COAIi-TAK COLORS. 75 COAL-TAR COLORS. inetliyl group (CH,) generally increases the violet tendency; the phenyl group (C'„H.,) pro- duces hluisli tints; the niqilithyl jirouj) (d,,!!;) a tendency toward brown-red, etc. The relative position of the groups likewise plays a large part in the determination of color. But, as we have already observed, a definite and allemlirae- ing rule does not exist. Frequently compounds nuist enter into combination with a base or an acid before they will fix themselves upon the fibre, and then the tints are frequently afi'ected by the dili'ercnt bases or acids to a varying de- gree. For example, alizarin dyes red with the hydroxide of aluminum, and black with the hy- droxide of iron. For the purposes of the present sketch, the coal-tar colors may be grouped in five classes : viz. the azo-colors; triphcnyl-carbinol deriva- tives; quinone derivatives; diphenyl-amine deriv- atives ; and indigo dj'es. Azo-C'OLOBS. The characteristic compound of this class is azo-benzene, C„H;N = NCoHj, al- ready mentioned above. We have seen that the introduction of either NH, or OH in place of a hydrogen atom produces a coloring matter — ^yel- low in the former^ orange in the latter instance. Replacing either or both of the phenyl groups (CJlj) by more complex hydrocarbon groujis deepens the tone (with a tendency toward the redder tiaits), increases the affinity for fibres, and diminishes the liability to fade. The earlier dj'es of this class, such as 'aniline yellow,' 'Bis- marck brown,' clirysoidin, etc., were singularly brilliant, but were not fast; whereas the bi-owns and the many reds, ranging from scarlet to ]iurple, which are now produced under the names of j>onecavix or bordeaux, eongos, quinoline red, etc., are exceedingly permanent. In manufac- turing this class of dyes, nitrous acid is allowed to act upon an ice-cold solution of the salt of any primary base (like aniline), and the 'diazo-salt' formed is allowed to act on another base or a phenol ; an endless variety of combinations is thus possible. TKipnENYL-CARBixoL Uerivati^'ES. These rep- resent the first discoveries in the aniline dyes, and some of them are still produced on the larg- est possible scale. The fundamental compound of the class is triphenyl-carbinol (C„H:,),COH, and its derivatives are properly subdivided into rosaiiiliiies, rosoUc acids, and phthalrhis. In the rosaniline group, two or three amido- groups (XH,) are introduced in place of hydro- gen atoms of the phenyls (CJL,). The di-amido- compounds are green: the /n'-amido-eompounds are red, violet, or blue. Strictly speaking, the compounds thus obtained are not themselves dyes, but are bases which must first be combined with suitable acids, and thus brought into a soluble form. Their salts are beautifully crys- talline bodies in the solid condition, showing colors quite different from those of the solutions, and having peculiar lustres like those of beetles' wings. The solutions have very intense colora- tions and stain animal fibres readily and per- manently, although they do not fix themselves easily upon cotton or linen. They are the most brilliant and lively dyes, but are strongly af- fected by sunlight, and are consequently less use- ful than some dyes of other classes. They are generally manufactured by oxidizing processes at a comparatively high temperature, wlu-reby two or three simpler compounds are welded, as it Vol. v.— C. were, into compounds of complex molecular structure. Thus, in the manufacture of the well- known magenta dye (a tri-amido-compound) ap- proximately equal quantities of aniline, ortho- toluidine, and para-tohiidine are healed from 8 to 10 hours with arsenic oxide to 100° C, in large iron kettles. . veiy thick mass results, Ahich can be extracted with hot water, and the compoimd thus obtained is found to be made up of molecvilar quantities of aniline, ortho-tolui- dine, and para-toluidine, chemically combined. Kosolie acid and its derivatives are made by the condensation of various i)henols, three phenols being eondcnscil into one compound of the rosolie acid group, just as three bases are condensed into one compound of the rosaniline group. The comparatively few dj-es of this group give various shades of red. The h3'droxyl groups, and hence the acid character of the phenols, remain unchangi'd in the products of condensation; the latter therefore combine with bases, and then thc' readily go into solution. The phthaleins differ from the rosolie acids in so far as one of the three phenyls of the triphenyl-carbinol is connected in them with a carboxyl group (COOH), the other two phenyls having one or more liydroxyls apiece, as in the rosolie acids. The phthaleins were discovered by Adolph Baeyer, and are chiefly remarkable for the fluorescence of their alkali salts in solution. They are prepared by heating phenols with phthalie anhydride and a little sulphuric acid; when resorcin is taken as the phenol, a very well-known compound is obtained, which has been called fhiorescr'in, while its sodium salt is known as uranin. Solutions of the latter are yellow by transmitted light, but bright green by reflected' light. This fluorescence is so intense that it is distinctly noticeable in extremely dilute solutions; so that this salt has been used to trace subterranean watercourses supposed to connect two neighboring bodies of water, the dye being thrown into one of these and fluorescence being subsequently noticed in the other. The potassium salt of a brominated fluorescein is eosin, C:„HjO.,Br,K,, with a magnificent red and yellow fluorescence. These fluorescences disap- pear on the fibre, but eosin and analogous sub- stances impart very brilliant flesh-tints to silk and wool. The Quinone Derivatives. These contain the characteristic nucleus —
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c II o and are almost invariably colored, although they become suitable for dyes only when they also contain several hydrox_yl groups. By far the most important substance of this class is alizarin (q.v.), which was already mentioned as identical with the active principle of madder. Anthracene (q.v.), a coal-tar hydrocarbon, is con- verted into anthraquinone by heating with potassium bichromate and sulphuric acid; the anthraquinone is acted upon by fuming sulphuric
