In colour making many acids are used for the solution of metals, the production of precipitates, for oxidations and so forth. Commercial acids, especially inorganic acids, generally contain not inconsiderable quantities of impurities which are injurious in the manufacture of many colours. =Hydrochloric Acid=, HCl = 36·5.—The commercial acid (muriatic acid, spirits of salt) generally contains large quantities of iron, which colour it yellow—fortunately, in many cases, this is not a disadvantage, and also at times the iron can be removed from solutions made in the acid. Another impurity is sulphuric acid. This can be detected by diluting and adding barium chloride; if sulphuric acid be present, a white precipitate, or, at least, a cloudiness, appears. Ordinary hydrochloric acid is a solution of hydrochloric acid gas in water. The strongest acid contains 42·85 per cent. of the gas, and has the specific gravity 1·21. The following table gives the strengths of acids of various specific gravities:— +------------+--------------+-------------+--------------+ | Specific | Hydrochloric | Specific | Hydrochloric | | Gravity. | Acid | Gravity. | Acid | | | per cent. | | per cent. | +------------+--------------+-------------+--------------+ | 1·21 | 42·85 | 1·10 | 20·20 | | 1·20 | 40·80 | 1·09 | 18·75 | | 1·19 | 38·88 | 1·08 | 16·71 | | 1·18 | 36·36 | 1·07 | 15·49 | | 1·17 | 34·34 | 1·06 | 13·86 | | 1·16 | 32·32 | 1·05 | 11·49 | | 1·15 | 30·30 | 1·04 | 8·97 | | 1·14 | 28·28 | 1·03 | 6·93 | | 1·13 | 26·26 | 1·02 | 4·89 | | 1·12 | 24·24 | 1·01 | 2·03 | | 1·11 | 22·22 | | | +------------+--------------+-------------+--------------+