From the above table it will be seen that different bodies require different quantities of heat to enable them to assume the vaporous state. An analogous fact is, that different bodies require very different quantities of heat to elevate their temperatures a given number of degrees. If a pound of water at 60o be mixed with a pound of oil at 90°, the resulting temperature will be 70° instead of the mean 750. And conversely, if a pound of water at 90° be mixed with a pound of oil at 60°, the temperature of the mixture will be 80o. In the first experiment we see that the oil lost 20°, while the water only acquired 10°; and in the second the oil gained 20°, while the water lost only 10°. Hence the specific heat of water is double that of oil; or the same quantity of heat that will raise the temperature of oil 20°, will only raise that of water 10°. The same fact may be shown- by placing mercury, oil, and water, in an oven; the mercury will be first heated, next the oil, and lastly, the water. An important practical illustration of the doctrine of specific heat is afforded by atmospheric air.

The specific heat of air diminishes more slowly than its specific gravity.

When air is expanded to a quadruple volume, its specific heat is 0.540; and when expanded to eight times the volume, its specific heat is 0.368. The densities 1, 1/2, 1/4, 1/8, correspond nearly to the specific heats 5, 4, 3, 2. Hence may be explained the intense cold that prevails at the tops of high mountains, and also the great heat developed in the compression of gases. A compression equal to four-fifths is sufficient to ignite tinder; and if a syringe of glass be used, a vivid flash of light is seen to accompany the compression.

We have now alluded to most of the phenomena of heat that may be useful in chemical investigations, except those which relate to the conduction and radiation, which we shall briefly illustrate.

It is well known that if a bar of iron, as a poker, be placed in the fire, the heat will in time be communicated to its remote end. It is also a matter of common observation, that a hot mass of iron, or a vessel of hot liquid suspended in a room, will gradually become cooler until it attains the temperature of the surrounding medium. If we suppose a mass of iron, heated red hot, to be placed on a metallic pillar or support, in a still room, we shall find that it will lose its heat in three distinct ways. 1. If the metallic support be felt, it will be found to be hot, and we may consequently infer that a portion of the heat has been conducted away by its means. 2. If the hand be held over the hot body, considerably above it, a current of hot air will be perceived, which must convey another portion of caloric from the hot body. 3. If the hand be held at some distance from the side of the body, a distinct sensation of heat will be experienced; and as this occurs when the hot body is inclosed in a vessel exhausted of its air, it is manifestly a different mode of cooling from the other two: in fact, a variety of experiments render it evident that the heat is projected from the hot body in right lines on every side.

In the first of these modes of cooling, the heat is conducted slowly along the iron bar, which is denominated a conductor; and the process is called the conduction of heat. In the second, the heat unites with the particles of air, and renders them specifically lighter, in consequence of which they ascend, and another stratum of cooler particles descend and occupy their place; these in their turn become expanded and rise, and thus a constant ascending current is maintained. Caloric, therefore, is conducted from bodies in two ways; it either imparts heat to the adjacent particles, which impart it to the next, and so on, without change of place, or it unites with the adjacent particles of the surrounding medium, and is conveyed upwards by the increased levity which it occasions. The third method of cooling in which the caloric is projected from the body in right lines, is called the radiation of caloric. The communication of heat by contact is manifest in solids and liquids, although in the latter it is chiefly propagated by the ascent of heated particles. If different solids be taken, and have one end exposed to a high temperature, one of them will become heated in a shorter time than another.

Thus, if a piece of copper or iron wire, 3 or 4 inches long, be held in the hand by one end, while a spirit lamp is applied to the other, it will soon become so hot as to be intolerable; while a glass tube, in similar circumstances, may be held within an inch of the flame with little inconvenience. The difference in the facility with which heat is transmitted through bodies, will appear from the following table: -

From this table it appears that the metals are the best conductors of heat, though even among them there are striking differences. The different kinds of wood have very little conducting power, and hence are well adapted for handles to vessels that are exposed to heat. Bodies of a porous or spongy nature, especially fibrous substances, as wool, silk, feathers, fir, etc. are the worst conductors of heat; and from this circumstance derive their value as articles of clothing. It is, however, probable that the warmth of these substances is attributable rather to the impediments they offer to the motion of the air than from any inherent heat-retaining power. Confined air is a bad conductor of beat; and if a quantity of it be enclosed among the interstices of the fur, wool, etc. it will furnish an effectual barrier to the egress of caloric. On this account double windows and doors are found effectual in maintaining an equable temperature in our apartments. The conducting power of liquids by contact is so exceedingly small, that for a long time it was doubted whether they conducted at all.