Steel, Lead, Next to metals, stones seem to be the best conductors ; but this property varies considerably in different stones. Bricks are much worse conductors than most stones.
Glass seems not to differ much from stones in its conducting power : like them, it is a bad conductor. This is the reason that it is so apt to crack on being suddenly heated or cooled. One part of it, receiving or parting with its caloric be fore the rest, expands or contracts, and destroys the cohesion. Next to these some dried woods.
Charcoal is also a had conductor ; ac cording to the experiments of Morveau, its conducting power is to that of fine sand :: 2 3. Feathers, silk, wool, and hair, are still worse conductors than any of the substances yet mentioned. This is the reason that they answer well for arti cles of clothing. They do not allow the heat of the body to be carried off by the cold external air. Count Rumford has made a very ingenious set of experiments on the conducting power of these substan ces. He ascertained that their conduct ing power is inversely as the fineness of their texture.
Having in the preceding sections con sidered the nature of caloric, the manner which it moves through other bodies and distributes itself among them, let us now examine, in the next place, the effects which it produces on other bodies, either by entering into them or separating from them. The effects which caloric produces on bodies may be arranged under three heads ; namely, changes in bulk ; changes in state ; and changes in combination.
It may be laid down as a general rule, to which there is no known exception, that every addition or abstraction of calo. ric makes a cortesponding change in the bulk of the body which has been subject ed to this alteration in the quantity of its heat.
In general the addition of heat increases the bulk of a body, and the abstraction of it diminishes its bulk ; but this is not uni formly the case, though the exceptions are not numerous.
Indeed, these exceptions are not only confined to a very small number of bodies, but even in them they do not hold, except at certain particular temperatures ; while at all other temperatures these bodies are increased in bulk when heated, and diminished in bulk by being cooled. We may therefore consider expansion as one of the most general effects of heat. It is certainly one of the most important, as it has furnished us with the means of mea suring all the others. See PyltOuET11n.
Though all bodies are expanded by heat, and contracted by cold, and this expansion in the same body is always proportional to some function of the quantity of caloric added or abstracted, yet the absolute expansion or contraction has been found to differ exceedingly in different bodies. In general, the expan sion of gaseous bodies is greatest of all ; that of liquids is much smaller ; and that of solids the smallest of all. Thus, 100 cubic inches of atmospheric air, by being heated from the temperature of 320 to that of 212°, are increased to 137.5 cubic inches : while the same augmentation of temperature only makes 100 cubic inches of water assume the bulk of 104.5 cubic inches : and 100 cubic inches of iron, when heated from 32° to 212°, assume a bulk scarcely exceeding 100.1 cubic inches. From this example, we see that the expansion of air is more than eight times greater than that of water ; and the expansion of water about 45 times greater than that of iron. See lJXreN•