Every substance, whether liquid or solid, may become an ingredient in this species of composition, and although judicious and experienced surgeons have of late very considerably improved the form of their cataplasms, yet the principles of medicinal combination, which it has been the object of the present work to investigate and expound, will suggest many important hints for the farther extension of their utility; and although the direction of them is more frequently left to the nurse than to the medical practitioner, yet in adapting them to each particular occasion some share of chemical address may be necessary; we have already seen that attention must be paid to the selection of ingredients, with respect to their powers of conducting heat, and it is evident that care must be taken not to reduce into pulp, by decoction, substances that contain volatile principles; while in the preparation of active liquids to be subsequently mixed with linseed meal, it is equally evident that we must be directed by the chemical nature of their composition. EMPLASTRA. L.E.D. _Plasters._ These are solid and tenacious compounds, adhesive at the ordinary heat of the human body; they owe their consistence to different causes, viz. 1. _To a due admixture of wax or fatty matter_, _and resin_, e. g. _Emplast: Ceræ_, &c. They may be said to differ only in _consistence_ from liniments, ointments, and cerates; Deyeux[312] accordingly proposes to distinguish them by the appellation of _Solid Ointments_. 2. _To the chemical combination of the semivitreous oxide of lead with oils or fat_, e. g. _Emplast: Plumbi_. 3. _To the chemical action of the component parts of the plaster on each other_, as _Emplast: Ammoniaci_, &c. Plasters are generally kept in rolls, wrapped in paper, and when to be used they are melted and spread on leather; in performing this operation the practitioner ought not to apply a heat above that of boiling water; for if metallic oxides be present, the fatty matter will, at a higher temperature, reduce them, in consequence of the powerful affinity of oil for oxygen at an exalted temperature; and if aromatic substances enter as ingredients they will thus suffer in their strength, besides which the fat itself will undergo a very injurious change by a mismanaged application of heat, and the plaster will be less adhesive.