The products of combustion may be fixed or gaseous, and either alkalies, oxides, or acids; or, when chlorine is the supporter, chlorides, &c. A few examples will be sufficient. By the combustion of metals, iron for instance, we obtain a fixed product, and in the present case an oxide of iron; by the combustion of antimony and arsenic, the antimonic and arsenic acids; by the combustion of charcoal, we have carbonic acid gas, a gaseous product; by the combustion of potassium or sodium, we obtain a fixed alkali, depending however on the quantity of oxygen; by the combustion of sulphur, phosphorus, &c. acids; and when metals are burnt in chlorine gas, chlorides are produced.
It is evident from facts, that, whatever theory may be assumed, combustion occasions the production of free caloric, or changes the condition of caloric, from quiescent to distributable heat. The conclusions drawn by Mr. Davy and others, appear to have been predicated on the absorption of the base, and development of caloric, as in oxygen gas, and the peculiar alteration in bodies implying a decrease in their capacity; and hence, as regards the products of combustion, they must necessarily possess a less capacity for heat than the mean capacity of their constituents.
Whether we regard heat as latent, in the acceptation of the term, as applied or used by Dr. Black, or quiescent, or in a state of rest, it is certainly evident, that combustion is a chemical change, and by it caloric passes from a combined to an uncombined state, and is thus made sensible, free, or thermometrical heat. Combustion may, as it certainly does, put quiescent heat in a distributable state; but this quiescent heat is the same in the present case, of which there can be no doubt, as latent caloric. The thermometer will only indicate as much caloric in the air as is in a distributable, or free state; but, if the same air be employed to supply, or support combustion, the heat, rendered appreciable by the senses and the thermometer, will be in the ratio of the decomposition of the oxygen gas of the atmosphere, and, of course, to the development of free caloric.
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.