SODA.— Hydrochloric Acid. 801
may bo beautifully shown by filling a tube, abouthalf an inch in diameter and 12 inches long, with themixed gases, and alternately shading it with anopaque cover and exposing it to the sun’s rays; andthe moment the tube is exposed even to the diffusedlight of day, a cloudiness appears within it, and thewater ascends more or less rapidly according to theamount of light (Henry).
Davy found that the heat evolved during the com-bination of a mixture of chlorine and hydrogenexceeded that produced by a mixture of oxygen andhydrogen, and that the former was inflamed at alower temperature. Oxygen and hydrogen ceased toexplode when rarefied 18 times, but chlorine andhydrogen retain their combustibility when rarefied24 times. Hydrochloric acid gas may be decomposedby the action of several of the metals—potassium,for instance, absorbs chlorine and the hydrogen isevolved. This experiment is best made in a smallretort, which is exhausted of air, and filled withhydrochloric acid gas from a jar over mercury.When cooled it will be found that the residuary gasis hydrogen, equal to half the original bulk of hydro-chloric acid. Tin and lead heated in the gas alsoeffect its decomposition, and evolve exactly half itsbulk of hydrogen. The greater part of the metallicoxides when heated in gaseous hydrochloric aciddecompose it, and metallic chlorides are formed, andthe oxygen of the oxide, combining with the hydro-gen of the acid, forms water. Solution of hydrochloricacid was first made known by Glauber as far back asthe middle of the seventeenth century; but hydro-chloric acid gas was first isolated by Priestley, in1772. It was procured by acting on common salt withsulphuric acid. The specific gravity is 1-262, and asit consists of equal volumes of hydrogen and chlorinecombined without change of volume, 100 cubic inchesmust weigh 39-36 grains (Brande). According toTiroiirsON, 100 cubic inches at 0°C 2 weigh 42-126grains. Faraday liquefied this gas by generating it ina sealed tube, so as to expose it to a pressure of about40 atmospheres at 10° C.; it was colourless, and pos-sessed a refractive power inferior to that of water,but he could not succeed in solidifying it.
Hydrochloric acid has a strong attraction forwater; and when it escapes into the air, it formsstrong dense fumes, from its absorption of waterfrom the atmosphere (Brande).
Water takes up 480 or 500 times its bulk ofhydrochloric acid gas, and has its specific gravityincreased from 1 to 1-210 (II. Davy).
Hydrochloric acid gas is rapidly absorbed bywater: 100 cubic inches of water will absorb no lessthan 48,000 cubic inches of this gas, whereby theliquid acquires a specific gravity of 1-210, and avolume of 142 cubic inches. This great condensa-tion is accompanied with a considerable productionof heat. In general muriatic acid of commerce hasa specific gravity of 1-15 to 1-20, and contains forthe most part less than 40 parts by weight of acidin the hundred. In general a very good approxi-mation may be found to the percentage of realmuriatic acid in any liquid sample by multiplyingVOL. II.
the decimal figures of the specific gravity by 200—thus, for example, at 1-162, we shall have by thisrule, 0-162 X 200 = 32-4 for the quantity of gas in100 parts of the liquid (Dr. Ure).
Uncombined hydrochloric acid exists in smallquantity in the human gastric juice. It is not foundin nature except occasionally as a volcanic product.The principal uses of this acid are in medicine, inthe production of chlorine, and in some of the arts,where it is chiefly used as a solvent for metals. Itis extremely injurious to vegetation; and mixedwith 20,000 times its volume of air, it proves fatalto plants, shrivelling and killing all the loaves intwenty-four hours (Ciiristison).
When metallic zinc is put into strong liquidhydrochloric acid, the latter is rapidly decomposed,and the hydrogen copiously evolved may be burnedas it escapes. Dioxide of lead added to hydrochloricacid immediately disengages chlorine, which may beshown by its bleaching power upon a piece of paperdipped into indigo blue. These experiments illus-trate the separation of two elements in the acid.
Hydrochloric acid may be produced in the labora-tory in a glass retort, from a cold mixture of 8 partsof common salt, and 13 parts of oil of vitriol, and 3parts of water. The retort is connected with asystem of Woulfe’s bottles, which are kept cool bybeing placed in water, and covered on the top withdamp cloth or by bibulous paper. In the first bottleis placed a very small quantity of water; in thesecond, as much distilled water as is equal in weightto the salt used, and the third bottle somewhat less.The retort may be gradually heated on a sand bathnearly to redness; and the solution of acid whichcollects in the first bottle is generally weak, andcontains most of the impurities—such as chloridesof arsenic, iron, &c. The 8 parts of water in thesecond bottle are converted into 13 parts of hydro-chloric acid of specific gravity 1-145. If the sul-phuric acid contains any nitric acid, the hydrochloricacid may contain free chlorine. The water in thethird bottle absorbs the last traces of the gas, andforms a weak solution, which may be again used inthe next operation (Gmelin).
Commercial hydrochloric acid is made on thelarge scale for employment in the arts; and whena high strength of acid is required, in an apparatusvery similar to that described under Nitric Acid .The salt is decomposed by sulphuric acid in a metalretort, about 6 feet long by 20 inches in diameter;and the hydrochloric acid which is generated isabsorbed in a series of earthenware bottles, produc-ing a commercial acid of about 32° Tw., or 1-16specific gravity. The chief impurities in this acidare chlorides of arsenic (got from the sulphuricacid used), and of iron (from the action of the acidson the metal retort). These impurities are mostlyarrested in the first bottle, and the second bottlegenerally contains a hydrochloric acid nearly pure,and which can be used in medicine and for otherpurposes where a chemically pure acid is necessary(It. C. Clapiiam).
The acid made by this process is chiefly manu-101