2. _The Silver Test._—Nitrate of silver yields, with hydrocyanic acid or cyanide of potassium, a white clotted precipitate, (cyanide of silver,) insoluble in cold but soluble in boiling nitric acid. If this precipitate be well dried and heated, cyanogen gas will be given off, which may be known by its burning with a purplish flame. This test is very delicate. 3. _The Iron Test._—Of the liquid collected in the receiver above-mentioned, or the suspected acid liquid, saturated with a few drops of caustic potass, a portion is to be taken, and to this is to be added a small quantity of a solution of sulphate of iron. A dirty brownish or greenish precipitate will fall, consisting of a mixture of the oxide of iron and prussian blue. On adding a few drops of diluted sulphuric or hydrochloric acid, and thus dissolving the oxides, the prussian blue will immediately be made clear if hydrocyanic acid be present. 4. _The Copper Test._—Sulphate of copper added to prussic acid rendered slightly alkaline by potass, gives a greenish-white precipitate, which becomes white by the addition of a few drops of hydrochloric acid to dissolve the blue precipitated oxide of copper. 5. _The Sulphur Test._—One of the most useful tests for prussic acid, whether in the fluid or volatile state, is the so-called sulphur or Liebig’s test. It is best adapted for detecting the acid in a state of vapor, and to this end a drop of yellow sulphide of ammonium in a watch-glass is held over the suspected liquid, which may be warmed by the hand to facilitate the evolution of the acid. In this position the watch-glass should be allowed to remain for some little time, after which a drop of solution of perchloride of iron is to be added, which will give rise to a blood-red color not discharged by corrosive sublimate. If the acid is in the liquid form a drop of the prussic acid and the yellow sulphide may be mixed and heated until they thoroughly combine. A drop of sulphate of iron is then added as before, but all the sulphide must be decomposed or a black sulphide of iron will be produced, even though prussic acid be present, instead of the ordinary blood-red color. Other substances give a similar reaction with iron; but their color is discharged by corrosive sublimate. CHAPTER XXVIII. DEPRESSANTS. NEUROTICS PRODUCING MARKED DEPRESSION OF THE HEART’S ACTION. DIGITALIS—CALABAR BEAN—TOBACCO—HEMLOCK. DIGITALIS PURPUREA (_Purple Foxglove_).—The seeds, leaves, and root of this indigenous hedge-plant are poisonous. _Digitalin_ is the principle which these parts contain. The officinal infusion of digitalis, made from the dried leaves, is used in doses of ʒij, ℥ss, or more; of the tincture m. v to m. xl are usually given. On the recommendation of the late Mr. Jones, of Jersey, half an ounce of the tincture is sometimes administered in cases of delirium tremens; this dose being repeated a second or even a third time, in the course of six or eight hours.