THE SELECTION OF DYES 169 



of the tropical logwood tree the blacks and grays, and the 

 fruit of certain palm and locust trees yielding the soft browns. 

 So great was the commercial demand for dyestuffs that large 

 areas of land were given over to the exclusive cultivation of the 

 more important dye plants. Vegetable dyes are now, however, 

 rarely used because about the year 1856 it was discovered that 

 dyes could be obtained from coal tar, the thick sticky liquid 

 formed as a by-product in the manufacture of coal gas. These 

 artificial coal tar, or aniline, dyes have practically undisputed 

 sway to-day. 



Wool and cotton dyeing. If a piece of wool is soaked in 

 a solution of a coal tar dye, such as magenta, the fiber of the 

 cloth draws some of the dye out of the solution and absorbs 

 it, becoming in consequence beautifully colored. The coloring 

 matter becomes " part and parcel/' as it were, of the wool 

 fiber, because repeated washing of the fabric fails to remove 

 the newly acquired color; the magenta coloring matter unites 

 chemically with the fiber of the wool, and forms with it a com- 

 pound insoluble in water, and hence fast to washing. 



But if cotton is used instead of wool, the acquired color is 

 very faint and washes off readily. This is because cotton 

 fibers possess no chemical substance capable of uniting with 

 the coloring matter to form a compound insoluble in water. 



That vegetable fibers, such as cotton and linen, act differ- 

 ently toward coloring matter from animal fibers, such as 

 silk and wool, is not surprising when we consider that the 

 chemical natures of the two groups are very different. Vegetable 

 fibers contain only oxygen, carbon, and hydrogen, while ani- 

 mal fibers always contain nitrogen in addition, and in many 

 cases sulphur as well. 



The selection of dyes. There are, of course, many excep- 

 tions to the general statement that animal fibers dye readily 

 and vegetable fibers poorly, because certain dyes fail utterly 



