+----------------+--------------+----------------+--------------+ |Specific Gravity| Nitric Acid |Specific Gravity| Nitric Acid | | at 15° C. | per cent. | at 15° C. | per cent. | +----------------+--------------+----------------+--------------+ | 1·530 | 100·00 | 1·323 | 55·00 | | 1·520 | 97·00 | 1·284 | 50·49 | | 1·509 | 94·00 | 1·251 | 45·00 | | 1·503 | 92·00 | 1·211 | 40·00 | | 1·499 | 91·00 | 1·185 | 33·86 | | 1·495 | 90·00 | 1·157 | 30·00 | | 1·478 | 85·00 | 1·138 | 25·71 | | 1·460 | 80·00 | 1·120 | 23·00 | | 1·442 | 75·00 | 1·089 | 20·00 | | 1·423 | 69·96 | 1·067 | 15·00 | | 1·400 | 65·07 | 1·022 | 11·41 | | 1·346 | 60·00 | 1·010 | 4·00 | +----------------+--------------+----------------+--------------+ Since the action of nitric acid chiefly depends on its oxidising properties, which are possessed by both kinds, it generally does not matter which is used. The usual impurities are chlorine and sulphuric acid; the presence of the first is shown by silver nitrate solution, of the latter by barium chloride, in each case added after diluting. When the acid is used for oxidations these impurities do not interfere, but nitric acid containing chlorine cannot be used to dissolve silver, because the chlorine would form insoluble silver chloride. The strength of nitric acid is gauged by its specific gravity as given in the table. =Aqua Regia.=—A mixture of 2 parts of hydrochloric acid and 1 part of nitric acid gradually turns orange or yellow and evolves chlorine. This liquid, which can dissolve gold in consequence of the free chlorine it contains (hence its alchemistic name, from gold, the “king of metals”), is used as a very powerful oxidising agent in colour making. =Carbon=, C = 12, is the only one of the non-metallic elements to be mentioned here; by itself it forms a group of very important pigments, which we shall describe in detail at a later stage. =Carbonic Acid Gas=, CO₂ = 44, is used in the manufacture of white lead, which it precipitates from lead acetate. This is, however, a particular branch of colour making carried on in special works. In describing this manufacture we shall return to the preparation of carbonic acid on a large scale. ORGANIC ACIDS. The organic acids which are important in colour making are acetic, oxalic and tartaric acids.