SYNTHETIC ORGANIC PRODUCTS 475 



able than are those on cotton, and when samples of the three 

 materials are dipped in a solution of a dye, the first two are per- 

 manently dyed, while from the last most dyes can be completely 

 washed out with water. 



Three modes of dyeing may be mentioned: 



1. Insoluble Dyes. If the colored body can be produced by 

 precipitation, after the solution has filled the capillary and wall 

 of every fiber of the goods, then, if the dye is sufficiently insolu- 

 ble, it is mechanically imprisoned in every fiber and cannot be 

 washed out. This plan may be applied to any kind of goods. 

 For example, if cotton, silk, or wool is first boiled in a solution of 

 lead acetate, and is then soaked in a boiling solution of potassium 

 chromate K 2 Cr0 4 , it is dyed a brilliant, permanent yellow. Lead 

 chromate is the colored body: 



Pb(C0 2 CH 3 ) 2 + K 2 Cr0 4 +2K(C0 2 CH 3 ) + PbCrO 4 J, . 



The part precipitated on the outside of the goods can be, and is, 

 at once washed off by rubbing in water, but the particles inside 

 the fibers can come out only by being dissolved, and they are 

 insoluble in water. Indigo Ci 6 Hi e N 2 2 , which is used in larger 

 amounts than any other dye, belongs to this class. The cloth 

 is saturated with an alkaline solution of indigo white Ci 6 Hi 2 N 2 O 2 , 

 a soluble, slightly acid substance, and the oxygen of the air subse- 

 quently oxidizes this and deposits the insoluble indigo blue within 

 the fibers: 



2Ci 6 Hi 2 N 2 O 2 4- O 2 -> 2Ci 6 Hi N 2 2 j + 2H 2 0. 



2. Mordant or Adjective Dyes. Since cotton is inactive chem- 

 ically and has but a slight tendency to adsorb dyes, it is usually 

 necessary first to introduce into the fibers of cotton some colloidal 

 substance with greater adsorptive powers. Substances of this 

 kind are tannic acid (p. 499) for basic dyes, and gelatinous col- 

 loidal hydroxides, such as those of aluminium, tin, iron and chro- 

 mium, for non-basic (including acid) dyes. They are called mor- 

 dants (Lat. mordere, to bite). Thus, if in three jars we place 



