In alum the whole acidity of the sulphuric acid is not neutralised; the solution has always an acid reaction; if soda solution is added, the escaping carbonic acid causes the liquid to effervesce. If soda solution is added until a further addition would cause a precipitate, a solution of so-called neutral alum is formed which has no longer an acid reaction. Neutral alum is occasionally required in colour making. In preparing it the soda solution must be added with great care when the liquid is near its point of neutralisation. Any addition of soda solution after this point is reached will cause a separation of alumina. This is not desirable, since it is generally only wished to precipitate the alumina in combination with colouring matters. =Roman Alum.=—Under this name, or that of “cubic alum,” a variety of alum is sold, generally at a rather higher price than ordinary alum, from which it is distinguished by its crystalline form. Ordinary alum forms octahedral crystals often the size of a child’s head, but cubic alum well formed cubes. The property of crystallising in cubes may be imparted to any alum solution by the addition of a little potash. Much so-called Roman alum is made in German works in this way. When this alum contains very little iron it is quite equal in quality to the best Roman alum, for the higher value of the latter is entirely due to its small content of iron. The alum prepared in the province of Naples is still better than Roman alum; it contains less iron. When alum is required for the preparation of lakes of bright and delicate shades, it is indispensable to use a preparation free from iron, because the brownish yellow oxide of iron would appreciably injure the shade. Alum, free from iron, is most simply prepared by dissolving alum in boiling water, running the boiling solution quickly through a cloth, and quickly cooling with constant stirring. The alum meal prepared in this way contains much less iron than the original alum, the iron salts remaining dissolved in the mother liquor. When this alum meal is collected and cold water poured over it to free it from mother liquor, it is generally sufficiently pure to be used for any purpose, but if not, it is again recrystallised. The alum liquors containing the iron are used for the preparation of colours which are not injured by the presence of iron.