Syn. Lunar Caustic.

Preparations and Properties. Nitrate of silver, for escharotic purposes, is prepared simply by melting the dry nitrate (i. 390) in a crucible, continuing the heat till ebullition ceases, and then pouring the liquid into moulds, where it is allowed to harden. it is in the form of cylindrical sticks, about as thick as a quill, and several inches long, which are usually kept wrapped closely in paper, sealed at the ends. These sticks are hard and brittle, with a crystalline radiated fracture, and at first of a white colour, which, on exposure, gradually changes to gray, and ultimately becomes nearly black. The chemical properties of the fused nitrate are the same as those of the crystallized (i. 390). it almost always contains impurities, some added fraudulently, as nitrate of potassa, others resulting from impurities in the silver employed in its preparation, as the nitrates of copper and lead. For a more particular account of these impurities, and for the modes of detecting them, see the article Argenti Nitras Fusa in the U. S. Dispensatory. As the preparation is found in the shops, it seldom fails to produce the desired effect. A good method for keeping the sticks is, as recommended by M. Dumeril, to coat them with wax, which, at the same time that it excludes the air and light, gives greater strength to them, and protects the fingers of the operator. When the application is to be made, the wax is simply scraped off from one end of the stick, where it is brought to a point.

Effects. We have here to consider only the topical effects of the lunar caustic, as applied to the surface, or parts within reach from without. Like all the other escharotic medicines of this class, it may be so employed, by varying its strength, as to be either simply excitant, epis-pastic, or escharotic; and, both for the first and last of these effects, it is very much used. Applied to the cuticle, it soon blackens it, probably through the partial deoxidation of the oxide of silver; but, on a mucous membrane, the skin destitute of epidermis, or the surface of an ulcer, it produces a pure white streak wherever it passes, owing chiefly to its union with the albuminous matter, but in part probably to its reaction with the chloride of sodium of the secreted liquids or the tissues, resulting in the formation of chloride of silver. These compounds, however, darken on exposure; and hence the surfaces, at first white, blacken if acted on by the light. Neither the blackening of the cuticle, nor the whitening of the moist surfaces, necessarily implies a caustic effect; as they are often produced by the salt too much diluted to affect the organization of the tissues. The stain of the cuticle disappears only with its ultimate exfoliation; that of the mucous surfaces gradually passes off with the altered secretion, or the separation of the epithelium.

When the dry nitrate is applied to the sound skin, also dry, little effect is produced; but a saturated solution, or the moistened stick, soon excites a smarting sensation, and, if continued, causes destruction of the cuticle, and superficial cauterization of the true skin. if the application is not continued long enough to destroy the cuticle, it often happens that, at the end of some hours, vesication takes place, which is usually less painful than a blister produced by cantharides. A similar application to a mucous membrane, or a denuded surface, immediately causes a superficial eschar. When vesication has taken place in the skin, the blackened cuticle breaks, and falls off in a short time, leaving a sound surface. When an eschar is produced, it separates in a few days, and not unfrequently, in the case of mucous or unprotected surfaces, in less than a day, when the effect has been very slight. The pain attendant on the escharotic operation is considerable, sometimes severe, but much less than that of caustic potassa; and in some surfaces it is scarcely felt. it differs very much with the susceptibility of the surface. Thus, in the conjunctiva the pain is often exquisite; while in the vagina, and at the os uteri, little or no disagreeable sensation may be felt.

Nitrate of silver operates as a caustic by its affinity for the constituents of the tissues, especially their albumen and fibrin, with which it forms insoluble compounds. These, in consequence of their insolubility, cannot be penetrated by the dissolved nitrate, and consequently protect the parts beneath from the uncombined portion of the salt. Hence, the escharotic operation of lunar caustic is always superficial, and it is almost impossible to make it act to a great depth. in this respect it differs entirely from caustic potassa; and the peculiarity constitutes its great recommendation. Without this property, it would be inapplicable to most of the important purposes for which it is used.

In the parts with which it may be in contact without destroying them, the salt produces, along with its excitant effect, contraction of the tissues, or, in other words, acts as an astringent; and this property also constitutes one of its therapeutic recommendations.

Therapeutic Uses. Nitrate of silver is one of the most valuable, if not the most valuable, of topical remedies; being used in a great number of affections, and with the happiest results. I shall treat first of its employment purely as an escharotic, and afterwards of those applications of it, which, though they may be connected with its caustic operation, depend also on other influences for their curative effect.

1. As a Caustic purely. in this capacity, its application is not extensive. it is unfit for the formation of issues, the opening of abscesses, or the removal of any considerable portion of structure, whether diseased or normal. it may, however, be used for the destruction of warts and small excrescences, whether venereal or otherwise, and for that of fungous granulations; in all of which its escharotic property depends probably as much upon the over-excitation it produces, as upon its chemically decomposing effect. in strictures it is sometimes beneficially used. in old and obstinate strictures of the urethra it has been much resorted to, though by some surgeons considered unnecessary. Strictures of the oesophagus and rectum have been beneficially treated by means of it. in these cases, care is taken, by suitably contrived instruments, to confine the operation of the caustic to the part affected. it is much safer than potassa, in consequence of the limited extent of its action. In those poisoned wounds where only a superficial effect is wanted, nitrate of silver is usually preferred, especially in dissecting wounds.