Page:The New International Encyclopædia 1st ed. v. 05.djvu/98

COAL-TAR. benzene, which is, in its turn, made from ben- zene. To separate its constituents, the crude naphtlia is tirst divided into three fractions by distiUation: each of these fractions is washed successively with sulphuric acid and caustic soda, as well as with water, and subjected to further fractional distillation.

(2) iliddlc Oil or CarhoUc Oil is the crude fraction distilliugr over from tar between the temperatures of 170° and 230° C. This fraction contains large quantities of naphthalene and carbolic acid, the fonner separating out in the form of a crystalline mass, while the latter re- mains liquid. The naphthalene thus obtained is purified by washing with caustic soda and sulphuric acid, and distilling. On the other hand, the cruije liquid is treated with caustic-soda solution, which takes up all of the carbolic acid and from which the latter is separated by adding sulphuric acid; the impure carbolic acid thus obtained is further purified by distillation. Jsaphthalene is extensively used in the manufacture of colors. Carbolic acid is extensively used as a disinfectant and for the manufacture of picric acid.

(3) Creosote Oil is the crude fraction distilling over from coal-tar between the temperatures of 230° and 270° C. This somewhat heavy oil is largely used for the presei'vation of timber. See Creosote.

(4) Aiifhracciie Oil passes over above 270° C. This fraction yields all the anthracene of com- merce; the anthracene crystallizes out from the oil, and is somewhat purified by washing with the solvent naphtha obtained from the first fraction. Anthracene is extensively employed in the manufacture of the beautiful alizarin dyes, which were formerly made from madder-root. See Alizarin.

(5) The Pitch remaining in the stills after the above fractions have passed over is used for making asphalt and varnishes, for protecting wood and metal work, etc. Coal-tar is produced in large quantities in the manufacture of illuminating gas, and while scarcely half a century ago it was looked upon as notliing but an otTonsive waste product, at present it constitutes the .source of innumerable substances of the greatest value to both science and the industries. See bibliogi'aphical references under Coal-Tab Colors; Gas, IllumxnatING; and see the articles on the various products mentioned above.

COAL-TAR COLORS. Coloring matters arti- ficially prepared from coal-tar. chiefly from the hydrocarbons extracted from it. (See Coal- Tab.) The first observation of a colored com- pound of this class was made by Runge in 1S34; but the real beginning of the great modern color industiy dates from 1856, when W. H. Perkins obtained a violet dyestuff by oxidizing impure aniline with chromic acid, took out a patent for it, and connnenced manufacturing it in Eng- land. Many other dyes were subsequently ob- tained from aniline and the substances related to it, by A. W. Hofmatm. Gries, Ciirard, Lauth, and many others. Ttut the most sensational step was the preparation by Graebe and Liebennann (1868) of a natural dyestuff — viz. the coloring principle of madder-root, from the anthracene of coal-tar. In 1880 indigo was first prepared, not from coal-tar products, but by a purely synthetic method, and other natural colors have since been prepared in a similar manner : so that natural dyestuffs reproduced by artificial means nee<l not necessarily originate from coal- tar. The artificial indigo and alizarin are not mere substitutes for the natural indigo and madder; they are chemically identical with them, and surpass them in purity, and their adaptability to special methods in dyeing and printing often makes them even more desirable. But as the cost of manufacture is high, they com])ete with the natural products on about equal terms. The color industry was first de- veloped in England and France, but the more thorough technical instruction at the German imiversities produced a body of skilled manufac- turers and investigators who soon took the lead. At present, in addition to the great factories near Berlin, Frankfurt. Elberfeld, and iIann- heim, and a host of smaller ones in various parts of Germany, German capital controls many of the establishments in France, Russia, and other countries. The United States possess few independent factories, and the list of their products is rather limited ; indeed, American dyers appear to call for a smaller range of dyestuffs than those of other countries. A peculiar development of the last fifteen years is the extension of the methods of the dye industry to the production of artificial drugs, such as antipyrin, antifchrin, etc., many of which are manufactured in the same establishments which control the dye patents.

Classipication. Artificial colors were for- merly classified merely according to the sources from which they were obtained. Thus, many of them, including magenta, 'aniline blue,' 'aniline green,' 'aniline yellow,' etc., were grouped together as aniline colors. At present somewhat different systems of classification are used by different authors, but all systems are based exclusively on the chemical constitution of the dyes.

Many attempts have been made to find a gen- eral answer to the question. What must Ije the chemical nature of a carbon compound in order that it may be a dye? An all-embracing answer to this question has not yet been found. But experience has shown that the true dyestuffs exhibit peculiar groupings of the constituent atoms. Such 'chromophore' groupings produce, however, only a temlcnct; toward color, but not necessarily colors ; indeed, niany com]>ounds containing them are perfectly colorless, and the majority of true dyes become colorless if de- prived of the small amount of oxygen they con- tain, although their chromophore groups may not be in the least affected. If. however, a chromophore group is combined with certain other atomic groups, the result is a dye. For example, the so-called azo-group ( — X=!N' — ) is chromophorie ; the compound called azobtn- zene, C,H; — X=X — OJi.^. although colored red and evidently containing the azo-grou]), is not a dye; but it becomes one when the so-called amido-group (NH,) also is introduced into its molecule, the compoimdC,.,n,,—X=X—C„H,XH., called amido - azobenzene, being a true dye. If, instead of the amido-group, a hydroxyl group (OH) is introduced, the result is again a dye (an orange one). Further, the tints of dyes are produced by variation in the 'substituting' groujis which replace hydrogen in the primi- tive molecule. Thus, the introduction of the