near. There are three species; viz. the serraticornis, hispicornis, and pygmæus. CALORIC, in chemistry, a word used to signify that substance or property, by which the phenomena of heat are produced. Concerning the nature of caloric there are two opinions, which have divided philosophers ever since they turned their attention to the subject. Some suppose, that caloric, like gravity, is merely a property of matter, and that it consists in a peculiar vibration of its particles; others, on the contrary, think that it is a distinct substance. Each of these opinions has been supported by the greatest philosophers; and till lately the obscurity of the subject has been such, that both sides have been able to produce exceedingly plausible and forcible arguments. The recent improvements, however, in this branch of chemistry, have gradually rendered the latter opinion much more probable than the former: and a recent discovery made by Dr. Herschell, has at last nearly put an end to the dispute, by demonstrating that caloric is not a property, but a peculiar substance; or at least that we have the same reason for considering it to be a substance, as we have for believing that light is material. Dr. Herschell had been employed in making observations on the sun, by means of telescopes. To prevent the inconvenience arising from the heat, be used coloured glasses: but these glasses, when they were deep enough coloured to intercept the light, very soon cracked, and broke in pieces. This circumstance induced him to examine the heating power of the different coloured rays. He made each of them in its turn fall upon the bulb of a thermometer, near which two other thermometers were placed, to serve as a standard. The number of degrees which the thermometer exposed to the coloured ray rose above the other two thermometers indicated the heating power of that ray. He found that the most refrangible rays have the least heating power, and that the heating power gradually increases as the refrangibility diminishes. The violet ray therefore has the smallest heating power, and the red ray the greatest. Dr. Herschel found, that the heating power of the violet, green, and red rays, are to each other as the following numbers: Violet.. Red = 16 22.4 = 55. that the illuminating power and the heat. ing power of the rays follow such different laws. The first exists in greatest perfection in the middle of the spectrum, and diminishes as we approach either extremity; but the second increases constantly from the violet end, and is greatest at the red end. This led him to suspect, that perhaps the heating power does not stop at the end of the visible spectrum, but is continued beyond it. He placed the thermometer completely beyond the boundary of the red ray, but still in the line of the spectrum, and it rose still higher than it had done when exposed to the red ray. On shifting the thermometer still farther,it continued to rise, and the rise did not reach its maximum till the thermometer was half an inch beyond the utmost extremity of the red ray. When shifted still farther, it sunk a little, but the power of heating was sensible at the distance of 13 inch from the red ray. These important experiments have been lately repeated and fully confirmed by Sir Henry Englefield, in the presence of some very good judges. From these it follows, that there are rays emitted from the sun which produce heat, but have not the power of illuminating; and that these are the rays which produce the greatest quantity of heat. Consequently caloric is emitted from the sun in rays, and the rays of caloric are not the same with the rays of light. On examining the other extremity of the spectrum, Dr. Herschel ascertained that no rays of caloric can be traced beyond the violet ray. He had found, however, that all the coloured rays of the spectrum have the power of heating; it may be questioned, therefore, whether there be any rays which do not warm. The coloured rays must either have the property of exciting heat as rays of light, or they must derive that property from a mixture of rays of caloric. If the first of these suppositions were true, light ought to excite heat in all cases; but it has been long known to philosophers, that the light of the moon does not produce the least sensible heat, even when concentrated so strongly as to surpass in point of illumination the brightest candles or lamps, and yet these produce a very sensible heat. Here then are rays of light which do not produce heat rays, too, composed of all the seven prismatic coloured rays. We must conclude from this well known fact, that rays of light do not excite heat; and consequently that the coloured rays from the sun and com It struck Dr. Herschel as remarkable, bustible bodies, since they excite heat,. must consist of a mixture of rays of light and rays of caloric. That this is the case was demonstrated long ago by Dr. Hooke, and afterwards by Scheele, who separated the two species from each other by a very simple method. If a glass mirror be held before a fire, it reflects the rays of light, but not the rays of caloric; a metallic mirror, on the other hand, reflects both. The glass mirror becomes hot; the metallic mirror does not alter its temperature. If a plate of glass be suddenly interposed between a glowing fire and the face, it intercepts completely the warming power of the fire, without causing any sensible diminution of its brilliancy; consequently it intercepts the rays of caloric, but allows the rays of light to pass. If the glass be allowed to remain. in its station till its temperature has reached its maximum, in that situation it ceases to intercept the rays of caloric, but allows them to pass as freely as the rays of light. This curious fact, which shews us that glass only intercepts the rays of caloric till it be saturated with them, was discovered by Dr. Robinson. These