476 SMITH'S INTERMEDIATE CHEMISTRY 



very dilute solutions of aluminium sulphate, ferric chloride FeCl 3 

 and chromous acetate Cr(C0 2 CH 3 ) 2 , then add a few drops of a 

 solution of a dye to each, and finally introduce a little of a base 

 (like sodium hydroxide) to precipitate the hydroxide of the metal, 

 this hydroxide will adsorb the dye and carry it into the precipitate. 

 Such a precipitate of mordant and dye is called a lake. With 

 the same dye, the three lakes have different colors. Thus, in 

 the above-mentioned experiment, if alizarin (madder) is used as 

 the dye, the colors are red (Turkey red), violet, and maroon, 

 respectively. This is probably due to the different degrees of 

 dispersion in the three colloidal materials. If aluminium hydrox- 

 ide is to be used, by first saturating the cloth with hot aluminium 

 acetate solution (p. 468), or by using first aluminium sulphate 

 and then ammonium hydroxide, the aluminium hydroxide is 

 precipitated within the fibers of the goods. When the material 

 is then dyed, the coloring matter is adsorbed by the mordant, with 

 which it forms an insoluble lake, within the fibers. Basic dyes, 

 like Malachite green and Methylene blue, behave similarly with 

 tannic acid, or an insoluble salt of tannic acid, as mordant. It 

 will be seen that, so far as the fabric is concerned, this process, 

 like the first, is a mechanical one, and is independent of the chem- 

 ical nature of the goods. 



3. Direct or Substantive Dyes. Most organic dyes are direct 

 dyes on silk or wool, and require no mordant with these materials. 

 The actions seem to be sometimes chemical, but more often cases 

 of adsorption by the silk or wool (both colloids) themselves. A 

 few dyes are also fast on cotton. Congo-red is fast both on cotton 

 and wool, but is no longer much used. Chrysophenin is now one 

 of the commonest dyes of this class. These dyes, which are 

 sodium salts of complex organic acids, are colloids like soap (p. 

 442), and are salted out within the fibers of the goods by adding 

 sodium sulphate to coagulate them and assist the adsorption by 

 the cotton. Once adsorbed in this way, unlike soap, they cannot 

 be washed out. 



