There is no good test for aconitia. HYDROCYANIC ACID (_Prussia Acid_), on account of its energetic and rapid action, is one of the most formidable poisons with which we are acquainted. In its concentrated state it is a limpid colorless liquid; possessing a somewhat acrid taste, and having an odor, when diffused through the air, resembling that of oil of bitter almonds. When diluted with water, it forms the acid kept by the druggist. The properties of this variety are similar to those of the pure form; except that, if kept in the dark, it is not so readily decomposed. It is in this condition that it is used as a poison. The diluted acid of the British Pharmacopœia contains about 2 per cent., and that known as Scheele’s from 4 to 5 per cent., of the strong acid; but all vary greatly with keeping. One of the salts of hydrocyanic acid, the _cyanide of potassium_, claims a short notice, since it is largely employed by photographers, workers in electrotype, &c. It has been taken as a poison. This salt is sold in the form of deliquescent white crystals, or in crystalline masses, which are very soluble in water, and possess the odor of prussic acid. From three to five grains will destroy life almost as rapidly as prussic acid itself, and in the same manner: a dose of five grains has proved fatal. Several vegetable substances yield prussic acid, such as the kernels of the peach, apricot, nectarine, cherry, &c., the leaves of the cherry laurel, and the pips of apples and pears. Cases of alarming illness have occurred from eating _bitter almonds_ too freely; while the essential oil obtained by distilling the pulp of these almonds with water is a powerful poison. This _essence_ or _oil of bitter almonds_ contains about ten per cent. of anhydrous prussic acid; and it is probable that from ten to thirty drops would prove fatal to an adult. The prussic acid may, however, be separated from it, and leave the oil harmless. A distilled water obtained from the leaves of the cherry laurel, which was formerly employed in medicine, proved dangerous from its very variable strength; it has been used as a poison. In the well-known case of Sir Theodosius Boughton, poisoned by Captain Donellan in 1781, _laurel water_ produced death within half an hour after two ounces had been swallowed.