facts are sufficient to convince us, that the rays of light and of caloric are different, and that the coloured rays derive their heating power from the rays of caloric which they contain. Thus it appears that solar light is composed of three sets of rays, the colorific, the calorific, and the deoxi. dizing. The rays of caloric are refracted by transparent bodies just as the rays of light. We see, too, that, like the rays of light, they differ in their refrangibili. ty; that some of them are as refrangible as the violet rays; but that the greater number of them are less refrangible than the red rays. Whether they are transmitted through all transparent bodies has not been ascertained; neither has the difference of their refraction in different mediums been examined. We are certain, however, that they are transmitted and refracted by all transparent bodies which have been employed as burningglasses. Dr. Herschell has also proved, by experiment, that it is not only the caloric emitted by the sun which is refrangible, but likewise the rays emitted by common fires, by candles, by hot iron, and even by hot water. The rays of caloric are reflected by polished surfaces in the same manner as the rays of light. This was lately proved by Herschel but it had been demonstrated long before by Scheele, who had even ascertained that the angle of their reflection is equal to the angle of their incidence. M. Pictet also had made a set of very ingenious experiments on this subject, about the year 1790, which led to the same conclusion. All the phenomena concur to shew, that the rays of caloric move with a very considerable velocity, though the rate has not been ascertained in a satisfactory manner. Some experiments of Mr. Leslie would lead us to conclude, that they move with the same velocity as sound. The following experiment of M. Pictet indicates a very considerable velocity. He placed two concave mirrors at the distance of 69 feet from each other; the one of tin, the other of plaster gilt, and 18 inches in diameter. Into the focus of this last mirror he put an air thermometer, and a hot bullet of iron into that of the other. A few inches from the face of the tin mirror there was placed a thick screen, which was removed as soon as the bullet reached the focus. The thermometer rose the instant the screen was removed without any perceptible interval, consequently the time which caloric takes in moving 69 feet is too minute to be measured. The velocity of caloric, if it is equal to that of light, would prove that its particles must be equally minute. Therefore, neither the addition of caloric, nor its abstraction, can sensibly affect the weight of bodies. Caloric agrees with light in another property no less peculiar: its particles are never found cohering together in masses; and whenever they are forcibly accumulated, they fly off in all directions, and separate from each other with inconceivable rapidity. This property necessarily supposes the existence of a mutual repulsion between the particles of caloric. Thus it appears that caloric and light resemble each other in a great number of properties. Both are emitted from the sun in rays, with the velocity of 200,000 miles in a second; both of them are refracted by transparent bodies, and reflected by polished surfaces; both of them consist of particles which mutually repel each other, and which produced no sensible effect upon the weight of other bodies. They differ, however, in this particular: light produces in us the sensation of vision; caloric, on the contrary, the sensation of heat. Upon the whole, we are authorized, by the above statement of facts, to conclude, that the solar light is composed of three distinct substances, in some measure separable by the prism, on account of the difference of their refrangibility. The colorific rays are the least refrangible, the deoxidizing rays are most refrangible, and the calorific rays possess a mean degree of refrangibility. Hence the rays in the middle of the spectrum have the greatest illuminating power; those beyond the red end the greatest heating power; and those beyond the violet end the greatest deoxidizing power: and the heating power on the one hand, and the deoxidizing power on the other, gradually increase, as we approach that end of the spectrum where the maximum of each is concentrated. These different bodies resemble each other in so many particulars, that the same reasoning respecting refrangibility, reflexibility, &c. may be applied to all; but they produce different effects upon those bodies on which they act. Little progress has yet been made in the investigation of these effects; but we may look forward to this subject as likely to correct many vague and unmeaning opinions, which are at present in vogue among chemists. From this account of the nature of caloric we learn, that it is capable, like light, of radiating in all directions from the surfaces of bodies; and that when thus radiated, it moves with a very considerable velocity. Like light, too, it is liable to be absorbed when it impinges against the surfaces of bodies. When it has thus entered, it is capable of making its way through all bodies; but its motion in this case is comparatively slow. Heat then moves at two very different rates. 1. It escapes from the surfaces of bodies. 