It should be remembered, in this connection, that the comparative fastness to light depends largely (a) upon the materials to be dyed, and (b) upon the depth and shade of color used in the test. For instance, if a skein of heavy cotton yarn, and one of very fine, brilliant, artificial silk are dyed the same color, and exposed to light under the same conditions, the cotton skein will hold its color longer than the silk. The latter, being semi-transparent, allows the sunlight to pierce it through and through, while the more opaque cotton gives some distinct protection to the color that has penetrated beneath the surface. So, too, a dark shade of any given color will stand the light much better than a very light or delicate shade, for the same general reason. The color beneath the surface is protected from the direct action of the sun’s rays by the surface color. _Fastness to Washing._—The test for washing-fastness is made somewhat differently. A skein dyed a full shade with the color is twisted up with two white skeins, one of wool and the other of cotton, and the three are thoroughly scoured for ten minutes in a strong bath of good quality laundry soap, heated to 140°F. This temperature is uncomfortably hot for the hands and yet is well below the boiling point. A _fast_ color is one where, with this treatment, neither the soap liquor nor either one of the skeins becomes colored. If the soap liquor is colored but neither one of the skeins, the dye is called _fairly fast_. If the soap bath is tinged, and one or the other of the skeins becomes colored at the same time, the dye is considered _not fast_. It must, however, be borne in mind that before making this washing-test, all excess of dye-liquor must first be removed by thorough rinsing. And it should be remembered that even the fastest of the Salt colors, as well as of the Acid and Basic colors described later, when applied directly to the fibre, without mordanting or after-treating, are never as fast to washing as those where the dyestuff is fixed or developed in an insoluble form in the fibre, by the action of the air, as are the Sulphur and Vat colors—or by the action of mordants, as with the Alizarine colors—or by after-treatment with certain special chemicals, as with the Salt colors in the process described below. All dyes can, sooner or later, be dissociated from the fibres to which they are attached. But if they are in an insoluble condition they drop off in the form of a powder, and are washed clean off, and leave sharp, clear outlines on the dyed goods. If, however, they have gone on in solution they will go off in solution, and are liable to _bleed_, and stain light-colored fibres near them. The earlier dyestuffs of this class were deservedly criticised as being, even when carefully applied, much given to bleeding, and also distinctly fugitive to the action of sunlight.