128 MODERN SCIENCE READER 



began the first serious competition between a natural and 

 an artificial dyestuff. The latter won, for a root yielding 

 one or two per cent, impure coloring matter and requiring 

 valuable land for its cultivation could not long compete 

 with factories producing the pure dyestuff from coal tar. 



In dyeing with most of the colors so far mentioned the 

 yarn must first be impregnated with a substance, called a 

 mordant, which has the power to fix the color. Different 

 dyes require different mordants. Dyes which are fixed by 

 compounds of alumina or chromium are called, simply, 

 mordant dyes. Tannin dyes are those which require the 

 application of tannin as a mordant. After a time the 

 chemists succeeded in producing dyes which color wool and 

 silk without the use of any mordant. As these dyes were 

 obtained by treating tannin dyes with sulphuric acid, and 

 are used in a slightly acid solution, they are called acid 

 dyes. In this class belong the azo dyes, the manufacture 

 of which, commenced about 1875, has developed amazingly. 

 They are now the most important of all dyes and have been 

 the most formidable competitors of cochineal and dyewoods. 



In 1884 appeared the first direct cotton dye; that is, a 

 dye which colors vegetable as well as animal fibers, without 

 mordanting. Many such dyes were soon produced and 

 were eagerly adopted by dyers, as they saved both time 

 and money. They were not as fast as the mordant dyes, 

 but this defect was remedied toward the end of the century 

 by the production of the sulphur dyes, which also require 

 no mordant on either vegetable or animal fibers and yet 

 leave little to be desired in point of permanence. 



Some of the direct cotton dyes can be changed in color, 

 after application, by treating the dyed goods with certain 

 reagents. Thus blue can be changed to black, and yellow 

 to bright red, and the new colors are more permanent than 

 the originals. This suggested the production of colors 

 within the fibers, by the reaction of two colorless substances. 

 Many shades are now produced in this way. They are 

 called ice colors because the goods are usually cooled with 



