4. A recent test for mineral acids has been suggested by Hager.[134] It consists in warming together two drops of East Indian copaiba balsam, and 30 drops of pure acetic acid, and subsequently adding to the mixture two or three drops of the vinegar under examination; if either sulphuric or hydrochloric acid be present, a blue-violet colour is produced. The free mineral acids in vinegar may be quantitatively estimated by saturating a weighed quantity of the sample with quinine, evaporating the mixture to dryness over the water-bath, and dissolving the quinine salts formed in alcohol, which is then removed by distillation. The second residue is next dissolved in water, and the quinine precipitated by addition of ammonia, and separated by filtration. The filtrate will contain the mineral acids present, and their amount is determined by the ordinary methods. The free sulphuric acid in vinegar can also be quantitatively estimated, according to Kohnstein,[135] as follows: 100 c.c. of the sample are shaken with pure and freshly calcined magnesia until completely neutralised. The mixture is filtered, the filtrate evaporated to dryness in a platinum dish and the residue ignited at a moderate temperature. By this treatment magnesium acetate is converted into the corresponding carbonate, while any magnesium sulphate present will remain unaltered. The ignited residue is moistened and evaporated with a little carbonic acid water, then digested with hot water, and the solution filtered; the insoluble magnesium carbonate remains upon the filter, the sulphate going in solution; the precipitate is thoroughly washed. After removing the traces of lime possibly present, the amount of magnesia contained in the filtrate is determined as pyrophosphate, from the weight of which the proportion of free sulphuric acid originally contained is calculated. The presence of metallic impurities in vinegar is detected by means of the usual reagents, such as hydrosulphuric acid and ammonium sulphide. In addition to water and sulphuric acid, the most common adulterants of vinegar are capsicum, sulphurous acid and various colouring matters. The presence of capsicum and other acrid substances is usually revealed by the pungent odour produced upon burning the solid residue obtained by the evaporation of the sample to dryness, and by the peculiar taste of the residue. Sulphurous acid is sometimes detected by its characteristic odour; its determination is described on p. 177. Caramel is recognised by extracting the solid residue with alcohol, and evaporating the solution to dryness; in its presence, the residue now obtained will possess a decidedly dark colour, and a bitter taste. Fuchsine, which is said to have been employed for colouring vinegar, is detected by the tests mentioned under Wine.