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.
Digitalis is very uncertain in its action. When given medicinally its effects should be watched, as in some cases it probably accumulates in the system, but in others, especially in heart disease, it may be given for months or years without hurt, and even with advantage. A poisonous dose seems to produce vomiting, purging, colic, headache, slowness and extreme irregularity of pulse, dimness of vision, dilated pupils, lethargy, prostration, convulsions, and coma. In two instances death occurred within twenty-two hours. The appearances found afterwards have been chiefly congestion of the cerebral vessels and slight inflammation of the stomach.