There is, possibly, not a single operation in chemistry that is not dependent on affinity; and although the wisdom and power of God may afford a more sublime and grander display in the harmony and arrangement of the heavenly bodies, yet they are not less visible nor less wonderful in the more minute parts of the creation.

CALORIC, HEAT, FIRE.

Definition and properties of Fire-Conductors of Caloric-Expansive power of Caloric in different substances-Caloric divided into two kinds, Free and Latent-Sun the chief cause or source of Caloric-Affinity of different Colours for Caloric-Condensation of Solar Heat by Mirrors and LensesCaloric produced by Combustion, Percussion, Friction, Chemical Mixtures, Electricity, &c.

Or all the blessings mankind enjoy, those derived from the agency of fire may be truly considered to hold the first place. Were it not for fire man would not have arrived at a state of civilization; manufactures could not have existed, nor could the arts have been cultivated; and instead of enjoying as we do the various comforts, luxuries, and elegances of life, we should have been, in our habits and conduct, scarcely removed above the brute creation.

Fire used to be considered a real substance, but that doctrine is now exploded: the more philosophical and correct opinion is, that it is the result of a high degree of excitement of the atoms of the heated body. Heat is properly the sensation produced by the agency of caloric; the term caloric being now used to express the cause of heat, whether caloric be a distinct substance, or whether it be a peculiar motion of the particles of bodies.

Fire is found to penetrate all bodies, even the hardest, which forms a peculiar feature of its character. It is also particularly disposed to form an equilibrium, so that a heated body will continue to give out streams of caloric until it has acquired the temperature of the atmosphere or of any body with which it may be in contact. Thus, if we touch a body hotter than our hand, a portion of caloric will pass from the body to our hand, and this will produce the sensation of heat; but if we touch a body colder than our hand, the contrary will take place; the caloric will be abstracted from our hand, and we shall experience

the sensation of cold. Some bodies are found to transmit ealoric more easily than others; they are thence called conductors of caloric: some do not transmit it at all, or in a very trifling degree, and these are called non-conductors of caloric. The best conductors are the metals; the worst conductors are fur, and caoutchouc. Air is also a bad conductor, and so is water.

One of the principal effects of caloric is expansion, and this is evident in all kinds of bodies, whether solid or fluid, but varying in degree. Thus, as it regards solid bodies, lead is more expansible than iron, and iron than platina. In order to show the expansion of metals, let a round piece of iron which has been made to fit a ring exactly when cold, be heated, and it will be found so increased in bulk as to be too large to pass through the ring. The expansion of fluids is prettily shown by filling a Florence flask with cold water to about the middle of the neck, and suspending it over a lamp; as it grows hot it will be found to expand gradually, so as nearly to flow over the neck of the flask. The principle of the thermometer depends on the expansive power of caloric; in proportion as the temperature increases, the quicksilver or alcohol, whichever it may contain, expands in proportion. Water in becoming frozen forms somewhat an exception to the above, for when it is cooled to a temperature of about 40° it begins to expand, and continues to do so until it becomes solidified into ice.+ The wisdom of this contrivance is strikingly evident; for if water followed the general law, and when frozen became of less bulk, and consequently of a greater relative weight, it would sink to the bottom as it was formed on the surface, and in the course of time the ocean, to a considerable extent, in the higher latitudes, as well as the ponds and rivers, would become a solid mass. The expansive power of water when converted into vapour or steam, is tremendously great. It appears

* In the higher latitudes thermometers containing coloured alcohol are used, as in intense cold the quicksilver may become frozen. + When water is frozen, the crystals form at an angle of 60o. Steam at 590° is said to have the force of one hundred atmospheres, or in other words, to press at the rate of 1500 lbs. on a square inch.

that solidity is the natural state of bodies, that when solids acquire a certain degree of caloric or atomic* motion, they become fluids or gases, and that fluids, by an increased degree of atomic motion, become vapour.

Caloric is popularly divided into two kinds, free caloric, and latent or combined caloric. By free caloric is meant caloric in a separate state, or, if attached to another substance, not chemically combined with it. Latent caloric is that which is chemically combined with any substance so as to make a part of it.

The chief source or cause of caloric is the sun; but the rays of the sun seem only to produce heat when they meet with an opaque substance, not when they pass through a transparent one, as glass, water, &c.

