to obtain any more than an approximation; secondly, on the assumption that the whole heat of bodies is retained in them by their capacity; and, lastly, on the supposition that while the body retains its form of existence, its capacity remains unchanged. Until these points be established, the theory is but an amusing speculation, in which the estimates of other philosophers do not materially differ from those of Irvine. Rumford, from experiments on the heat extricated by the combination of hydrogen and oxygen, placed the natural zero at 1552° Fahrenheit below the freezing point; Gadolin, from the cold produced by dissolving muriate of soda in water, inferred it to be at — 1432°.
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SECT. V.—VARIATIONS OF TEMPERATURE.

1. Artificial Means of Increasing Temperature.

Caloric may be excited by the sun’s rays collected by a lens or by a concave mirror, by friction and percussion, and by chemical action.

1. When we collect the sun’s rays by a lens, it is well known that combustibles may thus be fired; and if the lens be large, it produces the most intense temperature we can command. In the focus of the powerful lens made in London for Mr Parker (which measured three feet in diameter, three inches thick at the centre, and weighed 212 lbs.), the most infusible metals were instantly melted and dissipated in vapour, and most stony substances were vitrified. Another, constructed at Paris, is described in the Memoires of the French academy. Lenses of great power have been also made of two curved plates of glass joined together, and filled with spirit of wine. A remarkable lens of this sort, formed by bending two plates of glass on a parabolic mold, and filling the cavity between them with ninety quarts of spirit, was constructed by Rossini of Gratz, in Styria. The diameter of the plates was 3 feet 3 inches, and they were united by a strong ring of metal. The whole was mounted on a heliostat, which, with the lens complete, weighed 550 lbs. This fine instrument cost about L.1000; but became, a few years ago, the property of the French government for L.338. In its focus a diamond was instantly kindled and dissipated; and a piece of platinum, twenty-nine grains in weight, was melted and thrown into violent ebullition.

Concave metallic mirrors are capable also of concentrating the sun’s rays, so as to produce a powerful heat. Mirrors of hammered brass, or tinned iron, are used for experiments on the radiation of heat. It was by some combination of mirrors that Archimedes is said to have fired the Roman fleet at the siege of Syracuse; and Kircher, having found a description in Tzetzes, of the device of Archimedes, from which it would seem that the mirrors were placed on hinges, in order to adjust them to a focus, constructed a compound burning mirror of this kind possessing considerable power; but Buffon, by combining as many as 168 plane glass mirrors, six inches broad, showed that silver might be fused at the distance of sixty feet by such an instrument.

2. The capability of friction between two solids to excite heat is well known. In Rumford’s experiments water was made to boil by the friction in boring a cannon; and the simple experiment of rubbing a smooth metallic button on a board, by which much heat is produced, is familiar to every school-boy. The firing of carriagewheels, and of different kinds of machinery, whose parts, moving against each other, are not well oiled, is well known. This extrication of heat takes place in vacuo as well as in the air, and appears to be owing to the compression employed forcing the particles of the solid more closely together, and extricating their latent caloric. Berthollet showed that this extrication of caloric is not unlimited, as Rumford erroneously supposed; but that, if we repeatedly compress any body, the quantity of heat extricated rapidly diminishes by each application of the compressing force.

Percussion acts in precisely the same manner. A piece of iron, by smart hammering on an anvil, may become so hot as to fire combustibles. This process evidently diminishes the capacity of the iron for heat, its specific gravity becomes greater, and the loss of its latent heat renders it stiff and brittle. A similar change takes place in wiredrawing metals; so that, to restore pliability and ductility, we must subject them to the fire, which restores their latent heat, and renders them again ductile.

3. Chemical action is a fruitful source of increase of temperature. If we mingle together equal parts of sulphuric acid and water, or of alcohol and of water, the ' bulk of the mixture diminishes, and heat is given out. The temperature produced by chemical action will often ignite inflammables. Thus a drop of sulphuric acid on a mixture of chlorate of potassa and sugar will set the mixture on fire. Indeed, the process of combustion, the great source of artificial heat, is nothing more than the chemical union of the oxygen of the air with the combustible body. The source of the temperature is the liberation of the latent heat of the oxygen, on its entering into union with the carbonaceous matter of the fuel; and the increase of the temperature is in proportion to the air consumed in a given time. If we wish a higher temperature, we increase the quantity of air that passes through the fuel; hence the utility of bellows, and of the blowpipe, in exciting a higher temperature than the spontaneous combustion of the burning body would afford.

The construction of furnaces for the melting of metals depends on this principle; namely, that of causing a large quantity of air to pass through the fire in a short period.

