Caustics (Gr. kaίειv, fut. kavσw, to burn), or Eseharotics, substances which destroy the life of the tissues upon which they act. They may be classified according to the depth and nature of their action. Nitric and hydrochloric acids, acid nitrate of mercury, nitrate of silver or lunar caustic, and sulphate of copper remove only a very superficial layer of tissue. Arse-nious acid and chloride of zinc cause deeper sloughs, which require several days for their separation. The former agent may under some circumstances be absorbed and give rise to general poisoning. Caustic potassa and strong chromic acid melt down the tissues and leave but little solid slough; hence the neighboring parts should be protected during their application. The first class find a somewhat extensive application in modifying the unhealthy or specific character of an ulcerating surface, and substituting for it a condition of normal granulation. The articles of the second class are sometimes used to remove tumors, and are combined with other ingredients in various cancer cures. Though cases sometimes occur in which, for special reasons, this treatment is allowable, yet in general the knife, if properly used, possesses many advantages over it, especially in the avoidance of pain.

The third class are sometimes used for similar purposes, to remove an excess of indurated tissue, or to open an abscess. Caustics are much less used than formerly in the formation of issues for purposes of counter-irritation, or for removing imaginary impurities from the system.