This acid contains rather more than half as much anhydrous acid as Acidum Hydrocyanicum. Edin.

Prep. (Ferrocyanide of potassium, two ounces and a quarter; sulphuric acid, seven fluid drachms; distilled water, thirty fluid ounces, or a sufficiency. Mix the acid with four fluid ounces of the water, and to these, placed in a retort, when they have cooled, add the ferrocyanide of potassium, first dissolved in half-a-pint of the water. Put them into a retort, and adapt this to a receiver, containing eight ounces of the water, which must be kept carefully cold. Distil with a gentle heat till the fluid in the receiver measures seventeen ounces; lastly, add three ounces or as much water as may be necessary to bring the acid to the required strength.) The changes occurring in this process are rather complex, but in 4 substance may be shown as follows: Ferrocyanide of potassium is a double cyanide of potassium and iron (2 K Cy + Fe Cy + 3 HO); when acted upon by sulphuric acid and water, a portion of the cyanide of potassium of the salt is decomposed into hydrocyanic acid and sulphate of potash, and a salt containing two equivalents of cyanide of iron to one of cyanide of potassium, is left in the retort, called Everitt's yellow salt; the formula for the decomposition is as follows:

(K4 Cy4 + Fe2 Cy2) + 3 (HO, So3)=(Fe2 Cy2 + KCy) + 3 (KO, So3) + 3H Cy.

Anhydrous prussic acid can be prepared by passing sulphuretted hydrogen (hydrosulphuric acid) over cyanide of mercury: a sul-phuret of the metal and hydrocyanic acid are then formed. Hydrocyanic acid is also produced when amygdaline, a principle contained in the bitter almond and the kernels of allied fruits, is decomposed by the action of the albuminous principle contained in such fruit; vide Amygdala Amara.

Prop. & Comp. The dilute acid, prepared as above, is a colourless liquid of peculiar odour and taste, entirely volatilized by heat, with a very slight re-action, and the reddening produced on litmus paper fugitive in character. Sp. gr. 0.997. It contains 2 per cent. of real or anhydrous acid (H, Cy). Treated with a minute quantity of a mixed solution of sulphate and persulphate of iron, and afterwards with potash, and finally acidulated with hydrochloric acid, it forms Prussian blue. With nitrate of silver it gives a white precipitate, entirely soluble in boiling nitric acid. Half a fluid ounce of the acid, when treated with an excess of solution of soda, requires the addition of 80.66 measures of the volumetric solution of nitrate of silver before a permanent precipitate begins to form, which corresponds to 2 per cent. of anhydrous acid. The explanation of this test is as follows: When nitrate of silver is added to a solution of hydrocyanic acid with excess of soda, a double cyanide of silver and sodium is formed, which is soluble, and is dissolved when the solution is well stirred; this salt is formed until all the hydrocyanic acid present is exhausted, and then the further addition of nitrate of silver causes the precipitation of the oxide of silver, which is insoluble. As the double cyanide has a definite composition, by knowing the amount of nitrate of silver required to produce a permanent precipitate, we can calculate the amount of hydrocyanic acid present in the fluid. The decomposition may be illustrated by the formula, Ag O, No5 + 2 Na Cy=Na O, No5 + Na Cy, Ag Cy. 17 grains of nitrate of silver, or 100 measures of the volumetric solution, representing 5.4 grains of absolute hydrocyanic acid.

The dilute acid, when pure, is not coloured by sulphuretted hydrogen or precipitated by chloride of barium, showing the absence of metallic taint or sulphuric acid, and no red colour is produced on the addition of the iodo-cyanide of potassium and mercury, showing the absence of any foreign acid.

The acid known under the name of Scheele's prussic acid contains 4 per cent. of anhydrous acid.

The anhydrous acid is colourless, with a more intense odour than the dilute, sp. gr. 0.697. very volatile, and rapidly decomposed into a carbonaceous-looking matter. The dilute acid can be much longer preserved when a little mineral acid is present, as a trace of sulphuric or hydrochloric acid.

Therapeutics. Anhydrous prussic acid is one of the most intense and rapid of poisons, acting as a direct sedative, apparently from arresting the functions of the whole body; its effects are the same whether taken into the stomach or applied to other mucous membranes, as to the eye, or inhaled as vapour.

When much diluted, and in medicinal doses, it allays pain and spasm, and if the dose be large, induces giddiness, etc. It is given in painful affections of the stomach and intestines, as in gastrodynia, enterodynia, pyrosis, and vomiting; also in chest affections, as pertussis, asthma, and other cases where the character of the cough is nervous; occasionally it is used to allay palpitation of the heart, especially when connected with dyspepsia, and it has been prescribed in epilepsy, chorea, and other diseases of the nervous system.

Externally applied it allays irritation of the skin, and when freely diluted may be used in the form of lotion in cutaneous affections accompanied with much itching: great care should be taken that the skin is not abraded.

Dose. Of the acidum hydrocyanicum dilutum 2 min. to 10 min. Scheele's acid is twice the strength, and it is very desirable that it should not be employed in medicine. Aqua Lanro-Cerasi, or cherry-laurel water, which owes its activity to hydrocyanic acid, is described under Lauro-Cerasus.

Externally, in the form of lotion, 1 fl. drm. or more may be added to 10 oz. of water, lead lotion, or almond emulsion.

Incompatibles. It is often prescribed with alkalies, as liquor potasssae, etc.; then a cyanide of the metal is formed, which acts in the same manner as the acid; but if a salt of iron be also present, yellow prussiate or ferrocyanide of potassium is produced - a salt possessing none of the properties of prussic acid.