Another form in which mercury was very antiently used, and still continues to be applied with an intention of acting immediately upon affected parts, is, its being reduced into a state of vapour, by which means it acquires a very high degree of activity. This mode of applying mercury is attended with many inconveniencies, and, in this form, its application, while it is very sudden, can at best be but temporary. It must, however, be allowed, that the active form which the mercury here assumes is a very subtile one. By this means, it becomes capable of penetrating and resolving hard and indurated tumors, which cannot be pervaded by mercury in other forms. On this account, in such tumors, especially if they have been of long standing, and in obstinate ulcers which have resisted other mercurial preparations, the form of vapour may often be tried with peculiar advantage. For this purpose, cinnabar has commonly been employed. From its being resolved into fume, the mercury and sulphur are disunited, and the activity of the medicine totally depends upon the former arising in a state of vapour. Mercury, in whatever way it is used, is most active in these forms in which, from the addition of acids, it is reduced to a saline state. Some of these are in their nature so extremely corrosive, that they can scarcely, with safety, be used even as external applications. This peculiarly holds of those preparations which are formed by a combination of mercury with the nitrous acid. That they may be fitted for use, therefore, various means are employed to render them milder. Among other methods, one frequently practised, is, the abstraction of acids by calcination. In this manner are prepared the _mercurius corrosivus ruber_ of the London college, and the _praecipitatus ruber_ or _calcinatus_ of the Edinburgh. These are forms of employing mercury which are every day in use, for application to the part immediately affected. These preparations, even after having their action in this manner diminished, still possess a very considerable degree of corrosive power. On this account, they become particularly useful in many cases of venereal complaints, where corrosives are required. And hence it is easy to learn the foundation for the frequent and advantageous employment of these preparations in cases of fungous edged ulcers, of warts, and other such tumors. That these preparations may be applied more commodiously, they are frequently mixed with different ointments, particularly with basilicon; but, by this means, their power of action is greatly diminished. Where it is intended they should act as escharotics, it is perhaps in every case most proper to employ them in the form of dry powders. Such dressings may then be applied above them as are best fitted to the nature of the complaint.