samples are made to see if any samples sold under the label differ 

 from the one originally tested. In this way it is hoped before long 

 to have all the stains on the American market from batches which 

 have passed chemical tests and have also been tested and approved 

 by biologists skilled in their use. 



When stains are put on the certification basis, specifications for 

 them, based on the Commission's tests, are prepared and future 

 lots of stain submitted for certification are in all cases expected to 

 fulfill these specifications. So far as these specifications have al- 

 ready been drawn up they are listed in the appendix of this book. 



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 logTvood extract (see Chap. IX) are among the most valuable 

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

 ber that 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," altho 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. 



BENZENE 



All coal-tar dyes may be considered as derivatives of the hydro- 

 carbon, benzene, CeHe, which is the mother substance of the very 

 important aromatic series of organic compounds. It is an unusual 

 chemical compound in many respects, and it will be well, in order 

 to understand the structure of dyestuffs, to review briefly one 

 theory of its structure which accounts for many of its properties. 

 The molecule of benzene is composed of six carbon atoms combined 

 with six hydrogen atoms in such a way that each hydrogen atom is 

 identical in all its reactions with every other hydrogen atom in the 

 molecule. Now a carbon atom is considered to have in all cases 

 four valency bonds, that is it is capable of uniting chemically with 



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