(b) By precipitating metallic mercury from its solution, by the contact of a single metal. It should be generally known that, by virtue of superior affinity, certain metals will decompose the solution of corrosive sublimate, with different phenomena; in those cases where the precipitating metal is capable of forming a direct union with mercury, we shall find the precipitates to consist of an amalgam of the metal employed; where no such combination takes place, the mercury may be frequently seen standing on the surface as a metallic dew. This is particularly striking when iron or steel has been employed. In the evidence given on the trial of Mary Bateman[[283]], better known by the name of the “Yorkshire Witch,” Mr. Thomas Chorley, surgeon at Leeds, stated that he had received from his assistant, Mr. Hammerton, a jar which he had carefully preserved in his possession, and of the contents of which he gave the following account. “Upon tasting a portion, it was very acrid, styptic, and permanent upon the tongue; I then took a small quantity of it upon a clean knife, and rubbed it with my finger; a change of colour immediately appeared; further rubbing produced numerous globules of quicksilver, and the knife was, at the same time, blackened by it; this change of colour led me to suspect that it must be a mercurial composition, and having made a solution of it, and subjected it to a series of tests and experiments, it is my opinion, that the mixture in the pot did contain honey, and corrosive sublimate of mercury. In order, however, more fully to satisfy myself upon this point, a mixture was made of these ingredients, when it was found to yield the same results.” In the above experiment, the steel knife decomposed the sublimate, forming a chloride of iron, while the mercury, thus disengaged in its metallic form, being unable to amalgamate with the iron, appeared in globules[[284]] upon its surface. At the same time the knife became blackened owing to the precipitation of carbonaceous matter from the steel.
(c) Carbonate of Potass. A saturated solution of this salt, added to that of corrosive sublimate, will produce a deep brick coloured sediment, which is stated to consist of per-carbonate of mercury[[285]]; while a muriate of potass will be found to remain in solution. The sub-carbonate of potass will occasion a somewhat different precipitate, of a clear brick colour, and consisting of a mixture of the carbonate, and oxide of the metal.[[286]]
(d) Ammonia. A solution of the volatile alkali produces a white precipitate, which is an insoluble triple salt, composed of muriatic acid, ammonia, and oxide of mercury; being heated it grows yellow; it passes afterwards to red, and according to Orfila gives out ammoniacal gas, nitrogen, calomel, and metallic mercury. In this operation the oxide of mercury is supposed to be deoxidized by the hydrogen which results from a portion of the decomposed ammonia.
(e) Lime water. This reagent may be said to decompose corrosive sublimate more perfectly than any alkaline body; occasioning a precipitate of a deep yellow colour, which will be found to be a peroxide of mercury; unless indeed the quantity of lime water be very small, when it will be a sub-muriate of the peroxide.
(f) Nitrate of Tin. According to Dr. Bostock[[287]] this test is capable of detecting the three-millionth part of a grain in solution. A single drop will produce an immediate and copious dark-brown precipitation.
All the above precipitates, if rubbed on a bright plate of copper, will render its surface silvery white, in consequence of the amalgamation which takes place.
Brugnatelli has lately proposed a method of distinguishing corrosive sublimate from arsenic, which we have repeated to our satisfaction; but the experiment requires some nicety of manipulation to secure its success. Take a quantity of fresh wheat starch, mix with water, and add a sufficient quantity of iodine to give the liquid a blue colour; if either of the above poisons be now introduced into it, the colour will be destroyed, and assume a reddish tint; but if the change has been effected by the latter substance, a few drops of sulphuric acid will restore the blue colour; whereas if it has been produced by the former, it is not recoverable by such means.[[288]]
3. It is dissolved in various coloured liquids. Under this subdivision we have to consider the corrosive sublimate as existing in a state of solution, in liquids, whose colour will be liable to obscure the characteristic indications which the several reagents would otherwise occasion. It has been proposed to obviate such impediments by the previous addition of chlorine, which will discharge the colour in question. Orfila recommends such a process, where the salt has been dissolved in wine. The same objections which we urged against this mode of proceeding, under the consideration of arsenic, appear to us to apply to corrosive sublimate.
It will be preferable on these occasions to precipitate the salt by an appropriate reagent, and then to assay the precipitate for metallic mercury; or to evaporate the solution, and to submit the matter so obtained to the process of sublimation, when the sublimate may be dissolved in distilled water, and examined by the tests above described. This circuitous process may, however, in many cases be rendered unnecessary, by dropping the solution on the surface of white paper, and in such a situation proceeding to its examination by tests; when the colour of the precipitate will rarely be exposed to any optical fallacy. The Galvanic process of metallic reduction will also furnish a satisfactory solution of the problem.
4. It is mixed, or combined, with some medicinal body in a solid form. As persons have been poisoned by empirical remedies, and other medicines containing sublimate, accidentally or by design, it is necessary to point out the readiest mode by which the investigation may be pursued. If it should form part of a plaster, it will be adviseable to cut it up in small pieces, and boil them for a quarter of an hour in distilled water; this fluid, after standing for some time, should be filtered, and examined as we have before directed. It is evident that, if the sublimate is neither decomposed, nor strongly retained by the materials which compose the plaster, it ought to be found in the above solution; if, however, no such result can be obtained, the solid portion should be dried in a capsule, and mixed with potass; and in this state submitted, in the usual manner, to the process of sublimation, when the appearance of metallic globules will announce the existence of the salt in question, or, at least, of the presence of some mercurial preparation.