GALLS are excrescences produced on the _quercus infectoria_, a species of oak growing throughout Asia Minor. The gall grows on the shoots of the young boughs, and is produced by an insect, the _cynips quercusfolii_; this insect punctures the tender shoot with its sting and deposits its egg in the puncture; the egg is soon hatched, and the irritation of the maggot feeding on the plant produces the wen or gall-nut. When the nuts are gathered before the worm within changes to a fly, and not yet having eaten its way out, they are of a dusky green colour, and are called in commerce _blue_ galls, and are by far the best. Those collected after the fly has eaten its way out have a hole in each, are of a whitish yellow colour, considerably lighter than the blue-galls, and of an inferior quality: they are brought to this country chiefly from Aleppo. They are used in large quantities in the arts, principally for dyeing, and making ink. They contain a large quantity of _Tannin_ and _Gallic acid_. INDIGO is a well known deep blue substance, obtained from the _Indigofera tinctoria_ or Indigo bearing plant, a native of the East Indies, which is propagated by seed and will thrive in most tropical climates; hence we have good indigo from South America, the East Indies, Carolina, &c. The chief criterion of the goodness of indigo is, if, when cut with a knife, it exhibits a reddish copper-like appearance; where this shade is not, or only very slight, the indigo is of inferior value. It is prepared by macerating the leaves in water, whence is obtained the blue feculence or indigo. Indigo is insoluble in water, but soluble in sulphuric acid, hence a solution of it in this acid, forming a _sulphate of indigo_, is well known in the art of dyeing. The best indigo is that called _Flora_, which floats in water, all the other kinds sink in that fluid. The constituent parts of indigo are Carbon, 73.22, Nitrogen 11.26, Oxygen 12.60, and Hydrogen, 2.92, = 100. When indigo is digested in concentrated sulphuric acid, it is converted into a peculiar blue substance, commonly called _sulphate of indigo_; this colouring matter has been, however, lately named CERULIN, by MR. W. CRUM, who has made many experiments on it; (see notes to _Bertholet_, vol. ii. p. 357. et seq.) he observes that _cerulin_ dissolves more abundantly in sulphuric acid than water; but this does not prove the formation of a compound entitled to be called sulphate of indigo; that, such a solution differs in no respect from that of resins in acids or in alcohol. Another substance has been also obtained from indigo by MR. CRUM, of a purple colour, which he calls _Phenicin_; it dissolves both in water and alcohol. IRON rarely in its metallic state enters into the manipulations of dyeing, but its _sulphate_, _muriate_, _acetate_, &c. as well as its _oxides_ contribute largely to the dyer's art.