When a strong solution of hydrochloric acid is boiled at tho ordinary temperature and pressure of the atmosphere, it gives off a gaseous acid until it is reduced to a density of 1.10, then it distils over unchanged. In similar manner, a weak solution parts with its water until a density of 1.10 is aequired, and thon passes over with a constant composition, boiling at 112° (233° F.). The observation of this circumstance pointed at first to the conclusion that there was a definite hydrate of tho acid with this constant composition. It has, however, been established that by varying tho pressure, or vaporizing at different temperatures, other liquids with a constant compositiou are obtainable—liquids that distil over unchanged after reaching certain densities. The relation betweon the pressure under which the acid is distilled and the composition of the stable liquid is given in the following table, P m showing the pressure in metres of mercury, and the second column giving the percentage of real acid in tho constant liquid.
It is evident that there exists for each pressure a corresponding aqueous acid with a constant boiling point, which distils over, that is to say, under this pressure with unchanged composition.
• With regard to varying temperatures, when an aqueous solution is vaporized by passing dry air through it, similar varying liquids with constant boiling points are obtained. This will he readily seen from the following table :— The powerful affinity existing between hydroehloric acid and water, to which reference has been made, is shown by the well-known white fumes which are formed when the gas is allowed to come into contact witb the moisture of the atmosphere. As a pure gas it is invisible. Evolved into the air, it occupies a space several hundred times its original bulk. The aqueous solution is decom posed by all the metals whieh deeompose water at a red heat, the metal being dissolved and hydrogen set free. The liquefied acid dissolves bitumen. Neither gas nor solution possesses bleaehing properties.
A place in the arts and manufactures, only second perhaps to that oecupied by sulphurie acid, must be assigned to hydroehlmic acid, so largely does it enter into every-day commercial life. The great bulk is used in the manufacture of chlorine for various purposes, of the hypochiorites, and biearbonate of soda. It is also employed to a considerable extent in the manufacture of chloride of ZinC—used in the preservation of timber, &c., and as it disinfectant—of glue, the oxychloride
of lead, the chlorides of tin, antimony, ammonium, mercury, &e. ; in the preparation of certain freezing mixtures, in softening ivory, in cleaning tin plates, and in dissolving the incrustation upon boilers and pipes. In bleaching, hydrochloric is often substituted for sulphuric acid to make the " sours " whieh follow the applieation of chloride of lime, and it is thought to give better results, especially in cloths destined for garancine work. Finally it forms, with nitric acid, the solvent of gold, platinum, and various alloys known by the name of aqua regia, to which further reference will be made.
Hydrochlmie aeid is produced by the direct union of hydrogen and chlorine gases ; by the aetion of water upon certain chlorides—phosphorus, tin, antimony, &c.; by the action of chlorine gas upon various substances containing hydrogen—water, phosphuretted hydrogen, and numerous organic bodies, such as acetic aeid ; by the inverse aetion of hydrogen upon certain chlorine compounds, such as chloride of nickel ; but, chiefly, by the decomposition of common salt (chloride of sodium) by sulphuric aeid. By this last method the huge bulk of the acid of commerce is manufactured.
The direct union of hydrogen and chlorine is attended by some noteworthy circumstances. A mix ture of the two gases, in equal proportions, may be kept in the dark without any change taking place ; but if the vessel containing the gases he exposed to the sunshine, or if an electaie spark be passed through it, an immediate combination, with the formation of hydrochloric acid, will result. When the operation is condueted over mercury, no increase or diminution in the volume of gases is noticed, one volume of hydrogen uniting with one volume of chlorine to form two volumes of hydrochloric acid. This powerful attraction between chlorine aud hydrogen is further shown in the second method of producing hydrochloric acid cited—the, aetion of chlorine upon certain hydrogenized compounds. A solution of the gas in water exposed to the light becomes gradually converted into an aqueous solution of hydroehlorie acid. In the case of organic compounds, one portion of the attacking chlorine unites with the hydrogen, and another takes the place of the hydrogen removed. For example, in the decomposition of acetic acid,