=Application.=—These facts were discovered by Perkin while trying to introduce his Mauveine into the dyeing industry, and he discovered the methods, used to this day, for applying these dyes to the different textile materials. He found that the dyes of this class have a strong affinity for the different animal fibres, such as wool, silk, leather, etc., all of which seem to possess some acid properties of their own; but pure vegetable materials, like cotton, linen, and paper, from which all impurities such as vegetable acids, gums, etc., have been removed, have no affinity at all for even the most powerful of the Basic dyes. A cotton handkerchief, boiled for hours in a strong solution of Methyl Violet, can be washed in a few minutes clear of every particle of color, while a piece of silk or wool, soaked for an instant in the same dye-bath, will be permanently stained, deep and full. _Cotton, Linen, etc._—In order to fasten these dyes to vegetable fibre it is necessary to give the latter a distinctly acid character, and this was done by Perkin in a manner still used. He steeped the material for several hours in a hot bath of the acid vegetable compound, tannic acid or tannin, found so largely in hemlock and chestnut bark, sumac leaves, nut-galls, and the like; and then loosely fixed the tannin, thus absorbed, by a weak bath of tartar emetic. Cotton or linen fabrics, thus “mordanted,” will combine with the Basic dyes as readily and as firmly as any animal fibre, and the resulting colors, while not, as a rule, fast to light, are extremely fast to washing. Since the introduction of the direct cotton dyes, both Salt colors and Sulphur colors, this method of dyeing, for skeins or piece goods, has been largely discontinued; but, by using a modification of this process, enormous quantities of Basic colors are still employed, on cotton and linen, in the manufacture of calicoes, organdies, and other printed fabrics. Curiously enough the Salt and the Sulphur colors, in almost every instance, possess sufficient acid properties of their own to act as very fair mordants for the Basic colors. Accordingly, it is not uncommon for dyers to “top,” with Basic colors, cotton or linen goods dyed directly. In the case of the Salt colors, this increases their fastness to washing, and with Sulphur colors it makes the shades more brilliant. Most vegetable materials that are used in a more or less natural condition, like straw, raffia, grass, wood-shavings, jute, and the like, contain enough of this natural tannic acid to act as a mordant for the Basic colors, which may in this direction be used as direct dyes.