That bodies conduct heat, and with different degrees of power, so that some are called good and others bad conductors, is well known. This property depends on the quantity of caloric, which a body receives, before it changes its state. Metals are considered good conductors, and glass, charcoal, feathers, &c. bad conductors. Hence bad conductors, as wool, &c. preserve the temperature of the body, or keep it warm in winter; and snow, for the same reason, prevents the action of intense cold on the ground. Liquids also conduct heat. Whether we consider caloric in this case carried, or transported, as it is more properly defined, the fact may be shown by several experiments. Ebullition, or boiling, is a phenomenon, which depends on the increment of temperature; for as water, for instance, receives caloric, until the thermometer indicates 212 degrees, the boiling point, mere evaporation ensues; but that temperature, under the usual pressure of the atmosphere, causes the formation of bubbles at the bottom of the vessel, as that part receives the degree of heat necessary for ebullition before any other; and these bubbles, as they form, rise in succession, and pass off in the state of steam, while the circumjacent fluid takes its place, and the process continues till all is boiled away. Water, when it passes off in the state of steam, which requires a degree of heat equal to 212 degrees of Fahrenheit, receives also 1000 degrees of non-distributable caloric, or latent heat; and however singular the fact may appear, the wise Author of Nature, it seems, has reserved a store of caloric, in this form, ready to be put in requisition, when necessity demands it, in a distributable shape.
Caloric, when in a state of rest, exists in different proportions, and although the actual temperature may be the same, yet the quantity of caloric in a quiescent state may be variable. There are several experiments, which are adduced to illustrate this fact. It results from experiment, that bodies receive heat according to their several capacities for it; hence, when any number of bodies are differently heated, the caloric, which becomes latent, does not distribute itself in equal quantities, but in various proportions, according, as we remarked, to their several capacities. Caloric, therefore, in a state of rest, is in relative quantities; and as the capacity of bodies for heat is variable, and relative as to each other, the term specific caloric has been applied. From these conclusions, we may readily perceive what is implied by an equality of temperature. That it merely depends on the state of rest, which caloric necessarily comes to, and which is relative as respects the capacity of bodies, and nothing more, is a deduction very plain and obvious. Heat, in a state of motion, may be said to be progressing to a quiescent state; and equalization of temperature, although differently understood, may be considered an equalization of fixed caloric, according to the relative capacity of bodies, without regarding the equalization, which takes place of uncombined caloric, as is manifested by thermometrical instruments. In a word, by considering caloric in this view, that of tending to a state of rest, and uniting with bodies according to their respective capacities, we may account for many phenomena; as, for instance, the quantity of caloric which enters into ice, and becomes latent, during liquefaction. The quantity of caloric, in this respect, may be learnt by adding a pound of ice at 32 degrees to a pound of water at 172 degrees. The temperature will be much below 102 degrees, the arithmetical mean, viz. 32 degrees. It is evident that the excess of caloric has disappeared; and by deducting 32 degrees from 172 degrees, 140 degrees remain, which is the quantity of caloric that enters into a pound of ice during liquefaction, or the quantity required to raise a pound of water from 32 degrees to 172 degrees. This change of capacity appears to be absolutely essential to the well being of the universe, as affording a constant modification of the action of heat and cold, the effects of which would otherwise be inordinate. If this did not take place, the whole of a mass of water, which was exposed to a temperature above the boiling point, would be instantly dissipated in vapour with explosion. The polar ice, would all instantly dissolve, whenever the temperature of the circumambient air was above 32 degrees, if it were not that each particle absorbs a quantity of caloric in its solution, and thereby generates a degree of cold which arrests and regulates the progress of the thaw; and the converse of this takes place in congelation, which is in its turn moderated by the heat developed in consequence of the diminution of capacity, which takes place in the water during its transition to a solid state. The reason why boiling water in the open air never reaches a higher temperature than 212 degrees is evident, if we consider, that the capacity of those portions of liquid, which are successively resolved into a vapour, becomes thereby sufficiently augmented to enable them to absorb the superabundant caloric as fast as it is communicated.
The most obvious effect of caloric on bodies, is the change, which they undergo when exposed to its action.
That it acts constantly in opposition to the attraction of cohesion or of aggregation, by which bodies pass from a solid to a fluid, and from a fluid to an aeriform state, and produces also different changes in bodies—are facts that come under our daily observation.
It occasions changes in the bulk of bodies; hence solids, liquids, and gases are expanded. The expansion, and subsequent contraction of atmospheric air, give rise to various winds, which are currents of air rushing from one point of the compass to another to maintain an equilibrium. The theory of the winds is predicated on this fact, although some have asserted, that they depend greatly on the diurnal motion of the earth. The air thermometer of Sanctorius, and the differential thermometer of Leslie, are founded on this principle, of the expansion of air. Fluids expand until they arrive at the boiling point, as is the case with water, alcohol, &c. The expansion of mercury, in a glass tube, furnished with a graduated scale, forms the mercurial thermometer, by the rise and fall of which, the different variations of temperature are marked.
Notwithstanding caloric has the property of expanding bodies, there are some exceptions to this law, which may be proper to notice. Water, for instance, at the temperature below 40° contracts at every increment of temperature until it reaches 40°, which is its maximum of density. Above 40° it expands, until it arrives at the boiling point. Alumina, or pure argillaceous earth, also contracts by heat; hence it is used in the pyrometer of Wedgwood, to measure by its contraction intense degrees of heat. Various saline substances, in the act of crystallization, also expand. Several of the metals, when previously melted, on cooling exhibit the same character; and water, in the act of freezing, exerts a powerful force by its expansion, competent to the bursting of shells, and the splitting of rocks.
The changes in bodies, produced by caloric, we have already noticed. We will only add, that fluids require different temperatures, called the boiling point, to make them boil, under the same atmospheric pressure. Water boils at 212°. Many observations have been made with respect to water, both in the state of ice, and the state of vapour. Besides the accession of 212 degrees of caloric, appreciable by the thermometer, in water in the state of steam, there is also an accession of non-distributable caloric, called latent heat, which is calculated at 1000°. In consequence of this circumstance, steam has been judiciously applied to various useful purposes, and particularly in a certain manner for the drying of gunpowder.
That chemical changes are produced by the agency of caloric, is a fact well known. It is supposed to occasion decompositions, according to the laws of affinity, by changing previous affinities, and causing new affinities to take place. Hence the operations by fire, whether the substances themselves are exposed in a dry state to the action of heat, or otherwise, produce new results, or compounds, which could not be made without it. This truth has long been obvious. In consequence of a knowledge of this fact, Dr. Black (Lectures vol. i, p. 12,) defined "chemistry to be the study of the effects of heat and mixture, with the view of discovering their general and subordinate laws, and of improving the useful arts."
Caloric as a powerful auxiliary, performing as it does an innumerable multitude of changes and effects, an agent by which the operations of the universe are maintained in order and harmony