The heating of apartments by an open fire-place partly depends on somewhat different principles. We must not only secure a proper draught of air up the chimney to consume the fuel, but we must secure the radiation of heat into the room. Dark and rough surfaces radiate much of their communicated heat; and therefore, when any part of the grate is intended to afford the heat it receives from the fire to the air of the room, it should be of cast iron with some pattern on its surface, not of bright polished metal. This also forms the best material for a stove; but in open fire-places the heating of the apartment not only depends on what is radiated into the room, but also on what is reflected. As bright surfaces reflect best, the covings of the grate should be of bright and polished metal, and placed at about an angle of 45° to the back of the grate. By this disposition the heat of the fire, falling on the covings, is reflected at the same angle into the room; and we thus obtain a more considerable effect from the same expenditure of fuel.

For large apartments, and for public buildings, heat has been more effectually conveyed by the hot-air stoves of Dr Murray, and of Mr Strutt as improved by Sylvester. The principle of these inventions is to heat a current of air, by passing it between an interior cockle or furnace containing the fuel, and an exterior case, at the distance of a few inches from the cockle. The air so heated is admitted into the space to be warmed, and diffuses an equable warmth through the building. The only objection to hot air so introduced is the empyreumatic odour which the heat of the cockle, when not well managed, is apt to communicate to the air. But this objection is obviated by the contrivance of Mr Perkins, in which hot water is made to circulate through wrought iron tubes; and when more heat is required, these tubes are coiled up in several convolutions in the place where the most heat is wanted. In this city, the large premises of Messrs Cadell and Company, booksellers, are so heated; and the General Register House will in a short period be warmed by a similar apparatus.

2. Artificial Means of Diminishing Temperature.

There are three methods by which we can cool bodies; by placing them in contact with colder substances, by the evaporation of liquids, and by the liquefaction of solids.

1. The first method is very familiar, and dépends on the tendency of caloric to an equilibrium in contiguous bodies.

2. The conversion of a body into vapour causes, by the increase of its capacity for heat, an absorption of caloric.
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Thus the evaporation of water from the ball of a thermometer causes the mercury to fall. If we apply a still more evaporable fluid, aether, the fall of the thermometer will be still lower; and, if we accelerate this process by an air-pump, the cold produced will be intense; the degree of the absorption of heat, or, in other words, the production of cold, being in proportion to the quickness of the evaporation.

. 3. The most powerful means of reducing temperature is by what are termed freezing mixtures. All these depend on the rapid melting of solids by the addition of various substances. Many experiments have been made on this subject by Lowitz of Petersburg, and by Mr Walker of Cambridge. Salts are the solids most commonly used, and they are in general either mixed with snow or with acids. Thus, if we mix common salt and snow together, the temperature falls to 0° of Fahrenheit. If we pour two ounces of nitric acid diluted with an equal quantity of water on three ounces of sulphate of soda, the temperature sinks below the beginning of Fahrenheit’s scale. Equal parts of strong muriatic acid and of snow will produce a cold of — 30° Fahrenheit; and the same proportions of diluted sulphuric acid and snow, if previously cooled down to 20°, will cause the freezing of mercury, reducing the temperature to — 60°. Dry muriate of lime and dry powdery snow, in the proportions of two of the former to one of the latter, if previously cooled by immersion in salt and snow, will sink the temperature to — 66°; and three parts of muriate of lime and two of snow, similarly treated, will reduce the temperature to — 73°.

In all these experiments, it is the sudden conversion of sensible into, latent heat that lowers the temperature of the mixtures; the substances assume the liquid form, their capacity for heat is increased, and the disappearance of the sensible heat is manifested by the sinking of the thermometer.

3. Natural Variations of Temperature.

The temperature of the earth is affected by the absorption of the solar rays. The revolution of the earth on its axis brinġs successive portions of its surface under the immediate influence of the sun, and this portion is greatly extended on either side of the equator by the beautifully simple yet stupendous contrivance of the inclination of that axis to the plane of its orbit. These causes produce variations in natural temperature; and as they are uniform and constant in their operation, the temperature of any place is liable to little change, if compared for a series of years, however subject to inequalities in short intervals of time.

