+-----------------+--------------+-----------------+--------------+ |Specific Gravity.|Sulphuric Acid|Specific Gravity.|Sulphuric Acid| | | per cent. | | per cent. | +-----------------+--------------+-----------------+--------------+ | 1·8485 | 100 | 1·8043 | 89 | | 1·8475 | 99 | 1·7962 | 88 | | 1·8460 | 98 | 1·7870 | 87 | | 1·8439 | 97 | 1·7774 | 86 | | 1·8410 | 96 | 1·7673 | 85 | | 1·8376 | 95 | 1·7570 | 84 | | 1·8336 | 94 | 1·7465 | 83 | | 1·8290 | 93 | 1·7360 | 82 | | 1·8233 | 92 | 1·7245 | 81 | | 1·8179 | 91 | 1·7120 | 80 | | 1·8115 | 90 | | | +-----------------+--------------+-----------------+--------------+ _Nordhausen Sulphuric Acid_, H₂S₂O₇ = H₂SO₄ + SO₃, is generally a yellowish brown liquid, which gives off white fumes in the air. It contains varying quantities of sulphur trioxide dissolved in sulphuric acid. It often contains selenium, which separates as a red powder when the acid is diluted. The presence of this impurity does not interfere with the use of the acid for dissolving indigo, the only purpose for which it is required in the colour factory. =Nitric Acid=, HNO₃ = 63.—This acid, which is used in the preparation of many colours, is distinguished by the readiness with which it gives up part of its oxygen, and thus converts metals like antimony and bismuth into oxides, and transforms other compounds into a higher state of oxidation. There are two kinds of nitric acid: ordinary nitric acid, a colourless liquid which is more or less pure; and fuming nitric acid, a yellow or orange-coloured liquid, fuming strongly in the air, which consists of a solution of nitrogen peroxide, NO₂, and nitric oxide, NO, in nitric acid.