2. It is conducted, or passes through bodies. When bodies artificially heated are exposed to the open air, they immediately begin to emit heat, and continue to do so till they become nearly of the temperature of the surrounding atmosphere. That different substances, when placed in this situation, cool down with very different degrees of rapidity, could not have escaped the most careless observer; but the influence of the surface of the hot body in accelerating or retarding the cooling process, was not suspected till lately. For this curious and important part of the doctrine of heat, we are indebted to the sagacity of Mr. Leslie, who has already brought it to a great degree of perfection. To whose work we refer the philosophical reader for much useful and highly interesting matter. Although caloric is incapable of moving in rays through solid bodies, yet it is well known that all bodies whatever are pervious to it. Through solids, then, it must pass in a different manner. In general, its passage through them is re markably slow. Thus, if we put the end of a bar of iron, 20 inches long, into a common fire, while a thermometer is attached to the other extremity, four minutes elapse before the thermometer begins to ascend, and 15 minutes by the time it has risen 15°. In this case, the caloric takes four minutes to pass through a bar of iron 20 inches in length. When caloric passes in this slow manner, it is said to be conducted through bodies. It is in this manner alone that it passes through non-elastic bodies; and though it often moves by radiation through elastic media, yet we shall find that it is capable of being conducted through them likewise. As the velocity of caloric, when it is conducted through bodies, is greatly retarded, it is clear that it does not move through them without restraint. It must be detained for some time by the particles of the conducting body, and consequently must be attracted by them.Hence it follows, that there is an affinity or attraction between caloric and every conductor. It is in consequence of this affinity that it is conducted through the body. Bodies then conduct caloric in consequence of their affinity for it, and the property which they have of combining indefinitely with additional doses of it. Hence the reason of the slowness of the process, or, which is the same thing, of the long time necessary to heat or to cool a body. The process consists in an almost infinite number of repeated compositions and decompositions. We see, too, that when heat is applied to one extremity of a body, the temperature of the strata of that body must diminish equably, according to their distance from the source of heat. Every person must have observed that this is always the case. If, for instance, we pass our hand along an iron rod, one end of which is held in the fire, we shall perceive its temperature gradually diminishing from the end in the fire, which is hottest, to the other extremity, which is coldest. Hence the measure of the heat transmitted must always be proportional to the excess of temperature communicated to that side of the conductor which is nearest the source of heat. The passage of caloric through a body by its conducting power must have a limit; and that limit depends upon the number of doses of caloric, with which the stratum of the body nearest the source of heat is capable of combining. If the length of a body be so great, that the strata of which it is composed exceed the number of doses of caloric with which a stratum is capable of combining, it is clear that caloric cannot possibly be conducted through the body; that is to say, the strata farthest distant from the source of heat cannot receive any increase of temperature. This limit depends, in all cases, upon the quantity of caloric with which a body is capable of combining before it changes its state. All bodies, as far as we know at present, are capable of combining indefinitely with caloric; but the greater number, after the addition of a certain number of doses, change their state. Thus ice, after combining with a certain quantity of caloric, is changed into water, which is converted in its turn to steam, by the addition of more caloric. Metals, also, when heated to a certain degree, melt, are volatilized, and oxydated; wood and most other combustibles catch fire, and are dissipated. As to the rate at which bodies conduct caloric, that depends upon the specific nature of each particular body, the best conductors conducting most rapidly, and to the greatest distance. When bodies are arranged into sets, we may lay it down as a general rule, that the densest set conduct at the greatest rate. Thus the metals conduit at a greater rate than any other bodies. But in considering the individuals of a set, it is not always the densest that conducts best as bodies conduct caloric in consequence of their affinity for it, and as all bodies have an affinity for caloric, it follows as a consequence, that all bodies must be conductors, unless their conducting power be counteracted by some other property. All solids are conductors; because all solids are capable of combining with va rious doses of caloric before they change their state. This is the case in a very remarkable degree with all earthy and stony bodies: it is the case also with metals, with vegetables, and with animal matters. This, however, must be understood with certain limitations. All bodies are indeed conductors; but they are not conductors in all situations. Most solids are conductors at the common temperature of the atmosphere; but when heated to the temperature at which they change their state, they are no longer conductors. Thus, at the temperature of 60°, sulphur is a conductor; but when heated to 214°, or the point at which it melts or is volatilized, it is no longer a conductor. In the same manner ice conducts caloric when at the temperature of 20°, or any other degree below the freez ing point; but ice at 32° is not a conduc tor, because the addition of caloric causes it to change its state. With respect to liquids and gaseous bodies, it would appear at first sight that they also are all conductors; for they can be heated as well as solids, and heated too considerably without sensibly changing their state. But fluids differ from solids in one essential particular: their particles are at full liberty to move among themelves, and they obey the smallest impulse; while the particles