CHAPTER IX 



THE NATURAL DYES 



AS STATED above (p. 11) the group of natural dyes is shrink- 

 ing as more and more of them are being produced by artificial 

 means. Alizarin, for example, used to be a natural dye of 

 much importance; but now the artificial manufacture of this dye 

 is much more economical. The group of natural dyes, as ordinarily 

 recognized, contains only those which are not yet produced by 

 artificial means. Indigo, however, is listed in this chapter, be- 

 cause in its chemistry it does not fall in well with any of the groups 

 of artificial dyes. Indigo is still obtained from the indigo plant, 

 altho under present-day conditions its artificial manufacture is 

 ordinarilv the more economical. 



The chemistry of the natural dyes is less definitely known than 

 that of the artificial dyes. This is easily understood; for it will be 

 recalled that there are two ways of obtaining information as to the 

 chemistry of unknown compounds : the first by decomposing them 

 into simpler compounds of known composition; and the second by 

 manufacturing them from known compounds. In the case of dyes 

 not yet prepared artificially the second of these two lines of pro- 

 cedure is out of the question; hence the difficulty in learning their 

 exact chemical structure. 



The most important natural dyes for the biologist are haema- 

 toxylin, indigo, cochineal (and its derivatives), orcein, and litmus. 



The Indigo Group 



INDIGO c. I. NO. 1 17 7 



Synonym: Indigo blue. 



The plants from which indigo was formerly exclusively manu- 

 factured are largely species of the genus known as Indigofera, altho 

 some indigo-bearing plants are recognized by botanists as belong- 

 ing to different genera. In these plants is a glucoside, indican, 

 which is converted by fermentation into the dye indigo. Various 

 formulae have been given for indigo; the one favored at present is 

 based upon its method of artificial manufacture: 



CO CO 



\ / • 



c=c 



/ \ 



NH NH 



In this formula the exact chromophore group is uncertain; but the 

 ketone group (CO) in a closed ring occurs so often in dyes that it is 

 regarded as probably having chromophoric properties. 



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