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ELEMENTS OF MATERIA MEDICA.

Uses. —It has been employed both externally and internally in chronicdiseases of the skin (as herpes). When used externally, it is mixed withlard in the proportion of one or two drachms to an ounce of the latter.Internally the dose is ten or twelve grains to a drachm.

2. Car'bo Lig'ni .— Wood Char'coal.

History. —Wood charcoal must have been familiar to man from themost remote period of antiquity, and was probably known to the firstinhabitants of the globe. For an account of the ancient method of pro-curing it, I must refer the reader to the works of Theophrastus (cap. x.)and Pliny {Hist. Nat. lib. xvi., cap. vii).

Natural History. —Wood charcoal is always an artificial product.Some samples of Bovey coal have very much the appearance of woodcharcoal, but are readily distinguished by their containing hydrogen, inconsequence of which they bum with a yellow llame. Moreover,they are not good conductors of galvanic electricity (Kidd’s Outlines ofMineralogy, vol. ii. p. 47.)

Preparation. —Ordinary wood charcoal is prepared, on the largescale, for the purposes of fuel, by burning billet-wood piled in a conicalheap, covered by turf and sand, to prevent the access of atmosphericair, a few holes being left near the bottom and one at the top, to occa-sion a draught. The heap is then set fire to, and when the ilame has per-vaded the whole mass, the holes are closed. When cooled, the billetsare found converted into charcoal. For an account of the mode ofarranging the wood in heaps, consult Dumas, Traite de Chitnie, appliqueaux Arts, t. i., p. 561.

The wood charcoal used in the manufacture of gunpowder is obtainedby heating wood in cast iron cylinders, the gaseous products beingallowed to escape. The woods used for this purpose are Dogwood(Cornus sanguined), the common alder (Alms glutinosa), and the willow(Salix ).

Properties. —Wood charcoal is black, odourless, and insipid. It hasthe texture of the wood from which it has been obtained. It is brittle,and may be easily pulverized, especially when hot. Though a very badconductor of heat, it is an excellent conductor of electricity. • It isinsoluble, infusible, and incapable of volatilization. Its specific gravityvaries, according to the substance from which it has been obtained. Aremarkable property possessed by it is that of abstracting certain sub-stances (such as liydrosulphuric acid, organic colouring principles,various odorous matters, &c.) from liquids in which they are dissolved,or through which they are diffused. Another curious quality is that ofcondensing, within its pores, a certain quantity of any gas with whichit may be placed in contact. One volume of box-wood charcoal absorbsP75 volumes only of hydrogen gas, hut 90 volumes of ammoniacal gas.Some of the properties now mentioned (as that of decolorizing) arepossessed in a more eminent degree by animal charcoal.

Characteristics. —By burning in oxygen gas, charcoal yields car-bonic acid gas ; a property by which it is shown to consist of carbon.Its texture and appearance will distinguish it from other forms ofcarbon.

Physiological Effects.— Wood charcoal I believe to be an inert