_d._ _Arsenite of Potass._ As all re-agents must be considered as reciprocal in their operation, it is hardly necessary to state that this is one of the best tests for nitrate of silver. See the history of its effects at p. 240. If it should be necessary to discover the nitrate of silver amongst the fluids vomited, or those contained in the stomach of the deceased, we are very properly directed by _M. Orfila_ to filter, and then assay by the appropriate tests; if, however, the different aliments should disguise the characteristic colour and appearance of these precipitates, we must proceed to desiccate and calcine them in order to obtain the silver in a metallic state. THE CONCENTRATED ACIDS. These must be regarded as the most terrible of all corrosive poisons. Their action is so immediate and energetic, as generally to destroy the membranes of the stomach, before their peculiar antidotes can be applied. Notwithstanding the obvious suffering they must occasion, and the facility with which they may be detected, such bodies have frequently, especially in France, been the instruments of suicide and murder; whilst in this country, we have had many lamentable illustrations of their deadly force, by their ingestion from fatal carelessness. In conformity with our general plan we shall proceed to consider the individual substances included under this general class, although the symptoms do not materially differ in the different kinds, nor are the indications of cure peculiar to any of them. There are however chemical characters which exclusively belong to each acid, with which the forensic physician must be accurately acquainted, in order that he may be enabled to detect their presence. OIL OF VITRIOL. _Sulphuric Acid._ This acid, when perfectly pure, exists in the form of a colourless liquid, without smell, and of an oily consistence; whence its popular name. Its specific gravity is 1·85, so that, in round numbers, it may be stated that an ounce, by measure, will weigh fourteen drachms. It acquires a brown tinge from the smallest portion of carbonaceous matter; mere exposure to the atmosphere is sufficient to effect this change, in consequence of the acid disorganizing and carbonating the vegetable and animal matter suspended in the air. This fact sufficiently explains why we generally find the acid of commerce of a brown colour.