1902.] on Becent Vevelopments in Colouring-Matters. 103 



introduced into the molecule only changes the shade, not the charac- 

 teristic chemical properties. 



A greater variety still than by the achievements of modern syn- 

 thetical work will come into this group of mordant-dyes by the 

 progress of the elucidation of the constitution of the natural dye- 

 stuffs occurring in roots, barks and woods. A good many of them 

 are still unsolved mysteries, but there can be no doubt that they owe, 

 like alizarine, purpurine and the other madder dye-stuffs, their pro- 

 perty of dyeing metallic mordants to the presence of hydroxy 1 groups 

 in ortho-position in their molecule. 



A very large and varied group of colouring-matters, which for a 

 long time resisted all attempts at unravelling their constitution, are 

 the Saffranines, Eurhodines, Oxazines, Thionines, Indulines and other 

 allied groups. They are now completely understood, and have been 

 recognised as the amino- and oxy-derivatives of certain peculiar sub- 

 stances such as the azines and azonium-bases, the molecule of which 

 possesses a ring-structure. Here no longer carbon atoms only form 

 the closed chain, but nitrogen, oxygen and even sulphur atoms 

 participate in its structure and bring about the peculiar properties 

 of the compounds. When this fact was at first ascertained it seemed 

 suf&cient for the explanation of the behaviour of such compounds 

 as dyes. It was only somewhat later on that we recognised that in 

 these classes of dye-stuffs especially a quinonoid structure is essential. 



The greatest and most brilliant success of the chemistry of dye- 

 stuffs is however the industrial synthesis of indigo. This offers so 

 many points of general interest, that I am sure to meet with your 

 approval if I refer to it in some detail. 



The indigo problem is one of the oldest problems of chemistry. 

 When Baeyer took it up more than thirty years ago he found the 

 ground well prepared by others who had worked before him. But 

 his is the merit of having completely elucidated the constitution of 

 this extrordinary product of nature. He and others have also shown 

 various methods for the synthesis or artificial production of indigo. 

 In the laboratories artificial indigo has been known for the last 

 twenty years. 



But in this case the scientific synthesis of a natural product 

 proved to be by no means identical with the industrial one. Indus- 

 trial methods can only enter into competition with nature if they 

 work more economically than nature does. In the case of indigo 

 there seemed to be little hope for fulfilling this condition. The 

 most enthusiastic admirers of the modern synthetical industry could 

 not help seeing that all evidence in our hands went against the 

 probability of the practical synthesis of indigo, and just those who 

 understood most of these things could least of all close their eyes to 

 that fact. It could not be denied that every possible synthesis of 

 indigo, those known as well as those which might still be expected, 

 had to start from some aromatic derivative of benzene, containing 

 one carbonic and one nitrogenous side-chain in ortho-position. Of 



