HYDROCYANIC ACID 873 



air, emitting a corrosive vapour of a peculiar smell. The characteristic test of mu- 

 riatic acid in the most dilute state, is nitrate of silver, which causes a curdy precipi- 

 tate of chloride of silver. 



Liquid muriatic acid has a very sour corrosive taste, a pungent suffocating smell, 

 and aqts very powerfully upon a vast number of mineral, vegetable, and animal sub- 

 stances. It is largely employed in the manufacture of chlorine for preparing bleaching 

 powder and other purposes ; it is also used in the preparation of chloride of zinc and 

 other metallic chlorides used in the arts; it is employed in the generation of carbonic 

 acid gas from carbonate of lime ; and it is a constituent of certain freezing mixtures. 

 In combination with nitric acid, it forms the aqua regia of the alchemists, so called 

 from its property of dissolving gold. See SODA MANUFACTUHE. 



HYDROCYANIC ACID. Syn. Cyanhydric acid, Prussic acid. C 2 NH (CXTB). 

 This highly important acid is regarded by all chemists as being formed on the exact 

 type of the ordinary inorganic hydracids, such as the hydrochloric or hydriodic. The 

 compound radical analogous to chlorine, which is contained in it, has received the 

 . name of cyanogen, and possesses the formula C 2 N (CW). That this body is precisely 

 analogous in its relations to the simple salt-radicals is rendered certain by numerous 

 facts. It combines directly with metals to form compounds ; it possesses the same 

 vapour- volume, and unites with hydrogen to form a hydracid, which in its turn 

 decomposes the metallic oxides with formation of water. Tims we have, with metallic 

 oxides and hydrochloric acid (M standing for a metal), MO + HC1=MC1 + HO, and 

 with hydrocyanic and metallic oxides (Cy standing for cyanogen), MO + HCy = MCy 

 + HO. Two volumes of chlorine and two of hydrogen yield four volumes of hydro- 

 chloric acid gas, and two volumes of cyanogen with two of hydrogen yield four 

 volumes of hydrocyanic acid. The density of the vapour of hydrocyanic acid is con- 

 sequently 0-9476, the theoretical number being 0-9342. Its density in the fluid 

 state is 0-6967, at a temperature of 64-4. It boils at 80 F. at ordinary pressures. 



Hydrocyanic acid is never prepared in the anhydrous state, except as a curiosity 

 or for the purpose of scientific investigation. In fact, it cannot be long preserved of 

 great strength ; a somewhat complex decomposition invariably takes place in it. 

 with production of brown adhesive matters containing cyanide of ammonium, and 

 also a substance by some considered to be an acid, and known as the azulmic. Para- 

 cyanogen is probably formed at the same time. The constitution of azulmic acid is by 

 no means well known, and even its very existence, as a definite chemical substance, 

 is doubtful. It is singular that the presence of a mineral acid greatly retards the 

 decomposition of prussic acid, especially if it be dilute ; the pharmacopoaian acid con- 

 quently may be preserved of uniform strength, in well-filled and closely-stoppered 

 bottles, for almost any length of time. The deadly nature of prussic acid unhappily 

 causes it to be only too frequently resorted to by the despairing or the murderer. 

 Fortunately, however, in spite of its volatility, the chemist possesses excellent means 

 for its detection. 



Preparation. 1. Hydrated acid. As prussic acid is largely employed in medicine, 

 but in a very dilute form, it is usual to prepare it and dilute until of the proper degree 

 of strength. The following process for preparing it will be found to give a satisfactory 

 result, and, moreover, it may be performed on any quantity of materials. The apparatus 

 for the purpose will vary with the scale on which the experiment is to be made. If 

 on a few ounces, glass retorts and flasks answer well, if good condensation is ensured, 

 by means of a Liebig's condenser well supplied with very cold water. If a large 

 quantity of prussic acid is to be made, such as several gallons, the apparatus should 

 consist of a stoneware still, with head adjusted by grinding. The head should be 

 capable of adjustment with a stoneware adapter to a worm of the same material en- 

 closed in a tub of water. The joints are to be luted with a mixture of one handfull 

 of almond meal and five handfulls of linseed meal, worked with water to the con- 

 sistence of putty. A solution of rough chloride of calcium in water is to be made and 

 placed in a large iron pot, with a cover so contrived as to permit the still to drop in 

 up to the flange. 10 parts of yellow prussiate of potash are then to be bruised in a 

 mortar and mixed with dilute sulphuric acid prepared by adding 6 parts of sulphuric 

 acid (density 1-850) to 42 of water. The head being luted on, a fire is to be kindled 

 in the furnace under the iron pot, and the chloride of calcium bath is to be kept 

 boiling constantly until 36 parts of acid have distilled over. The beak of the still 

 should be placed in the funnel which conducts the acid to the Winchester quart bottles 

 which are to contain the product, and a piece of wet bladder is to be stretched over 

 the funnel to prevent evaporation of the acid into the laboratory. The worm used for 

 the purpose must be ascertained to be perfectly clean, and, if prussic acid is to be 

 frequently made, should be kept specially for that operation. To each Winchester 

 quart of the acid distilling over, one drop of sulphuric acid may be added to insure 

 its keeping, But the acid thus prepared generally keeps for a long time even without 



