_b._ _Prussiate of Potass_ produces a precipitate of a rather deep blue colour; which, consists of a mixture of the prussiates of zinc and iron. If the salt has been divested of all impurity, the precipitate will be white. _c._ _The Hydro-sulphurets_ instantly occasion a blackish precipitate, which, like the former, will be found to be a mixture of zinc and iron, in the state of an hydro-sulphuret. If the salt, however, is pure, its colour will be white with a tinge of yellow. SILVER. This metal does not exert any influence on the living body; but its oxide in combination with nitric acid constitutes one of the most corrosive of all the metallic salts. LUNAR CAUSTIC. _Nitrate of Silver._ _Lapis Infernalis._ The usual state in which this substance occurs is in that of small cylinders, having been cast into moulds for the purpose of imparting to it a form best adapted for the purposes it is designed to answer. Its action on animal matter is highly caustic, and when introduced in any considerable quantity into the stomach, will induce death by corroding the texture with which it may come in contact. At the same time there is reason to believe that the whole, or part of its composition, may be absorbed; for we have many instances on record where the frequent repetition of this metallic salt, in small doses, has imparted a blue tinge to the skin, which can only be explained on the supposition that the oxide of the metal has been actually deposited in the rete mucosum[319]. We are not aware that there is any modern case of poisoning by this salt[320]. The medical practitioner, however, ought to know, that common salt, is its true antidote; indeed so completely does it decompose and separate it from water, that if a saturated solution of nitrate of silver be filtered through common salt, it may be afterwards drunk with impunity. _M. Orfila_, by a series of experiments, has shewn that if the _muriate of soda_ be administered a very short time after the ingestion of lunar caustic, it will disarm it of its virulence by transforming it into an insoluble muriate, possessing no power of acting on the animal œconomy. _Chemical processes for the detection of Lunar Caustic._ If a small portion of the salt can be procured it may be dissolved in distilled water, and immediately identified by the following tests. _a._ _Muriatic acid, or any soluble Muriate_, will precipitate the muriate of silver, which is white, curdled, very heavy, insoluble in water, or nitric acid; but soluble in liquor ammoniæ; when exposed to the air it acquires a black colour. _b._ _Potass_, _Soda_, and _Lime water_, will occasion a precipitate of the oxide, of a deep brown colour. _c._ _Ammonia._ This alkali will form an _ammoniuret of silver_, and in consequence of the solubility of this new product, little or no disturbance is occasioned by the test.