HYDROBROMIC ACID, or HYDRO GEN BROMIDE, a compound having the for mula HBr, and analogous in its general prop erties to hydrochloric and hydriodic acids. Hydrogen and bromine do not combine directly, even in strong sunlight; but when hydrogen that is charged with bromine vapor is burned, hydrobromic acid and water are formed. Hy drogen and bromine may also be made to com bine by electric sparks, or by passing the mixed gases through a hot tube, or over hot platinum. The most convenient way of preparing the acid, however, is by the action of bromine upon water, in the presence of phosphorus, the reac tion being 41.130 SBr P = 5HBr H.PO4. Phosphoric acid, it will be seen, is formed at the same time; but the two are easily separated by heat. Pure hydrobromic acid, when free from water, is a colorless gas of pungent odor, having a density about 39.1 times as great as that of hydrogen. Under ordinary atmospheric pressure it condenses into a liquid at 99° below zero F., and at a slightly lower temperature it
crystallizes. It dissolves freely in water, a saturated solution, at 32° F., having a specific gravity of 1.78, and containing 82 per cent of hydrobromic acid, by weight. A concentrated aqueous solution of hydrobromic acid fumes strongly in the air, but does not decompose, When heated, the gaseous fumes are evolved until the solution contains about 47 per cent of the acid, when it boils continuously at Hydrobromic acid is a powerful acid, forming, with metallic bases and with organic radicals, definite salts called ((bromides? Hydrobromic acid is used in medicine in the treatment of the ear, and the bromides of the alkalis are ex tensively used as sedatives. Bromide of silver is also used in large quantities in photography, in the manufacture of sensitive dry-plates.