1886.] on Hecent Progress in the Coal-tar Industry. 457 



The several colours here chosen as examples are only a few 

 amongst a very numerous list of varied colour derivatives of each 

 groujD. Thus we are at present acquainted wdth ahout sixteen distinct 

 yellow colours ; about twelve orange ; more than thirty red colours ; 

 about fifteen blues, seven greens, and nine violets; also a number 

 of browns and blacks, not to speak of mixtures of these several 

 chemical compounds, giving rise to an almost infinite number of 

 shades and tones of colour. These colours are capable of a rough 

 arrangement according as they are originally derived from one or 

 other of the hydrocarbons contained in the coal-tar. The fifty 

 specimens of different colours exhibited may thus be classified, but 

 in this Table, for the sake of brevity, only the commercial names and 

 not the chemical formulso of these compounds is given. 



Azo-colours. — Amongst the most important of the artificial colour- 

 ing matters may be classed the so-called azo-colours. These colours 

 are chiefly bright scarlets, oranges, reds, and yellows, with a few 

 blues and violets. They ow^e their existence to the discovery by 

 Griess, in 1860, of the fact that the so-called azo-group — N = N — 

 can replace hydrogen in phenols and amido compounds. But it is 

 to Dr. 0. N. Witt that is due the honour of having given the first 

 start in a practical direction to the chrysoidine class of azo-colours 

 by the discovery of chrysoidine, and perhaps still more so by the 

 suggestions contained in a paper read before the Chemical Society. 

 Dr. Caro, of Mannheim, was also acquainted with several compounds 

 which belong to this class at the time Witt published his results, but 

 it does not appear that he made practical use of them until Witt 

 introduced the chrysoidines and tropeolines. To Koussin, of the 

 firm of Poirrier of Paris, is due the credit of having first brought 

 into the market some of the beautiful azo-derivatives of naphthol. 

 Griess, therefore, as the original discoverer of the typical compounds 

 and reactions by which the azo-colours are obtained, may be considered 

 as the grandfather, whilst Eoussin and Witt are really the fathers, of 

 the azo-colour industry. Nor must it be forgotten that it is to 

 Perkin we owe the recognition of the value of the sulpho group in 

 relation to azo-colours, a discovery patented in 1863. Moreover it is 

 interesting to note that changes in colour from yellow to red and 

 claret are eftected by the increase in the molecular weights of the 

 radicals introduced as well as by the relative positions occupied by 

 these groups. 



Indophenol. — Witt is also the discoverer of a new blue dye-stuff 

 termed indophenol, which has been used as a substitute for indigo. 

 Certain difficulties, however, have arisen in the adoption of this colour 

 on the large scale. The most important use indojohenol is at present 

 put to is for producing dark blues on reds dyed with azo-colours, 

 both on wool and cotton. The piece goods are dyed a uniform red 

 first, and then printed with indophenol white ; for like indigo itself 

 indophenol yields a colourless body on reduction, and this being a 

 very powerful reducing agent destroys the azo-coloui', being itself 

 transformed into indophenol blue. The process works with surprising 



