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Charcoal is fixed in the fire, no degree of being able to produce any change on it, so long as the air is excluded.
Obs. When newly made, it will absorb about 90 times its own bulk of ammoniacal gas, and certain, but less quantities, of any other gaseous substance.
148. It attracts from the air a quantity of water, which it strongly retains. The powder of fresh charcoal, is strongly disposed to unite to the odorous particles of bodies and the coloring matter of vegetables : hence it may be employed to correct the bad smell of corrupted water, of oiled silk bags, of ill conditioned ulcers, and of decayed teeth, when used as a tooth powder: it is employed to deprive vegetable infusions and other substances used in chemistry, of their colour, and for giving mellowness and maturity to newly distilled spirits.
Obs. Its principal use is as fuel, but it always ought to be remembered, ti at the gas which arises from burning charcoal, is the most insidious and deleterious that can be inhaled.
149. Charcoal when perfectly freed from saline and earthy matters, may be considered as pure carbon, differing from the diamond only in form and colour.
150. Charcoal has the quality of rendering putrid substances sweet, and preserving animal substances from putrefaction.
Ilius. 1. Let putrid water, as from a ditch, be passed through a few inches in depth, of pulverized charcoal. It will come through perfectly sweet, and fit to drink.
2. Finely powdered charcoal is of peculiar efficacy as a dentrifice, its particles being sufficiently hard to remove the concretions from the teeth, without hurting the enamel, while at the same time it destroys the fetor arising from a carious tooth.
3. If meat that is a little tainted be rubbed with fine charcoal, it becomes sweet. Fresh meat may be preserved for a considerable time, if surrounded with pulverized charcoal.
For all the above purposes, it is understood that the charcoal is to be fresh prepared, finely pulverized, and perfectly dry.
151. Neither air nor moisture affect charcoal at ordinary temperatures.
li/us. It is stated by Mr. Watson, that the beams of the theatre at He.culaneum, which were converted into charcoal above 1700 years ago, are still as entire, as if this event had happened only yesterday. It is also said, that there still exists charcoal made of corn in the days of Caesar, which is in so perfect a state, that the grains of wheat can be distinguished from those of rye.
152. Carbon unites to several substances, forming * class of compounds called carburets.
Illus. The substance used for pencils, and improperly callled black lead, is a carburet of iron, and contains about ten parts of carbon to one of iron. The name black lead was given it long before its real composition was known. Steel is a carburet of iron, or a compound of iron and diamond ; hence its hardness. The conversion of iron into steel depends on a quantity of carbon which it absorbs from the charcoal when it is heated. Case hardening is the superficial conversion of the surface "of iron into steel, by heating it in contact with animal carbon in close vessels. Bar iron is converted into steel, in the same way; only that powdered charcoal is the substauce in which it is imbedded.

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