By the method of firing, merely rubefacient or vesicating effects are produced. Firing is accomplished by the application of an iron disk provided with a suitable handle (Mayer's hammer). It is dipped into boiling water, and then passed, more or less rapidly, over the skin of the part to be acted on. Very brief contact suffices to vesicate, and this is the usual effect of the application.

Of the above preparations the most efficient is the ceratum extracti cantharidis, the most elegant the charta cantharidis, and the most convenient the collodium cum cantharide. The cerates should be spread on adhesive plaster, leaving a margin of the plaster to secure adhesion to the skin. Before the application of a blister, if prompt action is necessary, a mustard-plaster should be laid on long enough to produce rubefaction, or the skin should be rubbed with turpentine. When the skin is very thin and sensitive, the blister should be covered with tissue-paper. When the vesication is to be permitted to heal in a short time, or when young and irritable subjects are to be blistered, the cantharides-plaster should be removed when distinct redness of the skin is produced, and a poultice applied, which will develop the vesicles. The length of time required for a blister "to draw" is influenced by the age of the subject and the condition of the skin. From two to twelve hours, as a rule, will elapse before vesicles appear; and, when a very deep impression is intended, the blister may remain even twenty-four hours. In infants and in certain states of the constitution (scorbutus, purpura, scarlatina, etc.), prolonged contact of a cantharides-plaster may cause deep sloughing and very severe nervous symptoms, and adynamia.

When the vesicles are fully developed, they should be punctured at the most dependent point, and the serum, as it escapes, absorbed by a soft cloth. If the blistered surface is to be allowed to heal, a dressing of raw cotton suffices. If discharge is to be encouraged, resin-cerate or savine-cerate may be applied, spread on a cloth perforated to permit the discharge to exude, and covered with raw cotton to absorb the fluid. If the blister is slow to heal, boracic-acid lotion is an efficient application, or a weak lead-lotion may be used.