The mean temperature of any climate cannot be ascertained by a few observations on the heat of the air; but experience has shown that the mean temperature of the month of April, in our latitudes, will afford a near approximation to that of the whole year. The surface of the ground absorbs the heat of the sun, and becomes thereby warmed; but much of the heat so acquired is lost by radiation during the night, as we have already explained. The surplus heat is slowly transmitted downwards. The experiments made by Mr Ferguson, at Raith, in Fifeshire, by burying very large thermometers at different depths in his garden, showed that during summer the increasing heat penetrated through the ground about five feet each month, or about two inches daily; so that the whole heat of the summer six months would penetrate to about thirty feet, which agrees remarkably with the experiments made by Saussure on the banks of the Arve, near Geneva. There the heat of summer took six months to penetrate thirty-one feet; and beyond that depth the temperature of the earth underwent no change from the summer’s heat or the winter’s cold. Hence deep caves, or springs whose sources are from a considerable depth, may be considered' as affording a tolerable approximation to the mean annual temperature of the places where they occur. This, however, is not always true. The writer of this article, during several years, occasionally took the temperature of the largest spring in Great Britain, St Winifred’s, at Holywell, in Flintshire; and the temperature of its water was always considerably higher than 50°, the mean temperature . of that part of the island. In three successive years it was about 57°, and never but once, after heavy rain, observed so low as 55°, whilst another very copious contiguous spring, the sacred well of Einnon Asa, was no more than 50°.

Latitude must exert a great influence on the temperature of the earth, as is sufficiently obvious; but the extremes of temperature, at different times, in the same place, are less within the tropics than toward the polar limits of the temperate and in the frigid zones; for the difference of heat at different seasons must be less, in those regions where the sun is twice vertical annually, than in higher latitudes.

There are two other causes which exert no small influence on the heat of a climate, the vicinity of the ocean, and elevation above its level.

From the enormous mass of the ocean, its waters, except in the vicinity of land or polar ices, have a remarkable uniformity of temperature in almost every climate; and hence the ocean may be considered as the great equalizer of heat over the globe, tending to mitigate the fervour of tropical climates, and the rigour of the polar regions.

When cold air sweeps over the surface of the ocean, it tends to cool the particles of water at the surface; but by this process their specific gravity is increased, and they sink, whilst their place is supplied by fresh particles from below, which, becoming cooled in their turn, give rise to internal motions in the water. From its immense mass, it is evident that no partial application of cold air can reduce the mean temperature of its whole waters; and hence we find that the temperature of islands in our latitudes is not so low in winter as that of continents under the same parallel. Thus, the severity of winter in Britain bears no proportion to that of France or Germany in the same parallels of latitude. The most northern of the Orkney Islands is nearly in the latitude of Petersburg, yet in the former a continued frost for three or four days is a rare occurrence, whilst in the latter the rapid and spacious Neva is completely frozen over for several months every season.

On the other hand, the ocean exerts a great influence in mitigating the heat of tropical regions. It is true that when the upper-particles of water become heated, the internal motions from unequal temperature cease; but the rarefaction produced sets a current in motion, which, aided by the trade winds, becomes very manifest amongst the West Indian islands in the Gulf Stream ; the place of which is supplied by currents of cooler water perpetually setting in from the polar regions. But this is not all. The temperature of the atmosphere is mightily diminished by the increased evaporation from the water in warm latitudes; and we have seen, that during this conversion of water into vapour, as much heat becomes latent as would raise the temperature of an equal quantity of other matter by at least 900° Fahrenheit. Hence tropical islands are less warm than continents under the same parallel; and the intersection of continents by gulfs and seas tends to mitigate the extremes of temperature. The extended form of the old continent towards the eastern regions of Asia renders the inequalities of temperature much greater than under the same parallels in Europe. Thus, at [7:11:197]Pekin, the thermometer in winter often descends far below the freezing point, and averages, during the night, for several months, 20° Fahrenheit: yet this place is under the same parallel of latitude as the finest provinces of Southern Italy and the centre of Spain.

But of all the causes influencing the temperature of a country, there is none more important than that of elevation above the level of the sea. In every climate, as is well known, the temperature diminishes as we rise in the atmosphere; and if mountains be very lofty, their summits are the abode of perpetual winter. This is best explained by the fact, that as we rarefy air, its capacity for caloric is increased; the consequence of which is, that its temperature falls as its sensible is converted into latent heat. In passing through air, the sun’s rays do not sensibly heat it; but when they impinge on the opake surface of the earth, they are retained, raise its temperature, and a part of this increased heat is communicated to the contiguous stratum of air, which expands, ascends, and has its place supplied by a cooler stratum, which ascends in its turn. Thus the earth warms the air near the surface; but as the pressure of the column of air diminishes by its ascent, its redundant heat becomes latent as the volume of the air augments; so that, at moderate elevations, the atmosphere receives no genial influence from the warmth imparted to the earth; and very lofty mountains, in all climates, are covered with perennial snow, and where the temperature is steadily below the freezing point, there is perpetual congelation. The point at which this takes place is different in different latitudes. The limit is highest under the equator, and descends as we approach the poles. It is obvious too that the influence of the seasons, especially in those countries where there is a great difference between the temperature of summer and winter, will alter the limit of perpetual congelation. In such climates it will fluctuate with the seasons between two points, which form what are termed the higher and lower limits of congelation. The difference between these limits is least in tropical regions, but increases toward the poles. This circumstance is of great importance, inasmuch as it influences the formation of the stupendous glaciers, which excite the curiosity and admiration of the traveller, in the elevated regions of the temperate, and on the acclivities of the arctic circle.

