With regard to light-fastness, however, the case is different. A great many hundreds, possibly even thousands, of Acid dyes have been discovered, and scores of them, covering every shade, can be obtained in the open market. Most of these are of but little permanence, but a few products, from each of the great color houses, can be selected, whose fastness to light is extremely satisfactory. The dyes in the following list can hardly be considered as fast as the Vat dyes, previously described, but are probably faster, as a class, than any other class mentioned in this book. They would rank at the very top of the second class, and some at least would fairly enter the first class, being absolutely satisfactory against even the strongest sunlight. A series of skeins, dyed all colors of the rainbow, including many delicate light shades, with a red, yellow, and blue dye of those mentioned below, withstood an exposure test which quite ruined a similar set of skeins dyed with the very best natural dyestuffs. And a large hand-woven rug, made of wool dyed light shades with the same dyes, was placed for two weeks on a roof in New York, half of it being covered with boards and the rest exposed to the direct action of the July sunlight, and at the end of this time it was impossible to notice any difference in shade. The colors in the following list are to be used, principally, for wool. They will all dye silk, leather, and feathers, but in the chapters dealing with those materials some additional dyes may be mentioned, which are specially suited for them. _List of Selected Dyes._— Badische— Palatine Scarlet A, 3 R Palatine Light Yellow, R Tartrazine (yellow) Wool Fast Blue, B L Cassella— Brilliant Cochineal, R R Acid Yellow, A T, conc. Tetracyanol, S F Elberfeld—Azo Crimson, S Fast Red, A Fast Yellow, 3 G Alizarine Blue, S A P Cashmere Black, 3 B N Kalle— Biebrich Acid Red, 2 B Wool Yellow, T A Nero cyanine Blue, B Nero cyanine Black, D Metz— Fast Acid Red, M Fast Acid Orange, G Fast Acid Yellow, 3 G Fast Acid Blue, B B DYEING DIRECTIONS The Acid dyes, like the Basic, are used in an acid bath, but for a different reason. With the Basic dyes acetic acid or some other weak acid is added, for the purpose of readily dissolving the color. In the case of the Acid dyes, however, the dyestuffs are almost always put on the market in the form of the potassium or ammonium salts of the color acid. And the presence of some acid is always necessary, to liberate the color acid, and allow it to combine with the basic principles existing in the animal fibres.