This is a preparation of the cerate of cantharides just treated of, made, according to our national code, by melting together one part of the cerate and twelve parts of Burgundy pitch.

It is intended rather as a gentle rubefacient than as an epispastic, though it not unfrequently produces the latter effect, especially when carelessly or inaccurately prepared. To obviate this tendency, the strength of the plaster has been considerably reduced in the present Pharmacopoeia; twelve parts of the pitch having been substituted for seven. Whenever the particles of cantharides are brought into contact with the skin, each particle exerts its own full influence on the portion of skin touched, which may end in the vesication of that point. Hence the difficulty of diluting powdered cantharides so that it will not vesicate, if it produce any effect whatever. To obviate such a result, the best plan is to bring the active matter into the liquid form, in which its particles are infinitely divided, and thus become susceptible of indefinite dilution. It would be better, therefore, in preparing the warming plaster, to incorporate with the Burgundy pitch an oleaginous solution of the active matter of cantharides; and this can be accomplished by melting the portion of the cerate here directed, and allowing it to remain in this state until the oily matter has thoroughly dissolved the cantharidin, then straining, and incorporating it with the melted pitch. The British Pharmacopoeia meets the same indication by using an aqueous extract of cantharides; but, in other respects, the preparation is, I think, inferior to ours.

When this plaster can be made so as not to vesicate, it is an excellent application in cases of obstinate lumbago, and of chronic internal inflammations, as pleurisy, pneumonia, bronchitis, hepatitis, etc. It may vol. ii.-48 be continued for weeks, and, with occasional renewals, for months if necessary. As it is a plaster, it requires heat to spread it.