The ascent of heated air from the heated surface of the earth might be supposed to warm the superior regions of the atmosphere, were not this counteracted by the increased capacity of rarefied air for heat, by which much more is absorbed than the ascending air can carry with it; but, on the other hand, the too great loss of heat in the lower strata of air is provided against, by the change of capacity which takes place as cool air descends to supply the place of that which has ascended. Its density is increased by increasing pressure, consequently its capacity for heat is diminished; and the heat thus extricated is imparted to the contiguous strata of air. That these two processes counterbalance each other is very obvious, from the uniformity of mean temperature in any spot.

The influence of elevation above the level of the sea is capable of countervailing the effect of low latitudes on the vegetable productions of such regions. Tournefort had remarked the succession of plants on the ascent of Mount Ararat, which gave him, in Armenia, the vegetable forms of different parts of Europe. In Wahlenberg’s Flora Lap-pθiiica, this distribution of vegetable forms into zones is well characterized. The same is remarked iń Ramond’s examination of the botany of the Pyrenees; and the illustrious traveller Humboldt has given a striking illustration of the same distribution, in his examination of the equinoctial Andes. After rising through various parallel belts or zones of vegetation, each marked by some characteristic plants, if the mountains be sufficiently high, in all latitudes we arrive at the region of stunted shrubs, and of Alpine plants and grasses, amongst which the genera gentiana, ranunculus, and saxifraga, are conspicuous; and, finally, we trace byssi, hepaticae, and lichenes, the last outskirts of vegetable life, to the very verge of perennial winter.

These various causes affecting natural temperature render it impossible to estimate the mean heat of any country by its latitude; and as every formula hitherto devised for calculating the mean temperature of any region is now found to be erroneous, it is only by multiplied observations that we can arrive at any accurate conclusion. In the mean time Humboldt has conferred a great favour on meteorological science by his admirable paper On Isothermal Lines, and the Distribution of Heat over the Globe. Setting aside all hypothetical views, he has traced on a chart, from the best meteorological observations, lines of equal temperatures ; so that at a glance we can trace the places which are found to have the same mean temperature. From this map we immediately perceive how widely these isothermal lines differ from the parallels of latitude; and we remark, that the isothermal lines of Western Europe sink to a far lower latitude as they recede on either hand. Thus, the isothermal line of temperature 54° ∙4 in France, in longitude 0. 37. east, passes through latitude 45. 46. north; but in China, in longitude 116. 27. east, passes near Pekin, in latitude 39. 54. north; and in longitude 104. west, near the mouth of Columbia River, in America, it passes through 44. 40. north. The isothermal lines, however, on the eastern side of the Rocky Mountains, descend still lower, compared to their position in Europe, as the following table shows :—
[table]

In both continents the most rapid decrease of mean temperature is between the parallels of latitude 40° and 45°; a circumstance which, Humboldt justly remarks, has undoubtedly had a happy influence on the civilization and industry of the people inhabiting that zone, because these slight variations of latitude produce considerable changes in the vegetable productions that become objects of rural economy; and when contiguous countries differ much in their products, it stimulates the industry of each, and gives vigour to commercial intercourse; circumstances highly conducive to civilization.

For various important facts and observations on heat, see Black’s Lectures on Chemistry, vol. i.; Murray’s System of Chemistry, vol. i.; Dalton’s Chemistry ; Leslie on Heat ; Pictet sur le Feu; Rumford’s Essays ; Deluc sur les Modif. de l’Atmosph.; Saussure sur l’ Hygrométrie; Young’s Lectures on Philosophy ; Biot, Traite de Physique, i.; Martine on Heat ; Crawford on Heat ; Irvine’s Essays; J. and G. Murray’s Popular View of Chemistry ; Mrs Somerville on the Connection of the Physical Sciences ; Phil.· Trans. 1754, 1777, 1783, 1788, 1792, 1795, »1799, 1800, and 1801; Edin. Phil. Trans, vi. ix. x. xii. χiii.; Ann. de Chim. 3, 14, 22, 29, 71, and 75; Nicholson’s Journal, 4, 9, 11, and 12, 8vo series; Journ. de Physique, 61; Ann. of Phil. 2, &c.; Manchester Memoirs; Mémoires de ĭ Académie de Genève, &c. (ι. ι. I.) [7:11:198]