HYDROCHLORIC ACID. 371 HYDROCYANIC ACID. aqueous solution of which is extensively used in the arts. It was known to the alchemists, who refer to it as spiritus salis; Glauber pre- pared it by treating common salt with sulphuric acid. Priestley obtained it similarly and called it marine acid air. Lavoisier introduced the view that all acids necessarily contain oxygen, and hence hydrochloric acid was for years be- lieved to contain oxygen. About 1810 .Sir Humphry Da-y had established the elementary nature of chlorine, and hence the true nature of its hydrogen compound — hydrochloric acid; and within ten years the correctness of his re- sults became generally recognized. ( See Chem- istry.) Hydrochloric acid occurs in the ex- halations from active volcanoes, especially from Vesuvius and the fumaroles of Hecla. It is also a constituent of the waters of certain South American rivers that have their source in the volcanic districts of the Andes. Also it is a constituent of the gastric juices in man and animals, and plays an important part in the digestive process. When sodium chloride (com- mon salt) is decomposed by heating with sul- phuric acid, as in the .Le Blanc process for making soda, sodium sulphate (Glauber's salt) remains behind, while gaseous hydrochloric acid is evolved. The acid was formerly allowed to pass off into the air, and naturally had a very injurious effect on the vegetation in the vicinity of the manufacturing establishments. At pres- ent the acid vapors are carefully collected, and thus hydrochloric acid constitutes an important byproduct of the manufacture of soda. Hydrochloric acid is a colorless gas, with a pungent odor and taste, and fuming strongly in the air. On the application of pressure and cold it can be condensed to a colorless liquid; but the commercial product is usually dissolved in water. The commercial acid is generally colored •by the presence of some iron; it also usually con- tains arsenic and sulphuric acid, from all of which impurities it may be freed by distillation. The acid is used in preparing the chlorides of various metals, for extracting phosphates from bones, in dyeing and tissue printing, and in the manufacture of coal-tar colors. For works on hydrochloric acid, consult the various books on the manufacture of soda ash. C'lii.oRinES. The salts of hydrochloric acid are termed chlorides. By far the most important of these is the chloride of sodium or common salt, which is described in a special article. Very similar to it is the chloride of potassium, KCl, which occurs extensively (combined with mag- nesium chloride) in the mineral carnallite, and in smaller quantities in sea-water. It is manu- factured from carnallite, and is purified by re- crystallization from water. The chloride of aluminum, Al.Cl,,. is obtained by treating alumi- num oxide with charcoal in a current of chlorine. It is readily soluble in water, alcohol, and ether, but on evaporating its aqueous solution it com- bines with water to form aluminum hydroxide and free hydrochloric acid. Aluminum chloride is largely used in organic chemistrj- in the so- called Friedel and Crafts reaction. { See Fbiedel. ) For other chlorides, see Baru'M; Calomei. : Gold; JIanganese; MERcmic Chloride; and Sal Ammoxwc. See also Bleaching-Powder. HY'DROCOR'ALLFNA (Xeo-Lat. from Lat. hydrn. water-snake -f corallinu.t, coral-red, from coralium, coral). An order of Hydromedusse, in- cluding the important Paleozoic family Stroma- toporida;. See C(ELEXter.ta; Hydbozoa. HY'DROCYAN'IC ACID (from hydro-gen + cyan-ogen), UCX, or HCy. A chemical com- pound often called prussic acid, from its having been first obtained bj- Scheele in 1782 from the substance known as Prussian or Berlin blue. Hydrocyanic acid is of equal interest to the chemist, the physician, and the toxicologist. The pure anhydrous acid is a limpid volatile liquid of a strong, penetrating odor resembling that of peach-blossoms or oil of bitter almonds. At low temperatures it solidifies to a crystalline mass that melts at — 15" C.; at 18° C. it has a spe- cific gravity of 0.097, and it boils at 26° C. (78.8° F.). Its volatility is so great that if a drop be allowed to fall on a piece of glass, part of the acid becomes frozen through the cold pro- duced by its o«Ti evaporation. It burns with a pale-blue flame, reddens litmus paper slightly ( its acid properties being feeble ) , and is very soluble in water and in alcohol. In the anhy- drous state it is not affected either by the action of air or by that of light. In the presence of moisture, however, it is decomposed with forma- tion of a brown substance kiiown as azulmie acid. If its aqueous solution is e.xposed to light, decomposition rapidly takes place, with formation of a number of different substances, including ammonia, formic and oxalic acids, etc. Hydrocyanic acid is made by heating yellow prussiate of potash (potassium ferrocyanide) with dilute sulphuric acid, the reaction taking idace according to the following chemical equa- tion : 2K.Fe(CX),+ 3HjSO. = Potassitim Sulphuric ferrocyanide acid 6HCN + FeKjFe ( ON ) , + SK^SO. Hydrocyanic Potassium-iron Potassium acid ferrocyauide sulphate In this manner a more or less dilute solution of hydrocyanic acid is obtained, and by fractional distillation and drying over calcium chloride, the anhydrous acid may be prepared. An aqueous solution of hydrocyanic acid may also he obtained by shaking silver cyanide with dilute hydro- chloric acid. Hydroc}'anic acid occurs, both free and in combination, in various plants, and it is readily produced by macerating bitter almonds or cherry kernels with water. Bitter almonds, as well as many other vegetable products, con- tain a glucosido called amiifidalin. and a peculiar ferment called emulsin. When brought into con- tact with water, the amygdalin is acted on by the emulsin, hydrocyanic acid being formed along with other substances, according to the following equation: CsH^NO,, + 2H.0 = HCN -f CH.O + 2C.H„0, Amygdalin Hydrocyanic Denial- Glucose acid debyde The dilute acid of the U. S. Pharmacopoeia contains 2 per cent, bj' weight of anhydrous hydrocyanic acid; while what is known as "Scheele's Prussic Acid" contains from 4 to 5 per cent, of the absolute acid. The presence of hydrocyanic acid in solutions may be best proved by one of the following methods: (II To the solution in which the presence of hydrocyanic acid is suspected, caustic potash is added to strongly alkaline reaction, then a few drops of ferrous sulphate are added, and heat is
