in physic, an appellation given to medicines of so hot and fiery a nature, that, being applied, consume, and, as it were, burn the texture of the parts, like hot iron.

Caustics are generally divided into four sorts, the common stronger caustic, the common milder caustic, the antimonial caustic, and the lunar caustic.

The stronger caustic is prepared by boiling to a fourth part any quantity of the lees of almond-soap, adding lime that has been kept in a vessel pretty close stoppt for several months; the lime is to be added till all the liquor is absorbed, and the whole reduced to a paste, which is to be kept in a vessel well stoppt.

The common milder caustic is prepared by taking equal parts of soft soap and fresh quick-lime, and mixing them at the time of using.

The antimonial caustic is prepared thus: Take of antimony one pound, of corrosive sublimate two pounds; and being reduced separately into powder, mix them well, and distill them in a retort with a wide neck, in a gentle heat of sand; let what ascends into the neck of the retort be exposed to the air, that it may run into a liquor.

The method of preparing the lunar caustic is as follows: Dissolve pure silver by a sand-heat, in about twice its weight of aqua-fortis; then dry away the humidity with a gentle fire, afterwards melt it in a crucible, that it may be poured into proper moulds, carefully avoiding over-much heat, lest the matter should grow too thick.

Caustic curve, in the higher geometry, a curve formed by the concourse or coincidence of the rays of light reflected from some other curve.

Caustic glasses. See Burning-glasses.