Muriatic Acid, may be obtained by distillation from a mixture of common salt with clay or bole, which is the original process; but it is now only used where fuel and pottery earths are cheap, and oil of vitriol dear. The method most commonly practised at present to obtain it, is to decompose the muriate of soda, or common salt, by sulphuric acid, and condensing the muriatic acid gas in water, for which it has a great affinity. The annexed engravings represent the most approved apparatus for this purpose.
Fig. 1 is a transverse, and Fig. 2 a longitudinal section of a bench of cast-iron retorts a, (generally twenty in number,) resembling those used in gas-works. They are placed in pairs, each pair having a separate fire-place e, grate f, and ash-pit g. Every part of the cylinder should be equally heated, that the decomposition of the salt may be simultaneous, and the iron be as little as possible injured by the acid. For this purpose, a plate of cast-iron, k, is placed between the cylinders; and the flues h are constructed so as to produce an equal draught throughout every part of the furnace. The cylinders are closed at each end by a plate of cast-iron luted into the rim of the cylinder. Each end plate has a handle b, of cast-iron, and a small tube m, projecting from the upper part of the plate, for the purpose at one end of pouring in the sulphuric acid, and of conveying away the products at the other. The first cylinder communicates by the bent glass pipe c, with the earthen vessel d, which has three mouths, and which again communicates by two other bent tubes c, with two similar vessels.
All the gas not condensed in the first bottle d, passes into the other bottle d; at the same time the second bottle d receives the gas from the second cylinder, and transmits the gas which it does not condense to a third bottle, and so on to the last bottle, which receives the gas not condensed in all the others, as also the gas from the last cylinder.
From this bottle whatever gas is not condensed is transmitted through a second range of bottles half filled with water, which will absorb two-fifths of its weight of muriatic acid; and in this second range the whole of the gas is condensed. The first range, it should be observed, is placed in a trough I, through which is maintained a current of cold water. The purest acid is obtained in the second range of bottles; that which is condensed in the first range containing always a little sulphuric acid, and sometimes sulphate of soda and muriate of iron. Each cylinder is charged with about 160lbs. of common salt, and the end is luted with clay, and the fire is kindled; 1281bs. of sulphuric acid at 66°, Baume's areometer, are then poured on the salt. After the gas ceases to come over, the end plate is taken off, the sulphate of soda removed, and the operation repeated: thus 130lbs. of muriatic acid, of the spec. grav. 1.190, may be obtained from 100lbs. of salt. Muriatic acid, mixed with nitric acid, forms aqua regia, which is a solvent for gold and platinum: it is also used to prepare muriate of tin for dyers, to scour metals, and numerous other purposes.