It is therefore advisable when matching shades that are to be used at night not to use three-color shades wherever that is possible, but to get the desired soft effects by covering directly with grey (i.e., light shades of black) on top of a single or two-color shade. CHAPTER V THE SULPHUR COLORS Nearly thirty years ago one of the French color houses put on the market a new dyestuff which it named “Cachou de Laval”; Cachou being the same as catechu or “cutch,” the natural brown dyestuff long known and used in the East, and Laval being the name of the town in France where one of its discoverers was born. This dyestuff was made by heating sawdust, bran, turf, leaves, or other vegetable substances with the strongly reducing alkaline salt, _sodium sulphide_, in the absence of air. The product, dissolved in water, makes a dark green solution which, after standing in the air a short time, turns brown and deposits a fine brownish powder. Cotton or linen, heated in a fresh solution of this dyestuff, is colored green, but, when wrung out and exposed to air, the green color, which easily washes out, changes into a very permanent, though dull and uninteresting, shade of greyish brown. This Cachou de Laval was not a success, commercially, because of its poor color. It existed, however, as a chemical curiosity for some twelve or fourteen years; then suddenly, within a few months or even weeks of one another, all the great color houses put out a whole series of colors—chiefly browns, blues, yellows, and blacks—all formed, like this old “Cachou de Laval,” by the action of sodium sulphide or, which amounts to the same thing, of sulphur and caustic alkali, upon organic material, and all capable of dyeing cotton and linen, in one bath, colors extremely fast to washing and generally quite fast to light, after they have been “set” by exposure to the air. While in general these are known and identified as the Sulphur colors, the different manufacturers have given special class names to their own series thus: Immedial (_Cassella_), Katigene (_Elberfeld_), Kyrogene (_Badische_), Pyrogene (_Klipstein_), Thiogene (_Metz_), Thion (_Kalle_). These colors are used almost exclusively for dyeing cotton and linen, when shades fast to washing are required, without first putting them through a mordanting process. The dyeing is done in one bath, with little more difficulty than in the case of the Salt colors described in the last chapter; and, while not faster to light than the best of that class, they are not nearly so liable to bleed. On wool they are rarely, if ever, used. Wool is almost always dyed with the acid colors in an acid bath; and nowadays the range of these colors is so great and the best of them are so very satisfactory, that there is hardly ever a necessity for using colors of another class.