their way into the circulating current either through the branches of the thoracic duct, or those of the _venæ portarum_; when, as if by a species of election, each substance very frequently expends its venom upon some one particular system of organs. Many of the substances arranged under this second division, have moreover a local effect upon the structure with which they first come in contact; it is thus with _Colocynth_, and some other bodies; while on the contrary, several of those poisons which are distinguished for their _local action_, are subsequently absorbed, and are thus as it were enabled to ensure their work of destruction by a double mode of operation. We shall receive ample evidence of this truth, as we proceed in the history of particular poisons. The third class comprises such agents as inflict their vengeance upon the mucous membrane of the stomach, by actual contact, and destroy, by exciting local inflammation. _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.