202 ACIDS, ALKALIES, AND CLEANING 



so easily harmed by alkalies. In fact, if cotton is treated 

 with strong alkali for a short time, and is then washed 

 thoroughly, it is actually made stronger, and has a glossy, 

 silky appearance. Cotton so treated is called mercer- 

 ized cotton. 



Wool and silk (Fig. 198) are of animal origin, and con- 

 sist not only of carbon, hydrogen, and oxygen, as cellulose 

 does, but contain nitrogen also (cf. 

 123). They are the opposites of cotton 

 and linen in chemical behavior; for 

 they are not readily acted upon by 

 acids, but are easily destroyed by 

 FIG. 198. alkalies. The fibers of wool are entirely 



Co t c he siikworm. iber3 f different from those of linen and cotton. 

 Instead of consisting of long cells, wool 

 is made up of short, thick cells, with the projecting edges 

 of each lapping over part of the one next to it (Fig. 197). 

 The surface thus appears to be covered with horny scales 

 all lying in one direction. No method of washing wool 

 should be used that will force the cells closer together, 

 or the wool will shrink, and finally become stiff and 

 board-like. 



228. Dyes. Dyes are colored compounds that are 

 either absorbed by the pores of a fiber, or combine chemi- 

 cally with the fiber to form a colored, insoluble compound. 

 Silk and wool are much more active chemically than cotton 

 and linen. Hence silk and wool can combine directly 

 with many more dyes. We can think of the process of 

 dyeing cloth as a reaction much like that which occurs 

 when an acid and a base unite to form a salt (cf. 220). 



