Treatise on Poisons by Sir Robert Christison

of corrosive sublimate causes a dark, brownish-black precipitate, the bisulphuret of mercury. When the solution is not very diluted, the gas forms a whitish or yellowish precipitate before the blackening commences,—an effect which, according to Pfaff, distinguishes the salts of the peroxide of mercury from all other metals that are thrown down black from their solutions by sulphuretted-hydrogen.[835] The cause of this is that the particles of sulphuret first formed acquire a thin covering of corrosive sublimate by that property which chemists of late have termed superficial attraction. Hydrosulphuric acid is a very delicate test of the presence of mercury. It will detect corrosive sublimate, where its proportion is only a 35000th of the solution.[836] This test is not alone sufficient, unless reliance be placed on Pfaff’s criterion, which is rather a trivial one; for hydrosulphuric acid occasions a black precipitate in other metallic solutions, for example, in solutions of lead, copper, bismuth and silver. In mixed organic fluids its action is not liable to be prevented; but the precipitate formed is often kept intimately suspended, as in the instance of milk. It may be conveniently used in the form of hydrosulphate of ammonia. This test produces a dark-brown precipitate, which is said to pass slowly to a bright cinnabar red; but I have not been able to observe any transformation of the kind. _Hydriodate of Potass_ causes in solutions of corrosive sublimate a beautiful pale scarlet precipitate, which rapidly deepens in tint. The precipitate is the biniodide of mercury. This is a test of great delicacy when skilfully used, as it acts where the salt forms only a 7000th of the solution (Devergie). Care must be taken, however, not to add too much of the test, because the precipitate is soluble in an excess of the hydriodate, or too little, because the precipitate is also soluble in a considerable excess of corrosive sublimate. The action of hydriodate of potass is not liable to any important ambiguity: no other iodide resembles in colour the biniodide of mercury. It is not a certain test, however, when other salts exist in solution along with corrosive sublimate. Chloride of sodium, nitrate of potass, and probably also other neutral salts possess the power of dissolving the precipitate. Sulphuric and nitric acids, even considerably diluted, oxidate and dissolve the mercury, and disengage iodine, which colours the fluid reddish-brown. When corrosive sublimate is dissolved in coloured vegetable infusions or animal fluids, the hydriodate of potass cannot be relied on, the colour of the precipitate being altered, as in infusion of galls, or the action of the test being suspended altogether, as by milk. _Protochloride of Tin_ causes first a white precipitate, which, when more of the test is added, gives place to a grayish-black one. In very diluted solutions the colour struck is grayish or grayish-black from the beginning. In such solutions Devergie has found it useful to acidulate with hydrochloric acid before adding the test. The chemical action here is peculiar. The white powder thrown down at first is protochloride of mercury; a part of the chlorine of the bichloride of mercury having been abstracted by the protochloride of tin, which becomes in consequence the bichloride. On more of the test being added these changes are repeated, the chlorine is removed from the protochloride of mercury, and metallic mercury falls down. This test is one of extreme delicacy, affecting solutions which contain only an 80,000th of salt. It is prepared by acting on tin powder or tinfoil with strong hydrochloric acid aided by a gentle heat. The solution must be kept carefully excluded from the air; otherwise bichloride of tin is formed, which does not act at all on the solution of corrosive sublimate. The protochloride of tin is not liable to any fallacy. Neither is it liable to be suspended in its action by the co-existence of other saline substances. It causes precipitates with almost all animal and most vegetable fluids. But when corrosive sublimate is present, even in very small proportion, the precipitate is always darker than when no mercurial salt exists in solution, and frequently has its proper grayish-black tint. This property, as will presently be seen, is the foundation of a process for the detection of mercury in all states of admixture with organic matters. _Nitrate of Silver_ causes a heavy white precipitate, the chloride of silver, which darkens under exposure to light. This is a test for the chlorine of the corrosive sublimate, but not for the mercury, and is a necessary addition to the three former tests in order to determine how the mercury is kept in solution. It acts with very great delicacy. It is of no use, however, when chlorine or hydrochloric acid is present either free or combined with other bases. It is not of use, therefore, in animal fluids and vegetable infusions, because very many of them, besides organic principles which form white precipitates with this test, contain a sensible proportion of hydrochlorate of soda. Although the preceding liquid reagents when employed conjunctly are amply sufficient for determining the presence of corrosive sublimate in a fluid, many other tests hardly less characteristic and delicate have been used by medical jurists. These will now be shortly mentioned.