PLASTERS are a class of medicinal agents which are employed externally with var ious objects. They are solid and tenacious compounds, adhesive at ordinary tempera ture of the body, and owing their consisteney-1. To the chemical combination of oxide of lead, with one or more fatty acids; or 2, to a due admixture of wax, or fat, and resin; or 3, to the chemical action of the component parts of the plaster on each other. Strictly speaking, the term plaster should be restricted to the first class of compounds; viz., to combination of oxide of lead with fatty acids. In the British Pharmaeopceia, there are directions for making 12 plasters, viz., ammoniac and mercury plaster, belladonna plaster, cantharides plaster, chalybeate plaster, galbanum plaster, litharge (or lead) plas• ter, mercurial plaster, opium plaster, pitch plaster, resin plaster, soap plaster, and warm plaster. The I itharge (or lead) plaster, directly or indirectly, enters into the composition of all the twelve officinal plasters, excepting those of ammoniac and mercury, cantharides, and pitch. Lead plaster, which is ueually sold under -the name of diachylan, in com
bination with resin, constitutes the ordinary adhesive plaster. The best plaster of this kind for strapping is composed of a mixture of six drams of resin with a pound of lead plaster. The Ca ntharidcs plaster and the ammoniac and mercury plaster, are examples of the second and third varieties.
Plasters are generally kept in rolls; and when they are to be used, they are melted at a tempqrature of not more than 212°, and spread on soft leather. They are employed to answer two distinct indications, namely, to act mechanically, as by affording artificial support to weak muscular structures, by preventing threatened or tedious excoriations, by protecting parts already excoriated from the aelieu of the air, etc.; and to act medicin ally as stimulant, dtscutient, alterative, anodyne, etc.