428 OUR COAL-TAR DYES. 



is necessary for the elucidation of our subject, and subjoined is a table 

 giving a general but by no means detailed constitution of these light and 

 heavy oils. 



1. Liquid. — (a) Constituents of Naphtha or light oil. — Benzol, Toluol, 



Camol. 

 (b) Constituents of the heavy oil. — Aniline, Picoline, Leucoline, 

 Carbolic acid. 



2. Solid. — Naphthaline, Paranaphthaline, Pyren, Chrysen. 



Of the substances composing this list, Aniline is the one which it is the 

 object of the manufacturer of tar dyes to obtain, this being the basis of one 

 large class of colouring matters. Aniline was discovered in 1826 by 

 Undervorben by the distillation of indigo, and from which the name is 

 derived, anil being the Hindoostanee for indigo. It is also a product of the 

 distillation of coal, but requiring a tedious and difficult process to be 

 followed out for its separation, an indirect method from Benzole is found 

 to be a more economical source of production. Benzole is the principal 

 constituent of Naphtha, which affords it by purification, and it is, then, a 

 thin, limpid, colourless liquid, used for many purposes in the arts. 



The action of nitric acid is peculiar on this substance ; if a stream of 

 benzole mixed with the acid is made to trickle through a worm kept cool, 

 the two liquids react on each other, and a yellowish liquid, nitro-benzole, 

 appears at the end of the worm, having the odour of the essential oil ot 

 bitter almonds, which renders it useful in perfumery, especially for scenting 

 fancy soaps. The nitro-benzole procured in this manner is now put into a 

 retort with iron filings and acetic acid. Without the application of ex- 

 ternal heat, the mixture from the chemical action which ensues becomes 

 hot, and gives off a vapour which is condensed in a receiver kept well cool. 

 This distillate being treated with potash, the aniline separates as an oily 

 layer. Aniline is especially characterised by the various colours it dis- 

 plays under the action of different reagents ; in this respect none are more 

 marked than the hypochlorites ; they, it has long been known, produce, 

 with aniline, a purple solution of which little was known except that it was 

 very fugitive. Mr. Perkins, in some experiments on the formation of 

 quinine artificially, was led to use this substance, and attained an un- 

 promising black precipitate, which he subsequently found to contain 

 aniline, purple or mauve. With great industry and perseverance, he 

 followed up the subject, and, finally, perfected the process now employed for 

 the preparation of this dye ; which consists in mixing a cold solution of 

 aniline in sulphuric acid with bichlorate of potass, and the result is a black 

 precipitate. This is purified by solution in alcohol, and on evaporation the 

 colouring matter is left in the form of a beautiful bronze-coloured sub- 

 stance, which is soluble with difficulty in water, but readily in alcohol, 

 giving a solution having the well known purple tint. Two other colouring 

 matters, named from the character of their tints, Violine and Roseine, are 

 prepared from Aniline by the action of peroxide of lead, by processes similar 

 in character to the preceding, and giving use to a similar compound. 



