- Potassa Caustica cum Calce. Dub. - Weaker Common Caustic. - Vienna Caustic.

Though formerly recognized by all the British Colleges, this has been omitted in the existing British Pharmacopoeia. it is nothing more than hydrate of potassa, rendered milder by incorporation with an equal weight of lime. The U. S. Pharmacopoeia merely orders the two ingredients to be rubbed together. it is in the form of a powder, which may be kept for use in bottles. in Vienna, where it was introduced into use, it is prepared with six parts of quicklime and five of caustic potassa. {Diet, de Med., vii. 37.) By the use of this preparation, greater precision is said to be given to the outline of the eschar; as the alkali is not so much disposed to diffuse itself. When used, it is made into a paste with a little alcohol, and applied to a surface surrounded with adhesive plaster, as in the case of the stronger caustic. in five or six minutes the whole depth of the skin is usually cauterized; and, if it be allowed to remain from ten to twenty minutes, the effect extends to the underlying cellular tissue. On removing the caustic, the surface should be washed with vinegar to neutralize any remaining alkali; and the same caution should be observed when the stronger caustic is used.

M. Piedagnel states that this caustic may be rendered painless by mixing three parts of it, in the state of powder, intimately with one part of muriate of morphia, and then, by the addition of chloroform, alcohol, or water, forming a paste of such a consistence as to admit of being spread upon a piece of lead plaster. in five minutes, the skin underneath the application becomes of a dead white colour; in five minutes more, is surrounded by a whitish edematous border; and at the end of fifteen minutes, is brown and carbonized. The depth of the eschar increases with the continuance of the application, and becomes at length nearly equal to that of the layer of paste employed. {Journ. de Pharm. et de Chim., 3e sér., xxxiii. 470.)