The cause of the sensation of heat is denominated Caloric. Philosophers are not completely agreed whether it is a property only of bodies, such as a vibration of their particles1, or a peculiar substance; the latter opinion is the one more generally adopted.

Under this opinion, caloric is regarded as a very subtile, elastic fluid, which penetrates more or less all bodies, passes readily from one to another, yet cannot be wholly separated from any one: and is every where diffused. Its particles are supposed mutually to repel each other; and bodies into which it enters in any sensible quantity are increased in bulk, and undergo other changes of form, while their density is diminished. It is radiated in the same manner as light, and in this state forms a part of the solar ray.2 The rays are refrangible, and capable of reflexion, and of polarization, like those of light. It has no ascertainable gravity; and neither the addition nor the abstraction of it alters, sensibly, the weight of bodies.3 It exists in two different states : in a free state, and, in a latent state, or one of intimate union.

Regarding it as matter, the sources whence it may be obtained, the laws which regulate its motion and distribution, and its effects, require to be noticed.

Sources Of Caloric

The known sources of caloric are, the sun, combustion, mechanical action, chymical action, electricity, and the principle of vitality in animals.

a. The sun is an apparent source of caloric; but the direct action of the sun's rays upon bodies seldom produces a temperature exceeding 160°. When these, however, are concentrated by means of a concave mirror, or a lens; or when means are taken to prevent the communicated heat from being carried off by the surrounding bodies, a much higher.

1The idea of caloric being motion or vibration originated with Lord Bacon. 2Philsoph. Trans. 1807, 3 Ibid. 1799, p. 179.l temperature can be produced. This source of caloric is not resorted to for pharmaceutical purposes.

b. Combustion is a source of caloric highly interesting on account of its utility.

When a combustible is heated to a certain degree, it becomes still hotter of itself, and is consumed, emitting rapidly light and caloric, until the whole substance has suffered a change of properties.

The nature of this process was first attempted to be explained by Lavoisier, who laid it down as a chymical axiom, that "in every case of combustion oxygen combines with the combustible body." His explanation of combustion depends on two laws: 1st, That when a combustible body is heated to a certain temperature, it immediately begins to attract and combine with the oxygen of the atmospheric air. 2dly, This oxygen being in a state of gas, and combined with light and caloric, is decomposed during its union with the combustible, and its caloric and light are set free in a sensible form, while the oxygen itself combines with the combustible and forms a new compound. The truth of this theory was generally supposed to be proved by the following facts:-1. that combustion does not go on unless oxygen be present; and it is more brilliant in oxygen gas than in common air; 2. that the products of combustion are always heavier than the body con -sumed; and, 3. this increase of weight is exactly equal to the quantity of oxygen which the air loses.

One objection to this theory, namely, that every combination of oxygen with bodies does not produce the phenomena of combustion, was endeavoured to be explained by Brugnatelli, who supposed that oxygen combines with bodies in two states:-"1. Retaining the greater part of the caloric and light with which it is combined when in the state of gas; and, . After having let go all the caloric and light with which it was combined."

The above theory of combustion is, however, liable to objections : 1. It is a well known fact that heat is evolved in chymical action, when the products contain more insensible caloric than the substances that combine to form them: 2. The emission of caloric and light is not proportional to the quantity of oxygen that combines with the combustible, and the quantity of light that appears, depends, altogether, upon the combustible. Besides, the phenomena of combustion display themselves when no oxygen is present: thus phosphorus, and some metals in a state of minute division, undergo combustion in chlorine gas. Potassium also burns, emitting heat and light, in cyanogen gas. Berzelius regards the heat of combination as an electrical phenomenon, arising from opposite electrical substances neutralizing one another.

The caloric set free by the burning or combustion of coal, coke, coal gas, charcoal, oil, oil gas1, wax, tallow, and alcohol, is applied to the purposes of life, and is of the first importance in the practice of pharmacy: thence, endeavours have been made to ascertain the quantity of caloric evolved during the burning of different combustibles, and several experiments have been instituted, by the most able chymists, at different times, for this purpose. The following table exhibits the quantity of caloric evolved by the combustion of different substances, when all the circumstances are equal, the estimate being formed from the quantity of ice melted during the burning of one pound of each of the substances.2

1 Dr. Dalton states that the combustible gases give out caloric in proportion to the oxygen which they consume.

2 Thomson's Chymistry, 4th edit, ii.610.

The combustion of hydrogen in oxygen gas forms the most intense heat that can be produced.

c. Mechanical action consists of percussion and friction.

1. Percussion, as far as it applies to solid bodies, is another source of caloric. Smiths, for instance, are in the habit of kindling their fires by means of an iron rod smartly and quickly hammered until it become red-hot. This effect appears to arise from condensation, or forcing the integrant particles of the bodies closer together, so as to dislodge the latent caloric they contain, and give it out in the form of sensible caloric. The specific gravity of iron is increased .052 by being hammered; and it becomes so hard and brittle that it cannot again be heated by percussion until it has been exposed for some time to a red heat in the forge.

2. Friction is also a source of caloric. It is a well-known fact, that a considerable quantity of free caloric is disengaged when two substances are smartly rubbed together; but the real source of the caloric thus evolved still remains undetermined.

d. Electricity. The passage of this fluid through the air in discharging Leyden phials and batteries produces combustion.

e. Chymical action. The chymical union of two substances, in many cases, evolves caloric. This always takes place when the density or specific gravity of the mixture is greater than the mean of, the substances mixed; as in the mixture of alcohol and water, or of sulphuric acid and water: and much caloric is also evolved when water is thrown upon quicklime, owing to the solidification of the water when it unites with the lime. The caloric which is evolved in these and other instances of mixture is the latent caloric, which is the cause of the fluidity of the components; for, as the compound is less fluid, and consequently requires the presence of a smaller quantity of combined caloric, the superabundance which the more fluid components contained must be necessarily set free.

f. The vital principle in animal bodies. This is an obvious source of caloric; but it is never employed for pharmaceutical purposes.

Such are the sources from which caloric is obtained. Combustion is the most important of these; and the knowledge of the laws by which it is regulated, and of the modes of conducting it, is of the first consequence in the practice of pharmacy. (See Furnaces.)