chiefly used for the solution of gold and platinum and for "opening up"
sulphides. When solutions in aqua regia are evaporated, chlorides are
left.

~Bromine~, Br. (sp. gr. 3.0). Practically pure bromine.--It is a heavy
reddish-brown liquid and very volatile. It boils at 60∞ C., and,
consequently, must be kept in a cool place. It gives off brown
irritating vapours, which render its use very objectionable. Generally
it answers the same purpose as aqua regia, and is employed where the
addition of nitric acid to a solution has to be specially avoided. It is
also used for dissolving metals only from ores which contain metallic
oxides not desired in the solution.

~"Bromine Water"~ is simply bromine shaken up with water till no more is
dissolved.

~Carbonic Acid~, CO_{2}.--A heavy gas, somewhat soluble in water; it is
mainly used for providing an atmosphere in which substances may be
dissolved, titrated, &c., without fear of oxidation. It is also used in
titrating arsenic assays with "iodine" when a feeble acid is required
to prevent the absorption of iodine by the alkaline carbonate. It is
prepared when wanted in solution, by adding a gram or so of bicarbonate
of soda and then as much acid as will decompose the bicarbonate
mentioned. When a quantity of the gas is wanted, it is prepared, in an
apparatus like that used for sulphuretted hydrogen, by acting on
fragments of marble or limestone with dilute hydrochloric acid.

~Citric Acid~ (H_{3}[=C=i] or C_{6}H_{8}O_{7}.H_{2}O) is an organic acid
which occurs in colourless crystals, soluble in less than their weight
of water. The solution must be freshly prepared, as it gets mouldy when