aluminium are saturated with chlorine: aluminium hydrate, Al₂(OH)₆, is the hydrated oxide, and Al₂Cl₅OH, Al₂Cl₄(OH)₂, and so on are basic salts in which successively more of the Cl is substituted by OH. Generally, as a salt becomes more basic, its solution in water becomes more unstable, and very basic salts are either insoluble, or are precipitated from their solutions by very trifling causes, such as boiling, dilution, or the attraction of animal or vegetable fibres; separating into free acid and either hydrate or a still more basic and insoluble salt. On this property depends their importance in tanning and dyeing, many of the metallic mordants being solutions of basic salts. Basic salt solutions are formed in various ways, the most common being the direct solution of a hydrated oxide in a solution of the normal salt, or the neutralisation of a part of the acid of the normal salt by the addition of a stronger base. This is what takes place on the addition of soda to an alum solution. If the soda is added in excess, the whole of the alumina is precipitated as hydrate, or as an insoluble basic salt, but if a proportion not exceeding about four parts of crystallised sodium carbonate be dissolved separately, and added _slowly with constant stirring_ to the ten parts of alum dissolved in water, no precipitation will take place. In this solution leather can be tanned, either with or without addition of salt, the alumina is taken up more freely than from the normal alum, and the leather is more easily softened, and more resistant to water. In fact such leather bears a strong resemblance to the chrome tannages, standing a great deal of washing, and considerable temperatures without returning to a pelty condition. The more basic the solution that is used, the fuller and softer is the leather produced. The alumina-salt taken up by the skin from such basic solutions is always basic, while that absorbed from alum or alumina sulphate is _apparently_ the normal aluminium sulphate. It is probable however that the actual tanning salt is in both cases basic, and that the acid is fixed as free acid, as in the pickling process, as the proportions of acid and base found in the residual liquor are somewhat variable. Basic alumina solutions have hardly taken the place in practice which they deserve, though they were described by Knapp in 1858[112] and have since been patented by Hunt, but the patent (probably invalid) has been allowed to lapse. A good stock solution for practical use is made by dissolving 10 lb. of sulphate of alumina in 10 gallons of water, and 4 lb. of washing soda in 4 gallons, and gradually mixing the latter with the former. Salt can be used in addition if desired, and flour and egg-yolk may also be added. [112] ‘Natur und Wesen der Gerberei,’ Braunschweig, 1858.