CHAPTER II 



THE GENERAL NATURE OF DYES AND 

 THEIR CLASSIFICATION 



DYES are generally classed in two groups, the natural and the 

 artificial. The former class is now of relatively smaller im- 

 portance from the standpoint of the manufacturer and the 

 textile dyer; for the artificial dyes far outnumber them and the 

 advancement of science is gradually making it possible to produce 

 many of the formerly natural dyes by artificial means. It just 

 happens that one or two natural dyes, the derivatives of cochineal 

 and logwood extract (see Chap. IX), are among the most valuable 

 biological stains; but the natural dyes in general are so few in num- 

 ber than they can be practically disregarded in considering the 

 general chemical nature of dyes. 



Because the first artificial dyes were produced from anilin, all of 

 this class are often called "anilin dyes," although there are now a 

 large number of them which bear no relation to this compound and 

 are not derived from it. Therefore the term is now quite largely 

 being replaced by the more correct expression '*coal-tar dyes," 

 since all of them are made by chemical transformations from one 

 or more substances found in coal-tar. 



Comparatively little is known concerning the chemistry of the 

 natural dyes; but the synthetic dyes have been carefully studied 

 and volumes have been written concerning their chemical compo- 

 sition and its relation to their behavior. Much of this may w^ell be 

 ignored by the biologist who employs dyes merely for staining 

 microscopic objects. A certain familiarity with the general prin- 

 ciples is useful, however, in helping him to employ stains scien- 

 tifically. This is particularly true now that there is a growing 

 tendency to use dyes in biological w^ork as histochemical reagents. 

 The brief discussion that follows is intended to help the biologist 

 to understand merely the fundamental principles of dye chemistry. 



BENZENE 



All coal-tar dyes are organic compounds of the aromatic series. 

 In other words they may be considered as derivatives of the hydro- 

 carbon, benzene, CeHe, the formula for which is usually considered 

 to be 



CH 

 lie ^CH 



HC /CH 

 \^ 



and is usually written in abbreviated form 



