The filtrate, separated from the precipitate produced by the addition of nitrate of baryta, may contain hydrochloric, hydrobromic and hydriodic acids. In order to detect these compounds, some nitrate of silver is added to the solution, and the precipitate that may form carefully washed and decomposed by fusion with potassa. The mass is then dissolved in water, and the solution submitted to the following tests:
e. Hydriodic acid.—Some starch paste and nitric acid—containing nitrous acid in solution—are added to a portion of the solution: in presence of an iodide, the fluid immediately acquires a blue color.
f. Hydrobromic acid.—In case iodine has not been detected, chlorine water and ether are added to a second portion of the fluid, and the mixture well agitated. If bromine be present, the ether will assume a brown color. In case iodine is also contained in the fluid, and the detection of bromine is desired, it is necessary to acidulate the solution with hydrochloric acid, and then shake it with chloride of lime and bisulphide of carbon. The bisulphide of carbon dissolves the iodine, acquiring a violet color, which disappears upon a renewed addition of chloride of lime; whereas, in presence of bromine an orange coloration remains, even after the disappearance of the iodine reaction.
g. Hydrochloric acid.—Since the substance under examination will already contain hydrochloric acid, it is unnecessary, in most cases, to institute a search for this compound. Nevertheless, it may be well to take a quantity of the solution, corresponding to a known weight of the original substance, and precipitate the acid by adding nitrate of silver. The precipitate formed is dried and weighed. It is then heated in a current of chlorine, in order to completely convert it into chloride of silver, and its weight again determined. Only in case the amount of chloride found is very large, is it to be inferred that the poisoning has been caused by hydrochloric acid.
h. Hydrosulphuric acid.—(Sulphuretted hydrogen). If the precipitate produced by nitrate of silver possesses a black color, it may consist of a sulphide. Upon treating a portion with solution of hyposulphite of soda, all but the sulphide of silver is dissolved. In case a residue remains, it is calcined with nitrate of soda, and the sulphate formed detected by adding a soluble barium salt to its solution.
Sulphates, chlorides, carbonates and phosphates are most frequently met with in the preceding examination, and it should be carefully noticed which of these salts exist in the greatest abundance. If acids of comparatively rare occurrence (such as the oxalic and tartaric) are found, their approximate amount is also to be noted. These facts, together with the original acidity of the materials and the absence of other toxical bodies, would lead to the conclusion that the poisoning was caused by the reception of an acid, as well as to the identification of the special acid used. In subsequently effecting the detection of the poison by the determinative tests, the danger of destroying other poisons possibly contained in the substance will be obviated, as the question of the absence or presence of these latter will have been previously decided.
(5). The examination for acids concluded, the various fluids which have accumulated, and from which the acids present have been separated, are united and the whole evaporated to dryness. The organic substances, present in the residue obtained, are destroyed by means of nitric acid, and the residual mass examined for soda. If this substance has not been introduced into the portion of fluid examined, and is discovered in a quantity largely in excess of the amount normally contained in the organism, it is probable that poisoning has been caused by its administration, and that an acid has also been given, either in order to mask the poison, or to act as an antidote. In this case, it is necessary to carefully search for acetic acid, as this is the substance usually employed as an antidote for alkalies.
(6.) Whatever results have been obtained by the preceding examinations, the portion of the fluid which has been treated with soda (vide p. [87]) is evaporated to dryness. The organic matters possibly present are destroyed by means of nitric acid, or aqua regia, and the residue taken up with water. The solution so obtained is then examined for metals (including potassa, which salt has not been introduced into this portion of the fluid in any of the preceding operations) by the usual methods.
(7). The soluble portion of the suspected materials having been thoroughly tested, the undissolved substances remaining on the filter are next examined.