THE COAL-TAR DYE INDUSTRY 129 



ice during the operation. The process is cheap, as the 

 expense of heating the vats with steam, which is necessary 

 with most other dyes, is saved. It is also very rapid and 

 the colors are very permanent. 



Quite recently Professor Friedlander of Vienna has pro- 

 duced an indigo red, which contains sulphur and is there- 

 fore called thio-indigo. In its permanence, which greatly 

 surpasses that of indigo blue, as well as in its shade, the 

 new dye resembles the celebrated Tyrian purple of 

 antiquity. 



Now let us glance at the magnitude and importance of 

 the coal-tar dye industry. The first benefit it brought was 

 the utilization of coal tar, which cheapened the production 

 of gas and made it possible to establish gas works in small 

 towns. The second benefit was the scientific development 

 of the arts of dyeing and calico printing which had prev- 

 iously been conducted empirically and handed down from 

 father to son. The chemists who produced the new dyes 

 also extended their field of application and invented new 

 methods of treating cotton, linen, silk, wool, felt, jute, furs, 

 leather, paper, feathers, straw, tinsel, wood, and other 

 materials. The consequence was a greatly improved 

 market for dyed and printed goods of all sorts, and for 

 textile fabrics in general. Improved printing machinery, 

 capable of printing from six to twelve colors, instead of 

 two or three, was invented. The production of the new 

 dyes in very large quantities involved the treatment of 

 immense amounts of raw material and necessitated the 

 invention of machinery for crushing, disintegrating, mix- 

 ing, and heating. Some of the materials are heated in 

 iron or steel shells, called digesters or autoclaves, strong 

 enough to withstand a pressure of fifty or one hundred 

 atmospheres. Centrifugal machines and huge filter presses 

 are employed to separate the dyes in solid form. In the 

 manufacture of the azo dyes large quantities of ice are 

 used, and ice-making machinery is included in the plant. 

 All these things benefit the steel producer, the machine 

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