Of _chemical caustics_ a host exist; those most commonly employed are:—nitrate of silver, solid, or in saturated solutions (2 drachms to the oz. of water, &c.); fuming nitric acid; solution of nitrate of mercury in nitric acid; oil of vitriol made into a paste with powdered charcoal; chloride of zinc mixed with dry starch, then rolled into cakes and cut in slices; Vienna paste, that is, equal parts of potassa fusa and quick lime worked into a paste with spirits of wine; potassa fusa itself; solution of chromic acid. Some surgeons prefer one, some another; as a rule, the liquid caustics are employed where the surface to be destroyed is uneven and spongy, and solid caustics where the surface is smooth, and a long continued action is desired. _Vesicants and irritants._—Of the commonest are _mustard poultices_, made by mixing mustard flour in a basin with luke-warm water, _i.e._ about 100° F., to a paste and spreading it on muslin, which is again folded over the exposed surface of the mustard. Boiling water and vinegar should not be used, for they lessen the pungency of the poultice. If the full effect be desired the poultice should remain on the skin fifteen or twenty minutes. If only slight reddening is wanted, the mustard flour should be diluted with its bulk of linseed meal before mixing it with water. A stronger vesicant is _Corrigan’s hammer_, a button of polished steel with a flat surface, fixed to a handle; when used it should be plunged for a couple of minutes in boiling water, or heated over a spirit lamp, but care must be taken not to overheat it, or it will bring the cuticle away with it. It is pressed on the skin for ten or fifteen seconds; this is sufficient to cause reddening and vesication. _Blisters_ are raised by the emplastrum lyttæ, lin. cantharidis, or _pâte epispastique_, which is milder in its effect than the two preceding preparations of Spanish fly. Solution of iodine and iodide of potash, in three times their bulk of spirit of wine, also produces a blister when laid on freely. _Poultices_ are made of linseed meal, bread, or starch, and are means for applying warmth and moisture without absolutely wetting. Bread poultices sodden the parts to which they are applied most, and starch least, of the three kinds. Before making a poultice all the materials should be at hand and thoroughly warmed before a good fire. They are—boiling water, a broad knife or spatula, soft old linen or muslin, oil silk, tapes, strapping plaster, bandages, a piece of old blanket, flannel or cotton wadding, safety pins, or needle and thread.