The application of indigo to the coloring of textile fabrics requires the complete dissolving of the substance, for which the mechanical division is only a preliminary. There are only two known means of dissolving this substance: 1. By reduction; 2. By the action of concentrated sulphuric acid. The first means allows indigotine to be regenerated; and, when the dyeing is completed, it is pure indigotine which adheres to the colored fibre. By the second means, or dissolving by sulphuric acid, the coloring material enters into a new combination, from which it can never be separated: it becomes a new substance, endowed with new and special properties. _The fixing of Indigotine by means of Reduction_.—In this method the operator avails himself of one of the most remarkable qualities of indigotine: this is the facility with which this body takes up hydrogen, and becomes transformed into a colorless substance, which is soluble in favor of alkaline or alkaline-earthy bases, and is susceptible of reproducing indigotine by simple oxidation in contact with air. This hydrogenized substance is called white indigo. Blue indigo, or indigotine, is insoluble except by concentrated sulphuric acid; and this insolubility gives it its superiority to all other blue dyes. Not being soluble, it cannot, as blue indigo, attach itself to the material to be dyed; but in the soluble form of white indigo it can perfectly penetrate the fibre. If by any means of oxidation we can transform the white indigo into blue indigotine, the latter becomes insoluble, and is imprisoned in the pores of the fibre. This is, briefly, the whole theory of the use of indigo in dyeing or printing, although the reaction may be applied in different ways to the coloring of fibres, such as— 1. The indigo is dissolved by means of an alkaline reduction in a vat, and the fibre is immersed in the bath. This is the common blue vat. 2. The solution prepared beforehand is painted by a hair pencil and printed by a stamp or roller upon only certain parts of the tissues. This is the pencil blue. 3. The white indigo is precipitated under the form of a paste, in combination with a metallic oxide having strong reducing power, such as hydrated protoxide of tin, which prevents the too rapid reoxidation of the indigotine. The thickened paste is printed, and the tissue is placed in an alkaline bath (lime or soda), which, displacing the oxide of tin, forms a soluble combination of white indigo. The latter can then penetrate the fibre, and afterwards become fixed by reoxidation. This is the printer’s solid blue. 4. The finely ground, but not dissolved indigo, is placed upon the tissue in such conditions that it can be dissolved and reduced in place. This done, the fixing of the indigotine is effected by oxidation. This is the method for China blue or _bleu faïence_. Without dwelling upon the details of these methods, we hasten to a consideration of the most important of all the applications of indigo:—