Cylindrical cases of the required size, fitted with lids, are filled with wood. These cases are made to fit easily, and slide horizontally into iron retorts built in the wall, which admit of the accurate regulation of heat (communicated to them by furnaces underneath) throughout the operation of charring. A great saving of time and heat is effected by their use, as when the wood has been properly charred the case or slip containing it may be easily withdrawn, and another containing a fresh charge at once introduced into the retort, without allowing the latter to cool down, as would otherwise be necessary. When it has been sufficiently charred (which is known by experience, in watching the burning of the gas that is produced and is conducted into the fire), the slip is withdrawn by tackling, and at once lowered down into iron coolers or cases, which are immediately covered up with close-fitting lids, and then allowed to remain until all fire is extinguished. The goodness of charcoal is an essential point in the manufacture of gunpowder. About twenty-five to thirty per cent. is obtained; and one cord will produce about four cwt. of charcoal. If properly charred, it should have a jet black appearance, and when powdered a lustre resembling velvet; it should be light and sonorous when gently dropped, and its fracture should exhibit the same appearance throughout; it should be so soft as not to scratch polished copper, and ought not to exhibit any alkali when treated with pure distilled water. Charcoal is very porous, and absorbs very greedily gases and moisture from the atmosphere; no large store therefore is ever kept, and particular care is taken to prepare it only in proportion as it is required for use. At all times it must be kept exceedingly dry; when whole it will absorb about eight per cent. its weight of moisture, and when in powder 15 per cent., so that the fresher the charcoal is the better for the powder. SULPHUR. ~Sulphur.~ ~How purified.~ ~Flowers of sulphur unsuitable.~