3944. To Obtain Anhydrous Benzoic Acid

3944.    To Obtain Anhydrous Benzoic Acid. Add oxychloride of phosphorus to an excess of benzoate of soda; agitate together, and wash the mixture with boiling water. The anhydrous benzoic acid sinks like a heavy oil, and crystallizes on cooling.

3945. Chromic Acid

3945.    Chromic Acid. This consists of acicular crystals of a crimson-red color and an acid metallic taste, deliquescent, and very soluble in water, forming an orange-yellow solution. With the bases this acid forms chromates. Chromate of lead forms the pigment known as chrome-yellow.

3946. To Obtain Chromic Acid

3946.    To Obtain Chromic Acid. Take 10 measures of a saturated cold solution of bichromate of potassa, mix with it 15 measures sulphuric acid, and allow the mixture to cool. The chromic acid is deposited in crystals, which, after decanting the mother liquid, are placed on a tile to drain, covered with a bell glass.

3947. Hydrocyanic Acid

3947.    Hydrocyanic Acid. This is also called prussic acid, and consists of a thin, colorless, and volatile liquid, having a strong odor of peach kernels. It boils at 79° Fahr. and solidifies at 45°; its specific gravity is

.7058. It constitutes one of the most deadly poisons known. Its salts are hydrocyan-ates and metallic cyanides. Prussic acid, even when dilute, is very liable to spontaneous decomposition, and this speedily- occurs when it is exposed to the light. To promote its preservation, it is usual to surround the bottles containing it with thick purple paper, and to keep them inverted in an obscure situation. The addition of a very small quantity of muriatic acid renders it much less liable to change, and is generally made by manufacturers for that purpose.

3948. To Obtain Anhydrous Prussic Acid

3948.    To Obtain Anhydrous Prussic Acid. Pure crystallized ferrocyanide of potassium, 15 parts; water and sulphuric acid, of each 9 parts; distill in a glass retort into a well-cooled receiver, containing chloride of calcium in coarse fragments, 5 parts; stop the process as soon as the chloride in the receiver is perfectly covered by the distilled fluid, and decant the acid into a bottle furnished with a good stopper. Keep it in the dark, with the bottle inverted.

3949. Dilute Prussic Acid

3949.    Dilute Prussic Acid. Mix 41 grains muriatic acid with 1 fluid ounce distilled water, add 501/2 grains cyanide of silver, and shake together in a well stopped phial. When the precipitate has subsided, pour off the clear dilute acid and keep for use. (See No. 3947 (Hydrocyanic Acid).) (U. S. Ph.)

3950. Tests for the Presence of Prussic Acid

3950.   Tests for the Presence of Prussic Acid. It is distinguished by a strong odor of bitter almonds.

Neutralized by potash, and tested with a solution of sulphate or tincture of iron, it gives a blue precipitate, or one turning blue on the addition of dilute sulphuric or muriatic acid. This test may be applied by spreading a single drop of solution of potassa over the bottom of a white saucer or porcelain capsule, and inverting it over another vessel of the same size containing the matter under examination. After 2 to 5 minutes remove the upper capsule; add to the potassa upon it, a single drop of a solution of sulphate or tincture of iron, and expose it to the air for a few seconds. Next add 1 or 2 drops of dilute sulphuric acid, when a blue color will bo developed if hydrocyanic acid is present in the matter tested.

Nitrate of silver gives a white clotty precipitate, soluble in boiling nitric acid; and which, when dried and heated in a test tube, evolves fumes of cyanogen, which bum with a violet or bluish colored flame. A watch glass, moistened with this test and inverted over matter containing hydrocyanic acid, becomes opaque and white from the formation of cyanide of silver.

Liebig's test is considered the most delicate. Moisten a watch-glass or porcelain capsule with 1 or 2 drops of yellow hydrosulphuret of ammonia; invert it over the matter as before, and after a few minutes dry it with a gentle heat. A glass rod dipped in a solution of a persalt or sesquisalt of iron, drawn over the glass, will form a blood-red streak if the smallest quantity of hydrocyanic acid is present. (Cooley.)