138 MORRIS LOEB 



the resulting compound is melted with caustic soda, yielding 

 a sodium salt of alizarin. This is soluble in water with a fine 

 red color, but does not fasten upon any kind of fibre. If, how- 

 ever, cotton is previously impregnated with salts of alumi- 

 num, iron, or chromium, the alizarin will form insoluble salts 

 ("lakes") with these metals; and as the precipitation occurs 

 within the pores of the fibre, subsequent washing cannot re- 

 move it. Colors of this class of dyes are not suitable for 

 silk and wool, but are very intense and permanent when 

 properly applied to cotton. 



THE DIPHENYLAMINE DERIVATIVES. These include many 

 varieties of dyes, such as the indulins, indophenols, thiazins, 

 etc. Their chemistry is too involved to be disposed of in a 

 few words. It may, however, be mentioned that their char- 

 acteristic groups are similar to anthraquinone, excepting that 

 the oxygen of the latter is replaced by sulphur, imido-groups, 

 etc. The more important dyes of this class include methy- 

 lene blue and aniline black. 



INDIGO DYES. By far the most important of these is indigo 

 itself, a vegetable dye obtained from a tropical plant cul- 

 tivated in India since the earliest times. The sap of this 

 plant, when fermented under conditions excluding oxygen, 

 yields indigo white, a soluble material having the formula 

 Ci 6 Hi2N 2 O2; if the fermentation proceeds in the open air, in- 

 digo blue, Ci6HioN 2 O 2 , is produced. This substance is a deriv- 

 ative of the base called indol, C 8 H 7 N, which occurs ready- 

 formed, in small quantities, in many animal and vegetable 

 secretions. It can be prepared artificially from aniline and 

 chloraldehyde. When indigo was found to consist of two indol 

 molecules joined together and oxidized, the clue for the pro- 

 duction of artificial indigo was at hand. It has since been 

 found that any benzene derivative having a nitrogenous group 

 and a two-carbon group in the "ortho" position may give rise 



