208 THE POPULAR SCIENCE MONTHLY. 



chiefly in separating the tar by distillation into naphtha, creosote, oils, 

 and pitch. A few distillers, however, made small quantities of ben- 

 zene, which had been first shown by Mansfield, in 1849 to exist in 

 coal-tar naphtha mixed with toluene, cumene, etc. The discovery, in 

 1856, of the mauve or aniline purple gave a great impetus to the coal- 

 tar trade, inasmuch as it necessitated the separation of large quantities 

 of benzene, or a mixture of benzene and toluene, from the naphtha. 

 The trade was further increased by the discovery of the magenta or 

 rosaniline dye, which required the same products for its preparation. 

 In the mean time, carbolic acid was gradually introduced into com- 

 merce, chiefly as a disinfectant, but also for the production of coloring- 

 matter. 



The next most important development arose from the discovery by 

 Graebe and Liebermann that alizarine, the coloring principle of the 

 madder-root, was allied to antbracene, a hydrocarbon existing in coal- 

 tar. The production of this coloring-matter from anthracene followed, 

 and is now oue of the most important operations connected with tar- 

 distilling. The success of the alizarine made in this manner has been 

 so great that it has almost entirely superseded the use of madder, 

 which is now cultivated to only a comparatively small extent. The 

 most important coloring-matters recently introduced are the azo-scar- 

 lets. They have called into use the coal-tar hydrocarbons, xylene and 

 cumene. Naphthalene is also used in their preparation. These splendid 

 dyes have replaced cochineal in many of its applications, and have thus 

 seriously interfered with its use. The discovery of artificial indigo by 

 Professor Baeyer is of great interest. For the preparation of this 

 coloring-matter toluene is required. At present artificial indigo does 

 not compete seriously with the natural product ; but, should it event- 

 ually be prepared in quantity from toluene, a further stimulus will be 

 given to the coal-tar trade. 



The color industry utilizes even now practically all the benzene, a 

 large proportion of the solvent naphtha, all the anthracene, and a por- 

 tion of the naphthaline resulting from the distillation of coal-tar ; and 

 the value of the coloring-matter thus produced is estimated by Mr. 

 Perkin at 3,350,000. 



The demand for ammonia may be taken as unlimited, on account 

 of its high agricultural value as a manure ; and, considering the failing 

 supply of guano and the growing necessity for stimulating the fertility 

 of our soil, an increased production of ammonia may be regarded as a 

 matter of national importance, for the supply of which we have to 

 look almost exclusively to our gas-works. The present production 

 of 1,000,000 tons of liquor yields 95,000 tons of sulphate of am- 

 monia, which, taken at 20 10s. a ton, represents an annual value of 

 1,947,000. 



The total annual value of the gas-works by-products may be esti- 

 mated as follows : 



