DETERMINATIVE TESTS.
In many instances the tests we have termed indicative become determinative in their character. This is the case when the isolation of an alkaloid or a metal (unless mercury be found under the circumstances already mentioned) is accomplished; the results obtained are then conclusive. If, on the other hand,—not being able to separate either an alkaloid or a metal—upon saturating the originally acid fluid with potassa, or soda, the salts of these bases have been found in abundance, there is reason to infer that the poisoning has been caused by an acid; or, if, after the neutralization of the originally alkaline solution with an acid, potassa or soda are discovered in a large quantity, poisoning by an alkali is indicated.
In case the fluid is neutral, but more or less colored and odoriferous, and iodides or bromides are detected, we may justly suspect that the poisoning has been caused by the reception of iodine or bromine.
According to the indications furnished, iodine, bromine, one, or all of the acids, the caustic alkalies, etc., are then detected by means of the methods to be employed in cases where the expert has a clew to the poison present. In this manner, the presence of potassa and soda, and of bromine and iodine, even in mixtures, is easily ascertained. It only remains to mention the course to be pursued when suspicion exists that poisoning has been caused by the administration of a mixture of several acids. The suspected materials are boiled with water, and alcohol added to the solution in order to coagulate the animal matters. The solution is next filtered, the filtrate placed in a retort provided with a receiver and distilled until the residual portion acquires a pasty consistency. In this way, the acids present are separated into two classes: (a) those that are sufficiently volatile to have passed into the receiver, such as, acetic, nitric, hydrochloric and sulphuric acids (the latter acid will only be partially volatilized); and (b) those that remain in the retort. The former are detected by examining the distillate as previously directed.
The residue remaining in the retort is treated with absolute alcohol, the fluid filtered, and a solution of acetate of lead added to the filtrate: sulphuric, phosphoric and oxalic acids, if present, are precipitated. The precipitate is suspended in water and decomposed by means of sulphuretted hydrogen. The acids contained are now set free, and are detected by applying the tests already mentioned.
If there be reason to suspect the presence of both sulphuric and oxalic acids, the distillation is discontinued after a short time. The two acids are dissolved by shaking the moderately concentrated fluid remaining in the retort with ether, and, upon evaporating the solution, will be obtained in a state suitable for examination. Oxalic acid is then detected by means of sulphate of lime; sulphuric by means of oxalate of baryta.
The above examinations would fail to effect the detection of phosphorus, and it is necessary to examine a separate portion of the original substance for this body.