These operate, in the sound state of the skin, by so altering the epidermis as probably to render it impermeable to the air, and prevent the influence of oxygen on the parts beneath. Applied to the mucous membranes, they enter into combination with the superficial portion of the epithelium, or its whole thickness, forming a similar protecting layer. in ulcers they combine with the albumen of the secretions, and with the superficial tissue, and thus form an insoluble covering for the surface. These effects are produced by several of the escharotics; and in this way, as well as by an alterative influence, or the removal of the diseased structure, their efficacy in promoting the healing of ulcers may be explained. Three of this set of Protectives merit special notice; nitrate of silver, iodine, and creasote.