RESEARCH AND THE COAL-TAR DYE INDUSTRY 419 



Thus brominated indigos were prepared containing 2 to 4 

 bromine atoms per molecule, such as : 



Br 



NH 



NH 



Br 



Br 



C = C 



X/Xco 7 \o/\/ 



Br 



which are of a purer blue than the original dye and in addition 

 are somewhat faster, as also are the chlorinated derivatives. 



In 1905 Prof. Friedlaender in Vienna announced the 

 discovery of a curious and important new vat-dye closely 

 related to indigo but containing two sulphur atoms in place of 

 the NH groups : 



\ 



C = C 



\ 



\Ac6 



which he termed Thio-indigo. 



Though of a somewhat unpleasant bluish shade of red, 

 thio-indigo aroused the utmost interest owing to its fastness, 

 which is considerably greater than that of indigo. 



Still more recent researches, conducted largely by the 

 Hoechst works, have led to the production of a whole series 

 of such vat-dyes related to indigo, which are of a fastness and 

 brilliancy of colour previously unobtainable together. 



At the same time Dr. Bohn of the Badische Company dis- 

 covered accidentally in 1900 a new deep-blue vat-dye derived 

 from anthraquinone, to which the name " indanthrene blue " 

 was given, and which is stated to be the fastest dye known to 

 chemistry : when for instance it is mentioned that it withstands 

 heating with strong hydrochloric acid to 400 C. or with fused 

 caustic potash to 300 C. without change, some idea may be 

 gained of its resistance to chemical change. Again this dis- 

 covery was the incentive to further research work, yielding 

 in its turn a host of new vat-dyes related to indanthrene, of 

 almost unexampled fastness and ease of application, so that 

 indeed at the present day it is hardly too much to say that 

 if such colours could be produced as cheaply as the dyes of 

 the older classes, their wonderful properties would ensure 



