It appears scarcely necessary to describe the immediate and well known effects of the application of the lunar caustic to the surface of a wound or ulcer. It may, however, be shortly observed that the contact of the caustic induces, at first, a white film or eschar which, when exposed to the air, assumes in a few hours a darker colour, and at a later period, becomes black; as the eschar undergoes these changes of colour it gradually becomes harder and resembles a bit of sticking plaster; in the course of a few days, according to the size and state of the wound, the eschar becomes corrugated and begins to separate at its edges, and at length peels off altogether, leaving the surface of the sore underneath, in a healed state. In the formation of this eschar several things require particular attention. The application of the caustic should be made over the whole surface of the sore; and indeed no part requires so much attention as the edges; to make a firmer eschar the caustic should even be applied beyond the edge of the wound, upon the surrounding skin, for the eschar in drying is apt to contract a little, and in this manner may leave a space between its edges and that of the adjacent healthy skin. At the same time, much attention must be paid to the degree in which the caustic is applied. In cases of recent wounds unattended by inflammation, it may be applied freely; but when inflammation has come on, too severe an application of the caustic induces vesication of the surrounding skin, and the edges of the eschar may in this manner also be loosened and removed. If every part is touched, a slight application of the caustic is generally sufficient. The importance of avoiding all causes which might detach the edges of the eschar will be apprehended by the following interesting observation, which I have been enabled to deduce from very extensive trials of the caustic; it is, that, in every instance in which the eschar remains adherent from the first application, the wound or ulcer over which it is formed, invariably heals. Not only the cause just mentioned, but every other by which the eschar might be disturbed, must, therefore, be carefully avoided; and especially, as the eschar begins to separate from the healed edges of the sore, it should be carefully removed by a pair of scissors. To the surface of the wound the eschar supplies a complete protection and defence, and allows the healing process to go on underneath uninterruptedly and undisturbed. It renders all applications, such as plasters, totally unnecessary, as well as the repeated dressings to which recourse is usually had in such cases; and it at once removes the soreness necessarily attendant on an ulcerated surface being exposed to the open air. In many cases too, in which the patients are usually rendered incapable of following their wonted avocations, this mode of treatment saves them from an inconvenience, which is, to some, of no trifling nature.