MINERAL POISONS.
Under this head is included the greater proportion of those substances which are employed as the instruments of crime; for they are generally of easy access, require but little preparation, and are so destructive in small doses, and, at the same time, so little disgusting in flavour, as to furnish the assassin with the sure and secret means of destruction. Fortunately, however, for the ends of justice, such agents are pre-eminently the objects of successful analysis. In treating of the history of the individual substances derived from this kingdom, we shall consider, 1st. their external characters, such as form, colour, odour, taste, specific gravity; 2d. their chemical composition, and habitudes; 3d. the tests by which their presence may be recognised; 4th. the symptoms which they occasion; 5th. their physiological action; 6th. their different modes and forms of application; 7th. the lesions of structure they occasion; 8th. the phenomena presented on dissection.
Cl. 1. CORROSIVE POISONS.
ARSENIC.
The greek word Αρσενικον was employed by Dioscorides, and other writers of that period, to denote a particular mineral of a reddish colour, which Aristotle had already described by the name of σανδαρακη,[[214]] and his disciple Theophrastus, by that of αρρενικον. It was employed by the ancients both as a pigment and as a medicine, and appears to have been a compound of Sulphur, and a peculiar metal, to which the name of Arsenic is now exclusively applied. At what period this metal was first discovered seems very doubtful; and although a process for obtaining it is described in the Pharmacopœia of Schroeder, published in 1649, yet its peculiar nature was examined, for the first time by Brandt, in 1733.
The metal, Arsenic, is distinguished by the following properties, viz.
It has a bluish-grey colour, not unlike that of steel, and a considerable lustre; its texture is grained, and sometimes scaly; its hardness not very considerable, but its fragility is so great that it falls to pieces under a moderate blow of the hammer, and admits of being easily reduced to a very fine powder; according to Bergman its specific gravity is 8·31. When cold, it emits no sensible odour, but if heated, it yields a strong alliaceous, or garlic-like smell, which is to be considered as the most characteristic of its properties. Its point of fusion is unknown, for it is the most volatile of all the metals, and sublimes, before it melts, at the temperature of 540° Fah., and if the process be conducted slowly in close vessels, the metallic sublimate will assume a tetrahedral[[215]] form of crystallization; if the air be admitted, and the temperature still farther raised, it will burn with an obscure bluish flame.
Arsenic is extremely susceptible of oxidation, and, by mere exposure to the air, shortly loses its metallic lustre; and yet it may be kept under the surface of cold water, for any length of time without exhibiting the signs of oxidation, or solution; a covering of this fluid, or of alcohol, is therefore considered as affording the best means of preserving the metal in a state of integrity.
Arsenic is capable of combining with two proportions of oxygen, and of forming two definite compounds, which we shall hereafter consider under the title of Arsenious and Arsenic acids. The substance described by some authors as the black oxide of this metal would seem to be an indefinite mixture of the metal itself, and the arsenious acid.
Arsenic does not appear to possess any deleterious properties, but it is almost impossible to reduce the metal to powder, so as to adapt it for exhibition without its becoming oxidized. M. Renault therefore, in order to decide the question, had recourse to its alloys; and he found that Mispickel (an alloy of Arsenic and Iron), when given to the extent even of two drachms, scarcely produced any effect; a result which very satisfactorily accords with the conclusion drawn by Bayen, in his work on Tin, and which proves that the arsenic contained in that metal, need not excite the least alarm, since it exists in a metallic state. We have upon another occasion[[216]] observed, that the vapours characterised by an alliaceous odour are probably less noxious than the arsenical fumes which are inodorous; and that the little injury experienced by workmen who solder silver filligree with an arsenical alloy, may probably depend upon the deoxidized state of its fumes.