The art of dyeing does not simply consist in coloring the different substances; if this were the case, the color would be washed out as easily as imparted. Nearly all the colors used in dyeing being of vegetable origin, it is necessary to apply some substances to the fabrics to be dyed which shall fix the colors in the grain. Of these (called “mordants”) the chief are acetate of alumina, acetate of iron, and chloride of tin (substances well known to chemists), which have the property of making the coloring matter insoluble, so that it cannot be washed out. There are various ways of dyeing. If the fabric has to be dyed all one color, it is dipped in the mordant, dried, and afterwards boiled in a solution of the dye-stuff. If a colored ground with white figures is required, then the figures are printed on it with what is called a “resist,” that is, some substance thickened with gum, paste, or pipe-clay, which will resist the action of the mordant and dye-stuff, so that when the fabric is afterwards rinsed these figures remain white. If the ground is to be white and the pattern dyed on it, then this is printed with the mordant and the color adheres only to the parts printed, although the whole be boiled with the dye-stuff, what little may adhere to the ground being easily washed out in the rinsing. Sometimes the kind of mordant used determines the color of the dye; for instance, madder will dye red with the chloride of tin, and black with the acetate of iron. If we now suppose a piece of cloth to be printed in lines of acetate of iron, and figures of chloride of tin, when the whole is boiled in the madder vat, the cloth comes out with black lines and red figures on a white ground. Another point to be considered is the kind of stuff to be dyed, whether cotton, silk, or woollen, or a mixture of either, for what will take one kind of dye will not always take another, and advantage is taken of this to dye two colors at once, or part white and part colored. A table-cover, for instance, woven with worsted and cotton, can be boiled in various kinds of dye, and produce white and blue, or white and red, for that dye is selected which will attach itself only to one kind of fabric, and leave the other. Illustration: SCOURING. Many other chemical qualities of the substances used as dyes, and of the substances to be dyed, are also taken into consideration. One of the chief of these is shown in dyeing with indigo. This substance (a vegetable extract) as brought to England is quite insoluble in water and unfit for use as a dye; but it is made soluble, and at the same time almost colorless, by mixing with lime, sulphate of iron, and water. These substances deprive it of its oxygen, and the straw-colored solution is then used for dyeing, the substances boiled in it attaining--as the indigo regains its oxygen on exposure to the air--a deep blue color.