In these maladies the caustic action of arsenic is often extremely valuable, and the powdered drug may be used sufficiently strong to destroy diseased tissue without affecting the adjacent sound skin. For chronic superficial lupus, especially of the face, Hebra recommends "Cosme's paste," containing 20 gr. of arsenious acid and 60 gr. of cinnabar in 1 oz. of rose ointment (cold cream): this is spread on linen, and applied firmly for twenty-four hours, and then renewed for the same period, a third application being made if required. I have often used this with good results; at first there is little change produced, but by the second day the growth turns gray, and by the third day commences to slough, and may be separated in a poultice. Pain and oedema may occur, but can be relieved by sedatives and warm applications. Among many hundred cases thus treated no poisonous symptoms have been reported.

In epithelial cancer arsenic has long been used. Rousselot combined it with cinnabar, and Dupuytren with calomel, and Mr. Marsden has written in praise of its association with an equal part of mucilage. The paste commonly known in Ireland by the name of Miss Plunkett's is prepared with arsenious acid, sulphur, and two species of ranunculus: it often acts powerfully.

1 Arsenious acid 1 part, carbonate potash 20 parts, soap spirit 200 parts, water 2,000 parts. (Soap spirit is made with equal parts of soft soap and spirit of wine.)

As already stated, caution is required in the external use of arsenic: not that it should be applied in a more diluted form, for then its absorption would be even more probable, but only a limited area - not more than one square inch - should be covered at one time. Dr. Walshe has specially insisted that its use should be restricted to superficial cancer. From the internal administration of arsenic I have had good results in epithelioma (v. p. 66).