1902.] on Meceni Developments in Colouring Matters. lOl 



difficulty, which have not been solved so far. But fortunately these 

 difficulties have in no way interfered with the technical development 

 of this family of dye-stuffs, which has been for a whole quarter of a 

 century one continued and unparalleled series of successes. The 

 process for producing these dye-stuffs is of the greatest simplicity. 

 It consists in pouring together (Exp. VIII.) cold aqueous solutions 

 or suspensions of diazo-compounds and phenols or amines. The 

 dye-stuff is formed at once in a state of absolute purity, and with a 

 yield absolutely theoretical ; it need only be collected and dried to 

 form a saleable product. No wonder, then, that these dye-stuffs 

 gradually became the leading ones, and to a great extent superseded 

 the old empirical products which were concocted in many compli- 

 cated operations, with yields very far from satisfactory. As the 

 number of diazo-compounds and of phenols and amines at our dis- 

 posal is very large, the number of dye-stuffs which may thus be 

 prepared is quite extraordinary ; it has been computed, according to 

 the rules of permutation. 3,159,000 different individual dye-stuffs 

 have thus been proved to be at present easily accessible to our in- 

 dustry. Of these at least 25,000 form the subject of German patent 

 specifications and of corresponding specifications in England, France, 

 the United States, and other countries. Over five hundred are 

 regularly manufactured on the larger scale. 



The prolific nature of the azo-colour-reaction explains the fact, 

 that in this group we can choose, much better than in any other, 

 substances possessing that ratio of solubilities in water and in the 

 colloid substance of the various textile fibres, which we require. We 

 can produce, quite at will, azo-dye-stuffs which dye wool or silk or 

 cotton, which dye slowly or quickly, which will stand soap or acid 

 or alkali. This possibility of adjusting the chemical properties of 

 dye-stuffs with an almost mechanical precision has been the cause of 

 one of the greatest successes of the colour-industry, the introduction 

 of what is now known under the name of " substantive dye-stuffs," 

 an expression which means dye-stuffs that will dye cotton and other 

 vegetable fibres from a simple aqueous dye-bath without the use of 

 any mordant. The difference of the solvent power of cellulose and 

 of water is for the vast majority of dye-stuffs so small, that the pro- 

 cess of dyeing vegetable fibres with these ordinary colouring-matters 

 can only be compared to that case of the joint action of ether and 

 water on some substance soluble in both these solvents, where an 

 almost equal division of this substance takes place between the two 

 solvents. Such cases exist, as you saw in the first experiment. It 

 is amongst the azo-dyes that we have found compounds which are so 

 much more soluble in cellulose than in water, that they readily leave 

 their aqueous solution and take up their abode in the fibre. And we 

 have not only found these dye-stuffs but also the law which governs 

 this most valuable abnormal solubility : it appears in all azo-colours, 

 which are prepared with diazo-compounds derived from symmetrical 

 para-diamines. A novel and extremely fertile field for a systematic 



