Syn. Common Caustic. Caustic Potassa.

Preparation and Properties. For use as an escharotic, potassa is prepared by evaporating the officinal Solution of Potassa (Liquor Potassae, U. S., Br.) until it ceases to boil, and then pouring it into moulds to concrete. it is in cylindrical sticks, somewhat thicker than a quill, and several inches long, usually of a grayish colour, sometimes of a bluish or brownish tint, and occasionally white, when prepared out of very pure materials. On exposure to the air, it attracts moisture and carbonic acid, and rapidly deliquesces; and must, therefore, be kept in well-stopped bottles. it is soluble, with the exception of impurities, in less than its own weight of water; and is also readily dissolved by alcohol. The impurities are usually lime and oxide of iron, which are left behind by water, and carbonate of potassa, which is not taken up by alcohol. These, however, in any amount in which they are ordinarily found, do not materially impair the efficiency of the caustic. Caustic potassa is a hydrate of the pure alkali, consisting of one equivalent of pure potassa, with one equivalent of water, of which it cannot be deprived by heat.

Effects. When caustic potassa is placed in contact with the skin, it gradually liquefies, and soon destroys the life of the part, the colour of which is changed to a dirty grayish hue. its action is accompanied first with a sensation of warmth, which rapidly increases to a severe burning pain, more durable than that produced by the actual cautery, and diminishing only when the life of the part is lost. inflammation is excited in the adjoining tissues, ulceration takes place with suppuration, and the dead part, which begins to be detached in five or six days, is at length separated, at a period of time varying from two to four weeks, according to the energy of the inflammatory process. The action of potassa is deeper than that of most other caustic substances, extending through the skin, and, sometimes, if in large quantity, to a considerable depth beneath it. The effect is produced by a union between the potassa and the substance of the tissues; and, as the compounds thus formed are more or less soluble, no barrier is presented to the further penetration of the caustic until it is wholly expended in combination. This depth of action is the characteristic advantage of potassa as a caustic.

Uses. Caustic potassa is preferred for the formation of issues, and may be used for opening abscesses, decomposing the poison in wounds, and removing diseased structure, in fact, whenever it is desirable to destroy any considerable portion of tissue by escharotic measures; but for these latter purposes, the knife is now generally employed. Dr. Physick used this caustic in the treatment of carbuncles, and Mr. Travers, of London, adopted the same practice. (Am. Jour, of Med. Sci., N. S., xxxi. 518.)

Application. For preparing issues, potassa is applied in different methods. One is to cover the skin with a piece of adhesive plaster, having in its centre a circular or oval opening of the size of the proposed issue. A small piece of the caustic, from one to two lines in diameter, or more than one piece, when a large issue is wanted, are placed upon the exposed skin, and covered with another piece of adhesive plaster, and the whole secured by a compress. On the removal of the dressings at the end of four or five hours, a black moist eschar is seen, surrounded with a border of inflammation. Another, and probably preferable method, is, after having applied the first adhesive plaster as above directed, to moisten the end of a stick of caustic, and rub it upon the uncovered space in the centre, until the whole surface becomes discoloured. Dressings of resin cerate may be applied until the slough separates, or emollient poultices if there is much inflammation. To hasten the separation, a crucial incision is sometimes made through the eschar. To keep open the issue, it is customary to insert rounded bodies, called issue peas, most of which, from their porosity, absorb the liquids, and thus swell considerably. For this purpose dried peas may be used, or spherical pieces of orris root of the same size, or those minute bodies made from small unripe oranges. The two latter variety of issue peas have the advantage of an agreeable odour, and slight irritant properties. it will be found convenient to pass a thread through the centre of each pea, by which it can be withdrawn when deemed proper. if it be desired to render these little bodies more irritant, they may be covered with some stimulating ointment, as that of mezereon. The issue may be dressed with resin plaster. I have seen used, as a substitute for the peas, a thick piece of sole-leather, cut upon one of its surfaces into little quadrangular pyramids, the points of which were made to press upon the ulcer by applying to its surface the cut side of the leather.

At the suggestion of Dr. Manoury, of Chartres, in France, M. E. Robiquet has incorporated caustic potassa with gutta percha, and thus prepared a malleable paste, capable, when moderately warmed, of being moulded into cylinders, plates, or any other desirable form. The potassa is first finely powdered, and then quickly mixed with an equal weight of gutta percha, previously melted at the lowest possible temperature. The quantities employed at once should not exceed about an ounce and a half. The disadvantages of the deliquescence of caustic potassa are thus in some measure obviated. {Journ. de Pharm. et de Chim., 3e sér., xxxi. 255.) it has been stated above that, for other purposes than the formation of issues, and, among them, for the opening of abscesses, the knife is almost always preferred. There is one condition, however, in which potassa is habitually resorted to by some surgeons. I refer to abscesses of the liver, which it may be important to open, but in relation to which there may be a fear that the peritoneal surfaces may not have contracted adhesions, and that an incision might be followed by an escape of pus into the abdominal cavity. By removing with caustic potassa, carefully applied, successive portions of the tissue intervening between the cuticle and the peritoneal membrane, it is supposed that inflammation may be excited in that membrane, sufficient to lead to the desired adhesion, after the formation of which the lancet may be used with safety.