HY'DROCHLO'RIC ACID (from hydro-gen ch/or-ine), or M LIU AVM Ac-m. HCl. A eous compound of hydrogen and chlorine, the aqueous solution of which is extensively used in the arts. It was known to the alchemists, who refer to it as spiritus colic; 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 ISIO Sir Humphry Davy 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 by-product 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.