g. The detection of the presence of sulphate of quinidine is based upon the difference in the solubilities of the oxalates of quinine and quinidine. Oxalate of quinidine is sufficiently soluble in cold water not to be precipitated by double decomposition when solutions of oxalate of ammonia and sulphate of quinidine are mixed. Under the same circumstances, quinine is almost completely thrown down. The test is applied as follows:
The suspected salt is dissolved in water, a slight excess of oxalate of ammonia added, and the precipitate formed separated by filtration. If the salt be pure, the filtrate is scarcely rendered turbid by the addition of ammonia; when, however, sulphate of quinidine is present, it will be entirely contained in the filtrate, in which ammonia will produce an abundant precipitate.
EXAMINATION OF BLOOD STAINS.
This branch of legal chemistry formerly gave but very unreliable results. It is scarcely ten years since the reactions that are now regarded as only secondary and confirmative in their character, and far from conclusive, were the only ones in use: these are the tests based upon the presence of iron and albumen in the blood. Since then, great progress has been made in the methods employed. It must not be understood, however, that the question under consideration always admits of an easy and decisive solution: the stains are sometimes too greatly altered to be identified; but in cases where the distinctive reactions of blood can be produced, the real nature of the stains under examination can, at present, be determined with certainty.
Fig. 21.
Fig. 22.
The tests more recently introduced consist in the production of small characteristic crystals, termed haemin crystals, and in the use of the spectroscope. Crystals of haemin (first discovered by Teichman) are formed when dry blood is dissolved in concentrated acetic acid, and the solution evaporated to dryness: they are of a brownish-red color. Brücke first suggested an analytical method, based upon this property of blood, which is equally characteristic and sensitive: It is only necessary to dissolve a minute portion of the matter to be examined (dried blood, or the residue left by the evaporation of the fluid obtained by treating the stain, or the dried blood, with cold water) in glacial acetic acid and evaporate the solution to dryness in order to obtain crystals of haemin, which can be readily recognized by means of a microscope having a magnifying power of 300 diameters. If the crystals originate from fresh blood, they appear as represented in Fig. 21; crystals from old blood are represented in Fig. 22.
The former possess a reddish-brown, the latter a lighter color.