These are solid, tenacious compounds, adhesive in the ordinary heat of the human body. The base of the majority of plasters is a chemical combination of the semivitreous oxide of lead and oil; but some of them owe their consistence to wax and resin; and others contain no oily nor fatty matter whatsoever. Deyeux proposes1 to confine the name of plasters to the combinations of metallic oxides with oils or fat; and to give those not containing oxides the term solid oint-ments; but this definition would include among the plasters some of the ointments, and exclude many of the plasters.

1 Annales de Chimie, xxxiii. 52.

Plasters should not adhere to the hand when cold; they should be easily spread when heated; and should remain tenacious and pliant after they are spread; but should not be so soft as to run when heated by the skin. All plasters become too consistent and brittle when long kept; but in this case, those which are unctuous may be re-melted by a gentle heat, and some oil added to them. They are usually formed into rolls, which are wrapped in paper; and when they are to be used, they are melted and spread on leather, calico, linen, or silk. Those which contain metallic oxides ought to be melted by boiling water, for in a greater degree of heat the fatty matter is apt to reduce the oxide.

Plasters are employed as local remedies to answer various indications. When the materials of which they are formed are soft and bland, they are used simply as coverings to sores and abraded surfaces, to protect them from the action of the air, and give support to the parts; but in many instances they contain acrid and stimulating substances, and operate as rubefacients or as blisters.

Emplastrum Ammoniaci. Lond. Edin. Dub. Ammoniacum Plaster

"Take of ammoniacum, five ounces; distilled vinegar, eight fluid ounces. Dissolve the ammoniacum in the vinegar; then evaporate the solution with a slow fire, constantly stirring, until it acquire a proper consistence."

This plaster is stimulant and resolvent. It is applied to scrofulous and indolent tumours and white swellings.

Emplastrum Ammoniaci. Cum Hydrar-Gyro. Lond. Ammoniac Plaster, with Mercury

"Take of ammoniacum, a pound; mercury, three ounces; olive oil, a fluid drachm; sulphur, eight grains. Add the sulphur gradually to the heated oil, stirring constantly with a spatula until they unite; then rub the mercury with them until the globules disappear; lastly, add gradually the ammoniacum melted, and mix the whole together."

Dublin. "Take of pure gum ammoniacum, a pound; purified mercury, three ounces; turpentine, two drachms. Rub the mercury with the turpentine until the globules disappear; then add gradually the ammoniacum, previously melted, and melt the whole together."

In these plasters the mercury is probably in the state of oxide with a minimum of oxygen. They are discutients, and are applied to indurated glands, hydarthus, nodes, tophi, and indolent tumours.