Caustics, in medicine, are remedies, the operation of which resembles that of fire; by destroy-ing the texture of the part to which they are applied, and converting it into a substance not unlike burnt flesh.

Those bodies which possess causticity, are, when taken internally, mortal poisons : so powerful is their action, in general (for instance, arsenic), that cautious physicians hesitate to prescribe it, even externally. There are, however, several others, which may be in a great measure divested of this deleterious quality, such as the nitrous acid, or aqua fortis; lunar caustic, or a solution of silver in nitrous acid; common caustic, or pure kali; either of which are daily and successfully used, especially for extirpat-ing fungous flesh, removing warts, etc - See Cautery.

The causticity of bodies depends, principally, on the state of the sa-line and acid matters which they contain. When the latter are concentrated and attached to the substances with which they are combined, they possess great activity, H h 3 a-4 and are corrosive or caustic. In this manner, both fixed and volatile alkalis, though already caustic, acquire that property in a far greater degree, by being mixed with quick-lime; as this substance deprives them of a portion of unctu-ous and inflammable matter, and divests them of all their fixed air, which binds and restrains their saline principle.

The late Dr. Black, when treating on this subject, observed that the compounds produced by the. union of metals with acids, are in general corrosive. Many of them, when applied to the skin, destroy it almost as soon as mineral acids; and some of the most powerful cauteries are made in this way. Others are supposed to be more acrid than the pure acids themselves, and produce apparently more powerful effects, when taken internally. Thus, a person may swallow ten or twelve drops of spirit of salt, without feeling any inconvenience; but the same quantity of acid, previously combined with silver, quicksilver, copper, or regulus of antimony, will throw the whole body into violent disorder, or even prove fatal, if taken in one dose.

Caustics are not at present in general use; they are, however, applied occasionally in abscesses, to produce an orifice, and to give vent to the suppurating matter; as likewise to make issues in parts where incision is difficult, or dangerous, on account of the contiguous blood-vessels.