It has been stated, that the Metal Arsenic is susceptible of two degrees of oxidizement, the result of its first degree being Arsenious acid, and that of its second Arsenic acid. This latter compound, of which we are now to treat, may be obtained by the repeated distillation of white arsenic with nitric acid. In a solid state it is white, not crystallizable; of a sour, and at the same time, metallic taste; its specific gravity is 3·391; when exposed to the action of heat in a close vessel, it does not become volatile, but melts and vitrifies; thrown on burning coals, it swells, parts with its water, and becomes opaque; if the process of deoxidation be continued, it will, at length, rise in vapours, like those of arsenious acid, and which, like them, will yield an alliaceous odour, or not, according to the circumstances already explained. The Arsenic acid dissolves very readily in water, and is even indeed deliquescent. With alkalies, earths, and oxides, it constitutes a class of salts, called “Arseniates,” all of which, as well as the pure acid, are extremely active poisons; fortunately, however, they are not much employed[[263]] in this country, and are not likely to become the instruments of crime. These salts, like those of the arsenious acid, are obedient to the different re-agents which were enumerated under the consideration of this latter substance, but with different results; thus the silver test, instead of producing the yellow indication, occasions an equally characteristic precipitate of a red, or brick colour. The ammoniuret, and acetate of copper, furnish a bluish-white precipitate. The arsenic acid, in a solid form, or the arseniate, mixed with black flux, will, like white arsenic, furnish a metallic sublimate, when heated in a glass tube.
The Sulphurets of Arsenic.
There are two Sulphurets of Arsenic: the yellow variety known in commerce under the name of Orpiment, and the red sulphuret, termed Realgar. The bodies, as they occur native, do not appear to be endowed with the virulent powers which distinguish the other compounds of arsenic. M. Renault[[264]] gave as much as two drachms of the native orpiment to dogs of different sizes, from which they experienced no inconvenience. Hoffman[[265]] also offers his testimony of the inertness of this substance. The same observations apply to the Realgar. It is not a little singular that while these native sulphurets of arsenic should be so harmless, those which are produced by artificial fusions, are extremely virulent in very small doses. M. Renault supposed that this remarkable difference of effect was owing to the arsenic being oxidized in the latter compound, and in its metallic state in the former. This explanation, however, is not considered as satisfactory by M. Orfila, who states that it does not embrace all the varieties of the case, for that the sulphuret, which is artificially obtained by pouring the arsenious acid into a solution of sulphuretted hydrogen, is as inert as the native compounds; besides which, chemical analysis has proved that there is no oxygen in any of these sulphurets, and that they only differ from one another, by a greater or less proportion of their two ingredients. This apparent anomaly induced M. Orfila to institute a series of experiments for its investigation, but the results which he has obtained are too unsatisfactory to enable him to decide the question.
The presence of an Arsenical Sulphuret is to be sought for by calcination with caustic potass, in a small glass tube. The sulphuret is decomposed in a few seconds, yielding its sulphur to the potass, while its metallic element is volatilized with the usual phenomena.
Mercury.
Mercury, or Quicksilver[[266]], was known in the earliest ages. Its external characters are too familiar to require any particular description in this place. Its specific gravity is 13·568.[[267]] In its metallic state it exerts no action on the living system, except that which may depend upon its mechanical properties, although a different opinion has been entertained, (see Pharmacologia, art. Hydrargyrum.)
Several of the combinations of this metal are, however, highly destructive in small doses, and are consequently objects of forensic interest.
Corrosive Sublimate.
Oxy-muriate of Mercury. Bi-chloride of Mercury.
This metallic salt is by far the most active of all the mercurial preparations. According to the latest views of Chemistry it is a compound of two proportionals of chlorine, and one proportional of metallic mercury, and is therefore a bi-chloride of Mercury. It generally occurs in the form of a crystalline mass, made up of very small prismatic crystals, which undergo a slight alteration by exposure to air, becoming opaque and pulverulent. Its taste is extremely acrid, with a metallic astringency, occasioning a sensation of obstruction in the throat which continues for some time. Its specific gravity is 5·1398[[268]]. When pulverised and thrown upon burning coals, it is immediately volatilized, giving out a thick white smoke, of a very pungent smell, not at all resembling garlic, but which irritates the mucous membranes extremely, and is highly dangerous to those who breathe it. It is soluble in eleven parts of cold, and in three of boiling water; and this solubility may be farther increased by the addition of a few drops of rectified spirit, or of muriatic acid. When swallowed in small quantities it acts as a most virulent poison.[[269]]