Alums are only valuable in leather manufacture in proportion to the aluminium sulphate which they contain, the potassium or ammonium sulphate taking no part in the reaction, and since improved methods have rendered possible the production of aluminium sulphate practically free from iron, it has largely taken the place of alum, than which it is both cheaper and stronger. Crystallised aluminium sulphate, Al₂(SO₄)₃, 18Aq, has a molecular weight of 666, which is of equal value to 906 of ammonia-alum, and 948 of potash-alum. Iron is the most objectionable impurity in both alums and aluminium sulphate, and may be detected by the addition of potassium thiocyanate, which will produce a red colour, or potassium ferrocyanide (yellow prussiate of potash), which will produce a blue. As the iron may be present in the ferrous condition, it is safer first to boil the alum solution with a few drops of nitric acid or bromine water. For more accurate determination of iron see L.I.L.B., pp. 20, 136. No satisfactory leather can be produced with a solution of alum or aluminium sulphate alone, the skin drying horny, and incapable of softening by stretching. In practice, salt is always used in addition, the proportion being very variable, but averaging about half the weight of alum, or two-thirds the weight of sulphate of alumina employed. The mode of action of the salt has long puzzled chemists, and it has been supposed that its use was to convert the aluminium sulphate into chloride, a reaction which takes place to some extent, but which fails to explain the production of a soft leather, since aluminium chloride, though freely taken up by the skin, produces no more satisfactory leather than aluminium sulphate. The real explanation is found in Chapter IX. Alumina is a weak base, which readily gives up its acid to the pelt, becoming converted into a basic salt (see p. 187). The acid not only swells the pelt, and renders it incapable of producing a soft leather, but the swollen pelt is less ready to absorb the alumina salt, and so remains undertanned. The addition of salt prevents the swelling effect of the acid, and produces a partial pickling of the skin (p. 89), which, in conjunction with the tanning effect of the basic alumina salt formed, yields a satisfactory leather, though one which is readily affected by washing. If instead of adding common salt to the alum solution, an alkali such as soda is added, it combines with a portion of the acid, forming sodium sulphate, while the alumina remains in solution as a “basic salt.” As the term “basic salt” must be frequently employed in connection with mineral tannage, it may here be explained. Basic salts are compounds intermediate between the normal salt, in which the whole of the base is combined with acid, and the hydrated oxide in which the whole is combined with OH groups. Thus aluminium chloride, Al₂Cl₆, is a normal salt, in which the whole of the combining powers of the