Among the many changes made in the art of dyeing since the introduction of the coal-tar dyestuffs, perhaps the most important has been the gradual overcoming of the necessity for mordanting the textiles before coloring them in the dye-bath. Almost all of the old vegetable dyes were mordant dyes; that is, the color could not be fastened to the fibre, whether wool, cotton, linen, or even silk, unless the latter had been impregnated with some chemical which would act as a _mordant_ to—(i.e., would combine with and hold) the color. These mordants were, in general, the salts of some metal, aluminium, tin, chromium, and iron salts being the ones in common use; and the processes involved in properly mordanting the goods were in many cases—notably in the case of madder and the Turkey red process—far more difficult and tedious and expensive than the actual dyeing. The first dyestuffs discovered, the true aniline dyes, which were manufactured directly from aniline and from substances strongly resembling aniline in chemical composition, were at once found to act in a different manner on textile fibres. Animal fibres like wool and silk, fur and leather, were dyed by them directly, without the use of any mordant at all. If the dyestuff were dissolved in water (the addition of a little acid makes the color dissolve more readily, but is without other effect) and a wet skein of wool or silk were immersed in it, and a little heat applied, the color would leave the liquid, and fasten itself firmly on to the goods. But with cotton and linen and other vegetable fibres, these dyes would not work so well. When these materials are warmed in such a dye-bath, the color does not adhere to the fibres, but washes off directly in a hot soap bath, if not, indeed, under a stream of clear hot water. This was noticed by Perkin very soon after his famous discovery, and, wishing to use his new color for dyeing cotton and linen as well as silk and wool, he set to work to discover how to prepare these materials; in short, how to mordant them so that they too would take firm hold of the color. As will be described later in the chapter on Basic colors, his experiments soon led to the introduction and the use of tannic acid and tartar emetic combined, in a process widely used to this day. The next class of dyestuffs discovered were the so-called acid colors, thus named because they all exhibited distinctly acid properties—that is, they would form salts with the substances known as bases (of which last, by the way, aniline is an important member). These colors, like the earlier ones, would dye the animal fibres directly, but would not color the vegetable fibres, unless the latter were carefully mordanted with alumina, or iron oxide, or some similar metallic base. And even this treatment does not give colors that are fast to washing, so these acid colors are never used on cotton or linen.