During the next ten or twelve hours, considerable at tention is necessary to prevent any mischief resulting from the sudden disengagement, and consequent com bustion of carburetted hydrogen, which take place dur ing that period of the manufacture. The explosion is announced by a rumbling noise, and seldom does any other injury than that of throwing of some portion of the covering, and through the opening thus formed flame and smoke issue. It is necessary to close up all such openings with a few spadcfuls of dry earth. When the smoke decreases, and the explosions have entirely ceas ed, the interstices between the sods at the lower part of the pile must be closed. At this stage little attention is required, the combustion gradually extends to the sur face, and in about thirty or thirty-four hours after the process commenced the whole pile becomes a glowing mass. The wood is thoroughly charred, the whole is covered with dry earth, and in four or five days it may be taken down. The particular stage at which it is pro per to do so, is determined by making a small opening into the pile : if no flame appears, it is fit to be taken down; if it bursts forth, the aperture must be again closed, and allowed to remain so for another day.
Great nicety is requisite in the preparation of char coal for the manufacture of gunpowder and other deli cate chemical processes. And the manufacturers se lect the stems of the willow, alder, dogwood, and some others, which they prepare with peculiar care. In most of the large manufactories, the charcoal is distilled from iron vessels; by which means it is obtained in a state of considerable purity, and the other products are saved.
As all charcoal contains minute poi tions of earthy and metallic substances, lamp black is commonly used in nice chemical experiments. Lamp black is obtained by the turpentine manufacturers, from the combustion of the refuse of their operations in furnaces appropriated to that purpose. The smoke deposits itself on the sack ing which is hung up ; it is swept off, and sold for com mon use without further preparation. The lamp black in this state contains some oil, which is separated by being heated to redness in a close vessel.
The chief consumption of charcoal is as fuel. It is also employed as a tooth powder, and to purify tainted meat. No mode of preparation for the first of these ob jects is at all necessary, and for the two last, it must merely be reduced to a fine powder.
It forms a part of all reducing fluxes. It is an indis pensible constituent of gunpowder. (See GUNPOWDER.) It is the basis of most black paints and varnishes. It is used to polish brass and copper, and is an excellent clarifier.
Powdered charcoal must be heated to redness in a covered crucible, with an opening in the middle of the cover, and kept in that state till no flame issues out ; it must be then withdrawn, allowed to cool, and then put into close vessels and kept for use. Whenever either wine, vinegar, or any other fluid is to be clarified, it is simply to be mixed with the liquor; a froth appears at the surface, and after infiltration, it is pure and colour less. See Philosophical Magazine, vol. iii. p. 1. No. 17. 1kc.; Annales de CIthyde, vols. xxxi. xxxii. xxxvi. xlii. ; Nicholson's Journal, 4to. vol. iv. (e. N.)