_Sulphurous Acid._--For the detection and estimation of sulphurous acid, the following methods have been recommended:--500 c.c. of the wine are placed in a flask, the exit-tube of which dips into a test-tube which is suitably cooled, and subjected to distillation. When about 2 c.c. have distilled, a few drops of a _neutral_ solution of silver nitrate are added to the distillate: in presence of sulphurous acid, a white curdy precipitate will be formed, which differs from silver chloride in being soluble in nitric acid. According to Haas,[104] this test is not invariably decisive, as pure wine may cause the precipitation under certain conditions; moreover, acetic acid is said to render silver nitrate turbid in strong alcoholic solutions. Sulphurous acid can be quantitatively determined by adding phosphoric acid to 100 c.c. of the wine, and distilling it in an atmosphere of carbonic acid gas. The distillate is received in 5 c.c. of normal iodine solution. When one-third of the sample has passed over, the distillate (which should still contain an excess of free iodine), is acidulated with hydrochloric acid, and the sulphuric acid formed precipitated with barium chloride. _Colouring matters._--Very numerous processes have been published for the detection of foreign and artificial colouring matters in wine. Among those suggested are the following:-- 1. A few drops of the sample are placed in succession on the smooth surface of a piece of white calcined lime, and notice taken of the tint produced. The following colours are stated to occur with pure and artificially coloured wine:-- Natural red wine yellowish brown. Wine coloured with fuchsine rose colour. „ „ „ Brazil wood „ „ „ „ „ logwood reddish violet. „ „ „ black hollyhock yellowish brown. „ „ „ poke-berries yellowish red. 2. If ammonium hydroxide be added to the suspected sample to distinct alkaline reaction, then a little ammonium sulphide and the liquid filtered, the filtrate from genuine wine will possess a green tint, whereas that obtained from artificially coloured wine will exhibit other colours, such as red, blue, violet, or brown.