of solids, from the very nature of these bodies, are fixed and stationary. One of the changes which caloric produces on bodies is expansion, or increase of bulk; and this increase is attended with a proportional diminution of specific gravity. Therefore, whenever caloric combines with a stratum of particles, the whole stratum becomes specifically lighter than the other particles. This produces no change of situation in solids; but in fluids, if the heated stratum happens to be below the other strata, it is pressed upwards by them, and being at liberty to move, it changes its place, and is buoy. ed up to the surface of the fluid. In fluids, then, it makes a very great difference to what part of the body the source of heat is applied. If it be applied to the kighest stratum of all, or to the surface of the liquid, the caloric can only make its way downwards, as through solids,by the conducting power of the fluid; but if it be applied to the lowest stratum, it makes its way upwards, independently of that conducting power, in consequence of the fluidity of the body, and the expansion of the heated particles. The lowest stratum, as soon as it combines with a dose of caloric, becomes specifically lighter and ascends. New particles approach the source of heat, combine with caloric in their turn, and are displac ed. In this manner all the particles come, one after another, to the source of heat; of course the whole of them are heated in a very short time, and the caloric is carried almost at once to much greater distances in fluids than in any solid whatever. Fluids, therefore, have the property of carrying or transporting caloric; in consequence of which they acquire heat independently altogether of any conducting power. If we take a bar of iron and a piece of stone of equal dimensions, and putting one end of each into the fire, apply either thermometers or our hands to the other, we shall find the extremity of the iron sensibly hot long before that of the stone. Caloric, therefore, is not conduct. 1 ed through all bodies with the same ce- Gold, Copper, nearly equal Tin, Platinum, Iron, Steel, them. The effects which caloric produces on bodies may be arranged under three heads; namely, changes in bulk; changes in state; and changes in combination. It may be laid down as a general rule, to which there is no known exception, that every addition or abstraction of caloric makes a corresponding change in the bulk of the body which has been subjected to this alteration in the quantity of its heat. In general the addition of heat increases the bulk of a body, and the abstraction of it diminishes its bulk; but this is not uniformly the case, though the exceptions are not numerous. Indeed, these exceptions are not only confined to a very small number of bodies, but even in them they do not hold, except at certain particular temperatures; while at all other temperatures these bodies are increased in bulk when heated, and diminished in bulk by being cooled. We >much inferior to the others. may therefore consider expansion as one Next to metals, stones seem to be the best conductors; but this property varies considerably in different stones. Bricks are much worse conductors than most stones. Glass seems not to differ much from stones in its conducting power: like them, it is a bad conductor. This is the reason that it is so apt to crack on being suddenly heated or cooled. One part of it, receiving or parting with its caloric before the rest, expands or contracts, and destroys the cohesion. Next to these some dried woods. Charcoal is also a bad conductor; according to the experiments of Morveau, its conducting power is to that of fine sand 2 3. Feathers, silk, wool, and hair, are still worse conductors than any of the substances yet mentioned. This is the reason that they answer well for artieles of clothing. They do not allow the heat of the body to be carried off by the cold external air. Count Rumford has made a very ingenious set of experiments on the conducting power of these substanHe ascertained that their conducting power is inversely as the fineness of their texture. ces. Having in the preceding sections considered the nature of caloric, the manner which it moves through other bodies and distributes itself among them, let us now examine, in the next place, the effects which it produces on other bodies, either by entering into them or separating from of the most general effects of heat. It is certainly one of the most important, as it has furnished us with the means of measuring all the others. See PYROMETER. Though all bodies are expanded by heat, and contracted by cold, and this expansion in the same body is always proportional to some function of the quantity of caloric added or abstracted, yet the absolute expansion or contraction has been found to differ exceedingly in different bodies. In general, the expansion of gaseous bodies is greatest of all; that of liquids is much smaller; and that of solids the smallest of all. Thus, 100 cubic inches of atmospheric air, by being heated from the temperature of 329 to that of 212°, are increased to 137.5 cubic inches: while the same augmentation of temperature only makes 100 cubic inches of water assume the bulk of 104.5 cubic inches: and 100 cubic inches of iron, when heated from 32° to 212°, assume a bulk scarcely exceeding 100.1 cubic inches. From this example, we see that the expansion of air is more than eight times greater than that of water; and the expansion of water about 45 times greater than that of iron. See EXPAN SION. All substances in nature, as far as we are acquainted with them, occur in one or other of the three following states; namely, the state of solids, of liquids, or of elastic fluids or vapours. It has been ascertained, that in a vast number of cases, the same substance is capable of existing successively in each of these states. |