The solution of the pure salt is precipitated white by the caustic alkalis, an oxide being thrown down, which is soluble in an excess of ammonia. The alkaline carbonates also precipitate it white, the carbonate of ammonia being the most delicate of these reagents. The precipitate is soluble in an excess of carbonate of ammonia, and is not thrown down again by boiling. The precipitate produced both by the alkalis and by their carbonates becomes yellow, when heated nearly to redness; and on cooling it becomes again white. This is a characteristic property, by which the oxide of zinc may be known from most white powders. But oxide of antimony is similarly affected. The ferro-cyanate of potass also causes a white precipitate. A stream of sulphuretted-hydrogen likewise causes a white precipitate, the sulphuret of zinc, the colour of which distinguishes the present genus of poisons from all those previously mentioned, as well as from the poisons of lead. The precipitate is apt to be suspended till the excess of gas is expelled by ebullition. The action of this test will not distinguish sulphate of zinc from the salts of peroxide of iron, by which white sulphur is disengaged from the gas in consequence of the peroxide of iron being reduced to the state of protoxide. The same decomposition takes place wherever there is free chlorine, as in impure samples of muriatic or nitric acid.
When the sulphate of zinc contains iron, the alkalis throw down a greenish-white precipitate, the alkaline carbonates a grayish or reddish-white, the ferro-cyanate of potass a light-blue, but sulphuretted-hydrogen the usual white precipitate. Tincture of galls, which merely renders the pure salt hazy, causes a deep violet coagulum if there is any ferruginous impurity.
The sulphate of zinc is acted on by albumen and milk precisely in the same manner as the sulphate of copper. The salt is decomposed, and the metallic oxide forms an insoluble compound with the animal matter.
When the sulphate of zinc has been mixed with vegetable and animal substances, the action of the tests mentioned above is modified. In such circumstances I have found the following process convenient.
The mixture being strained through gauze, it is to be acidulated with acetic acid, and filtered through paper. The acetic acid dissolves any oxide of zinc that may have been thrown down in union with animal matter. The filtered fluid is then to be evaporated to a convenient extent, and treated when cool with sulphuretted-hydrogen gas,—upon which a grayish or white milkiness or precipitate will be formed. The excess of gas must now be expelled by boiling, and the precipitate washed by the process of subsidence and affusion, and collected on a filter. It is then to be dried and heated to redness in a tube. When it has cooled, it is to be acted on by strong nitric acid, which dissolves the zinc and leaves the sulphur. The nitrous solution should next be diluted, and neutralized with carbonate of ammonia; after which the liquid tests formerly mentioned will act characteristically. The effect of carbonate of ammonia, and that of heat on the carbonate of zinc which is thrown down, ought to be particularly relied on.
I have tried this process with the matter vomited after the administration of sulphate of zinc, in a case of pretended poisoning, and found it to answer exceedingly well.
Orfila has lately suggested the following method. Boil the suspected substance in water, evaporate the filtered decoction to dryness, char the residuum with nitric acid as directed for copper in similar circumstances, digest the charcoal in diluted muriatic acid, and subject the filtered solution to hydrosulphuric acid. If the sulphuret be not white, but yellowish from iron, heat it with strong nitric acid, dry the product, and heat it to redness; dissolve it in weak nitric acid; throw down the oxide of iron by an excess of ammonia, which retains the oxide of zinc; and then having filtered the fluid, separate the oxide of zinc by neutralizing the ammonia.[[1194]]
Orfila has furnished the only accurate information hitherto possessed regarding the effects of sulphate of zinc on the animal system.[[1195]] He found that dogs might be made to swallow 7½ drachms without any permanent harm being sustained, provided they were allowed to vomit; for in a few seconds the whole poison was invariably discharged, and the animals, after appearing to suffer for four or five hours, gradually recovered their usual liveliness. But the result is different if the gullet be tied: violent efforts to vomit ensue, and death follows in three days, the intermediate phenomena being those of local irritation chiefly, and the appearances in the dead body those of incipient inflammation of the stomach, without corrosion.—When injected into the veins, the effect of sulphate of zinc is much more violent, in an inferior dose. Forty-eight grains occasioned almost instant death; and half the quantity proved fatal in three minutes. Orfila does not appear to have ascertained the cause of death in the last two experiments. But Mr. Blake found that when this salt is injected into the veins in the dose of three grains, it causes some depression of the heart; that thirty grains arrest the action of the heart in eight seconds, leaving that organ exhausted of irritability and full of florid blood in its left cavities; and that when injected into the arterial system in the dose of sixteen grains, it seemed not to cause any obstruction of the capillaries, but to act on the nervous system, producing extreme prostration, without insensibility or convulsions.[[1196]] These experiments, when taken together, show that sulphate of zinc, though a moderately active irritant, is more indebted for its activity to a remote operation on some vital organ.
Sulphate of zinc is absorbed in the course of its action; for Orfila has lately found it by his process for complex mixtures in the spleen, liver, and urine of animals.[[1197]]
The effects of the preparations of zinc on man in large doses have not been particularly studied. In the dose of a scruple or a drachm, the sulphate is the most immediate emetic known; and it is to be inferred, that if larger doses are rejected, as is the fact, with equal rapidity, they will in general cause no more harm than the medicinal dose.