222. Why the Color Returns. Some of the compounds formed by the sulphurous acid bleaching process are gradually decomposed by sunlight, and in consequence the original color is in time partially restored. The portion of a hat protected by the band retains its fresh appearance because the light has not had access to it. Silks and other fine fabrics bleached in this way fade with age, and assume an unnatural color. One reason for this is that the dye used to color the fabric requires a clear white background, and loses its characteristic hues when its foundation is yellow instead of white. Then, too, dyestuffs are themselves more or less affected by light, and fade slowly under a strong illumination. Materials which are not exposed directly to an intense and prolonged illumination retain their whiteness for a long time, and hence dress materials and hats which have been bleached with sulphurous acid should be protected from the sun's glare when not in use. 223. The Removal of Stains. Bleaching powder is very useful in the removal of stains from white fabrics. Ink spots rubbed with lemon juice and dipped in bleaching solution fade away and leave on the cloth no trace of discoloration. Sometimes these stains can be removed by soaking in milk, and where this is possible, it is the better method. Bleaching solution, however, while valuable in the removal of some stains, is unable to remove paint stains, because paints owe their color to mineral matter, and on this chlorine is powerless to act. Paint stains are best removed by the application of gasoline followed by soap and water. CHAPTER XXIII DYEING 224. Dyes. One of the most important and lucrative industrial processes of the world to-day is that of staining and dyeing. Whether we consider the innumerable shades of leather used in shoes and harnesses and upholstery; the multitude of colors in the paper which covers our walls and reflects light ranging from the somber to the gay, and from the delicate to the gorgeous; the artificial scenery which adorns the stage and by its imitation of trees and flowers and sky translates us to the Forest of Arden; or whether we consider the uncounted varieties of color in dress materials, in carpets, and in hangings, we are dealing with substances which owe their beauty to dyes and dyestuffs. The coloring of textile fabrics, such as cotton, wool, and silk, far outranks in amount and importance that of leather, paper, etc., and hence the former only will be considered here; but the theories and facts relative to textile dyeing are applicable in a general way to all other forms as well.