OUR COAL-TAR DYES. 429 



The three substances enumerated seem to he closely allied, and are found, 

 under like circumstances, by the action of oxidising agents with the presence 

 of water, and are likewise similarly acted upon by most re-agents. 

 Deoxidising substances deprive them of colour, but this is again restored by 

 exposure to oxygen. Sulphuric acid forms with them green solutions, 

 whilst their original colour is restored by dilution ; tannin forms insoluble 

 compounds with them. They are soluble in alcohol, and but slightly 

 soluble in water, from which they are thrown down by alkalies. 



Another well-known dye, Magenta, is also prepared from aniline, but 

 possesses different properties, and belongs to a different series of compounds. 

 It was practically introduced into the arts by Dr. Hofmann. On a large 

 scale, it is prepared by the action of perchlorides of tin, or mercury, or the 

 nitrates of mercury, on aniline. Chemically, Magenta is basic, so that 

 by the former process, a hydrochlorate, by the latter, a nitrate, of Magenta is 

 obtained. It is precipitated by ammonia as a red paste, which, on drying, 

 becomes a red powder, soluble with difficulty in water, but tolerably 

 soluble in alcohol, which, on evaporation, leaves the colouring matter as a 

 brittle mass with a golden grain metallic reflection, appearing red by trans- 

 mitted light. Allied to Magenta are two other products of aniline, namely, 

 bleu de Paris and emer aldine. The former is a beautiful blue dye, pre- 

 serving its colour by artificial light, resisting the action of acids, and being 

 darkened by alkalies. The latter, called also aniline green, is formed by 

 oxidising a hydrochloric acid solution of aniline with chlorate of potash, 

 or perchloride of iron. This colour may readily be impressed on a fabric 

 by printing the design with a thickened solution of chlorate of potash and 

 then passing it through a solution of an aniline salt. Under the action of 

 an alkali, the colour becomes dark blue. 



It is a curious and interesting fact that the production of these beautiful 

 pigments is not confined to aniline. The bases Toluol and Camol yield 

 similar colouring matters ; but, for practical purposes, neither the colour 

 nor general results are so good as those obtained from the aniline products. 

 But amongst the constituents of coal tar there is one substance — carbolic 

 acid — which is eminently useful to the dyer, on account of several colours 

 which the chemist extracts from it. Picric acid has been known since the 

 latter part of the last century, as a product of the action of nitric acid on 

 indigo and other substances, and was introduced for dying purposes five or 

 six years since, but is now more economically prepared by treating car- 

 bolic acid with nitric acid. It occurs in shining lamina, having a light 

 crimson colour, and forming with water a beautiful yellow solution. The 

 fashionable orange colour, capucine (the French name for the Nasturtium), 

 which before the late mourning rivalled mauve in popularity, is this 

 substance. By combination it affords a goodly list of other tints ; with 

 oxide of iron it yields a red ; and with oxide of copper a yellow green, long 

 known to the dyer. 



Rosolic acid, obtained likewise from carbolic acid, has been used as a 

 red dye, especially for printing muslins, but is now, in a great measure, 

 superseded by other tar colours. 



