Chemical combination, such as occurs by the mixture of fluids, as alcohol and water, sulphuric acid and water, some of the gases, as muriatic acid gas with water, &c. evolves heat, and sometimes sufficient to boil water. In cases of spontaneous combustion, it would seem, that quiescent heat passes to the state of distributable heat; for if nitric acid, for instance, contains so large a quantity of quiescent heat, or fixed heat, as the experiments of Mr. Lavoisier make it appear, we may readily explain why spontaneous combustion ensues, when that acid is brought in contact with spirits of turpentine; because the chemical action of the acid on the carbon and hydrogen of the turpentine, which takes place, produces at the same time a corresponding change in the caloric itself, from a quiescent to a distributable state. If the same data be admissible with regard to the combination of the nitric acid with potassa, which we may judge to be the case by the experiments of Lavoisier and Laplace, (quoted in our article on Gunpowder), then, indeed, its mechanical union with charcoal, and sulphur, although in a common temperature no combustion ensues, will, at the temperature required to inflame the mixture, (about 700 degrees according to some), produce a decomposition altogether chemical; and while new products are formed, the caloric, necessary also for their generation, passes from a quiescent to a distributable state; and a portion of it goes into a new state of combination, also quiescent. We mean that portion which is necessary for the constitution of gaseous fluids. This fact is remarkable. By referring to the original state or condition of the caloric, if we admit that state in the present instance, (which appears the only mode of accounting for the emanation of free caloric by the combustion of gunpowder), it is easily perceived, that chemical changes, besides the usual supporters of combustion being concerned, as in ordinary cases of combustion, must produce a similar change in the state of combined or quiescent heat.
Predicating this opinion on the results of the experiments of MM. Lavoisier and Laplace, and seeing that gunpowder inflames per se, or without the aid of a gaseous supporter, we have no hesitation in risking it, in the present state of our knowledge concerning heat as our present belief and conviction. Although there is no satisfactory theory offered to explain all the instances of spontaneous combustion, yet it seems reasonable to conclude, that in many cases at least, that effect may take place by some chemical action, which, like the instances already quoted, may change quiescent into distributable heat. We have stated (See [Gunpowder]) some instances of spontaneous combustion, which have taken place merely in consequence of the charcoal. Some have attributed the effect to pyrophorus, and others to the presence of hydrogen in the coal, which, by absorbing and combining with oxygen and forming water, sets the caloric of the oxygen gas at liberty, and thus produces combustion. However this may be, there are other instances, that of cotton and oil, some kinds of wood, wood-ashes and oils, &c. which have produced spontaneous combustion.
We will only add, however, that until we can give a better theory, the effect in these instances may be attributed to chemical action, and with it, the change of caloric in the manner already mentioned. Chemical action in such cases appears necessary, although mechanical means, as percussion will produce heat.
Quiescent heat is also put in motion by electricity; but in what manner it acts, so as to produce that effect, is unknown. It is a powerful agent in nature, and calculated for important ends, of which we are ignorant. It is unnecessary to notice opinions concerning it. All electrics will yield it, such as glass, rosin, &c. and it may be collected in the usual manner by the prime conductor and Leyden jar. Galvanism, called also Galvanic electricity, produced by an arrangement of zinc, and copper plates in a pile, or trough, and placed in contact with some oxygenizing fluid, has the same effect of causing quiescent heat to become distributable, and is undoubtedly the result of chemical action. The peculiar character of this fluid, the nature of the two opposite poles, &c. have been, and continue to be, a subject of interest to the philosopher. The deflagrator of professor Hare of Philadelphia is an apparatus well calculated for many interesting experiments on galvanism. To that gentleman, we are also indebted for the compound blowpipe, which produces a very intense heat by the combustion of hydrogen in contact with oxygen gas. Notwithstanding professor Clark of England has laid claim to the apparatus, and the use of hydrogen gas in this way, the merit of the discovery is due to our learned and ingenious countryman.
Since heat is put in motion as a consequence of the increased capacity of a body, and, by combining with a substance whose capacity has been increased, becomes by degrees quiescent, according to the respective capacities of bodies; cold is an effect, which is occasioned by this change from a free to a combined or quiescent state. The absorption of heat, necessary for the generation of cold, if so we may consider it, takes place in every instance, where that effect is observed. The heat of surrounding bodies, in a distributable state, is now no longer characterised as such; and the consequence is, therefore, that that particular sensation, or effect follows.
Cold may be produced by saline mixtures, the salts for which having their full quantum of the water of crystallization; and by the evaporation of fluids, as water, alcohol and ether. In the one case, that of the freezing mixtures, we have seen, that the effect is produced by the absorption of heat; and with regard to the cold produced by fluids, even in vacuo, (where the effect is more instantaneous), the cause is attributable to evaporation; for the fluid changes from a liquid to an aeriform state, and during this transition robs the body, with which it was in contact, of a part of its caloric, and thereby reduces its temperature. Artificial ice is made on this principle.
The next subject with regard to heat, is the different modes in which it tends to a state of rest. There are some facts in relation to this subject worthy of notice; and particularly, that, in the tendency of caloric to become quiescent, after having been put in motion, bodies often increase in temperature. This tendency to a state of rest is effected either by the conducting power of bodies, or radiation. Heat radiates in all directions, and in quantities, according to the experiments of Leslie, more or less variable, which depend on the nature of the radiating surface. Hence that power, which bodies possess, called the radiating power, varies in different substances. Thus, the radiating power of lampblack is 100, while gold, silver, copper, and tin plate are 12, from which it appears that the metals distribute less heat by radiation. That caloric obeys the same laws as light, is obvious from Pictet's experiments with concave mirrors, where the calorific rays move in the same order, the angle of incidence being equal to the angle of reflection. It is also refracted; hence the concentration of the solar rays in a focus by the burning glass. Various experiments have been made with mirrors, and concave reflectors. The effect of the former in destroying the fleet before Syracuse, an experiment made by Archimedes, is a fact well authenticated in history. Concave reflectors have inflamed gunpowder. This subject, however, is noticed at large, when speaking of mirrors as an incendiary in war.
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.