Colours have a striking difference in their affinity for caloric; it has been found by experiment that black has the greatest degree of affinity, then blue, next brown, white having the least: the black absorbs the rays of light, while the white reflects them. Although black has this particular affinity for heat, it radiates it, or distributes it to surrounding bodies sooner than any other colour; thus, a blackened vessel will become heated if placed over a fire sooner than a bright one; but when removed from the fire it will not retain its heat so long. If the surface of a metallic vessel be scratched or roughed, the radiation will be increased, and this is attributed to the exciting atoms which cause the heat having more points for distributing it to surrounding bodies.

Various experiments have been made in order to show the powerful effects of the sun's rays when condensed or multiplied. Archimedes appears to have been the first who attempted this with any success; he so multiplied the sun's heat by means of mirrors as to set fire to the Roman fleet at Syracuse. As one plane mirror, for instance, reflects a certain portion of the sun's heat, so two, or three,

*It is supposed that the cause which produces the sensation of heat is like gravity, a property of matter; and that it consists in the peculiar motion or vibration of the particles or atoms of bodies. According as this motion is more or less rapid, a higher or lower temperature is produced; and bodies are, in proportion to the intensity of the atomic motion, either in a solid, fluid, or gaseous state.

or any other number, will proportionately increase it if directed to the same object. Leonard Digges, who lived in the reign of Elizabeth, asserts, that by a combination of mirrors he fired bodies half a mile distant. Buffon, by a combination of forty plain glass mirrors, of six inches by eight, set on fire a tarred beech plank sixty-six feet distant; and with a combination of one hundred and sixtyeight, he performed the same at the distance of two hundred feet. He is said to have melted all the metals at thirty or forty feet. Concave mirrors concentrate or condense the sun's heat by reflection into a focus, and the effect is increased in proportion as the surface of the mirror exceeds that of the focus. A concave mirror of four feet in diameter, made of a mixture of copper and tin, melted a sixpence in less than eight seconds, and a half-penny in twenty seconds. Convex lenses, also, by concentrating the sun's heat into a focus, increase the heat in a similar proportion to concave mirrors. One of the largest and most powerful lenses of this nature was made some years since by a Mr. Parker, of Fleet-street; it was formed of flint glass, and when fixed in its frame exposed a clear surface of two feet eight inches and a half in diameter; its weight was 212 lbs; its focal length six feet eight inches, and the diameter of the focus one inch: a second lenswas used to receive the focus of the first, which reduced it to half an inch. By these lenses the sun's rays were condensed in the ratio of four thousand two hundred and twenty-five to one, and the effects produced were astonishing. Iron, when exposed to the focus, melted in a moment; slates, tiles, &c., became red hot, and vitrified instantly; and wood, acted on under water, became burnt to a coal. Mr. Parker had the curiosity to try what the sensation of burning at the focus was, and having passed his finger through it, he described it to be like that of a cut with a lancet.

Caloric is also produced by combustion, through the Oxygen of the atmosphere becoming decomposed, and evolving or setting it at liberty. It may be observed, that oxygen owes its gaseous state to the caloric which it contains, and in the process of combustion the oxygen combines with the base of the body consumed and forms an

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oxide, while the caloric escapes. The strongest heat produced by combustion is that by means of the combination of the oxygen and hydrogen gases, a small stream of which ignited, and made to fall on watch-spring or even platina wire, will make it to melt like wax.

Percussion and friction are well-known mechanical means of producing caloric. A blacksmith will, by hammering a piece of iron, very soon make it sufficiently hot to light a match. The heat produced is attributed to the compression of the particles of the body forcing out a portion of its latent caloric. In strong percussions the first blow produces the greatest degree of heat, which gradually decreases until it is exhausted; it will not again produce heat until it has been exposed to the fire, or has had some means of imbibing free caloric; when this is effected, the caloric will continue in a latent state until it is again acted upon. In the collision of the flint and steel, so much caloric is disengaged that the metallic particles struck off are actually melted; this is evident from the pear-like form that such particles assume. Friction is considered a continued series of percussions, and the heat sometimes arising therefrom is very great. Forests have been known to be fired by the friction of the branches of the trees; and coaches have been burnt by the friction of the wheels. Most savage nations produce fire by the friction of two pieces of wood. Count Romford, by boring cannon under water, so heated it by the friction that he made it boil, and actually boiled in it a piece of beef.

Caloric is also sometimes produced by chemical mixture, although not always; for in some instances chemical mixture produces cold. It may, however, be taken as a general rule, that when substances become more condensed by mixture heat is evolved; and when they become more expanded cold is produced. If one ounce of sulphuric acid be mixed with four ounces of cold water, a degree of heat greater than that of boiling water will be produced; but if the mixture be measured it will be found, for the above reason, not to amount to as much as five ounces. Bodies in changing from a liquid to a solid form always

* See article Gaseous Bodies.