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 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.

1. Lime-Water throws down the binoxide of mercury in the form of a heavy yellow powder. The precipitate first thrown down is lemon-yellow, an additional quantity of the test gives it a reddish-yellow tint, and a still larger quantity restores the lemon-yellow. This test is characteristic, but not so delicate as those already mentioned.—2. Caustic Potass has precisely the same effect as lime-water, except that the tint of the precipitate is always yellow—3. Caustic Ammonia causes a fine, white, flocculent precipitate of intricate composition, commonly called precipitate. It is a very delicate test; but ammonia likewise causes a white precipitate in other metallic solutions.—4. Carbonate of Potass causes a brisk-red precipitate, by virtue of a double decomposition, the precipitate being carbonate of mercury.—5. The Ferro-cyanate of Potass causes at first a white precipitate, the ferro-cyanide of mercury. The precipitate becomes slowly yellowish, and at length pale-blue, owing, it is believed, to the admixture of a small quantity of iron with the corrosive sublimate.—6. A polished plate of Copper immersed in a solution of corrosive sublimate becomes in a few seconds tarnished and brownish; and in the course of half an hour a grayish-white powder is formed on its surface. This powder, according to Orfila, is a mixture of calomel, mercury, and a copper amalgam. If it is wiped off, and the plate then rubbed briskly where tarnished, it assumes a white argentine appearance.—7. A little Mercury put into a solution of corrosive sublimate is instantly tarnished on the surface; the solution in a few