COLORS, the coloring mat ters or dyes artificially prepared from substances derived from coal tar. They are exceedingly numerous, and are mostly of complex chemical constitution, belonging to the group known as aromatic compounds (q.v.). They consist chemically of the natural element carbon in association with hydrogen, nitrogen, oxygen, sulphur and other elements. The coal-tar colors are of the greatest importance in the arts, and are used in enormous quantities in dyeing and calico-printing. In popular language they are often called aniline colors. This designation is inexact, however, for although they include the colors of the aniline class some of them are derivatives of phenol, anthracene and other substances. Some prejudice exists against the coal-tar colors in the public mind, because they are believed to be less permanent than the natural animal and vegetable colors, for which they are substituted. This criticism was quite justifiable some years ago, but a considerable number of coal-tar colors are now known which are fully as °fast° as the natural ones, and in some cases the coal-tar color has a decided ad vantage in this respect. The coal-tar color in dustry had its origin in England, where for 14 years it maintained pre-eminence, but the centre of the industry soon passed to Germany, then distilling enormous quantities of coal for other purposes. The first coal-tar dye was made by the English chemist Perkin in 1856. He called it °Inauveine.° In 1858 A. W. Hofmann published a paper in which he showed that magenta (aniline red) can be prepared by the action of carbon tetra-chloride upon aniline, and in 1859 Verguin first separated fuchsine, and manufactured magenta in quantity. During the next few years various other colors, notably aniline blue, were discovered and placed on the market, and aniline black was discovered by Lightfoot in 1863. The coal-tar color in dustry was greatly stimulated when Graebe and Lieberman effected the synthesis of alizarin from anthracene in 1868, thereby opening up a new and important field of chemical activity. Previous to this date alizarin was obtained from madder-root, but it is now almost exclusively manufactured from anthracene. Caro pro duced the first eosin dye in 1874. The synthe sis of indigo was the result of a long series of investigations, the final step in which was taken by Baeyer in 1878; and in 1880 a German com pany placed on the market a substance known as nitrophenylvropiolic acid, for depositing arti ficial indigo upon fibres by Baeyer's method. Other methods for the artificial manufacture of indigo have since been discovered, but the natural dye is still largely used.
The marks that are used in commerce for designating the shade of a color consist usually of certain letters affixed to the name of the color to be described. Thus R is used for red, 0 for orange, J or G for yellow (Fr. jaune; Ger. Gelb), B for blue and V for violet. Thus °Scarlet RR° or "Scarlet 2R° signifies a scarlet whose tint inclines toward red, and the doubled R signifies that an intermediate color is recog nized, whose tint lies between that specified and the pure scarlet. Aniline blue occurs in a reddish shade which is designated as "aniline blue while the finest quality of aniline blue is designated as "aniline blue 6B.° Much attention has been paid to the connec tion between the color of a substance and its chemical constitution. It is known that color depends on the structure of the molecule, but.
no absolute rule can be given for predicting the color of a compound whose formula is known. In the case of the aromatic series, however (which is of special interest to the student of coal-tar colors), the following may be said: All of the aromatic hydrocarbons are colorless (or white), and the same is true of such of their mono-substitution compounds as are obtained by replacing one atom of hydrogen with OH, NO, or NH,. If two hydrogen atoms are re placed, the resulting compound is also colorless if the radicals introduced are alike. If they are unlike, and one of them is NO,, then the result ing compound is colored. For example, benzene, Ca., is colorless, and so also are its derivatives, aniline (C.H,.NH,) and nitrobenzene (CAM. NO,), which are obtained by replacing one atom of hydrogen by NH, and NO,, respec tively. But if a second atom of hydrogen in the benzene nucleus of aniline be replaced by NO,, we have the compound • NO..C..H..NH,, which is known as nitroaniline and is yellow. The presence of certain atomic groups in the molecule of a substance appears to have a strong influence, also, upon the color of the compound. This is particularly noticeable in the quinones, which contain the group .0.0., and in the azo compounds, which contain the group .N :H. . The difference between a colored substance and a coloring matter, or dye, must be borne in mind, however. A dye is a sub stance which unites directly with the animal or vegetable fibres of the fabric to which it is applied, or with a mordant with which those fibres are itnpregmated. According to Witt's theory, a true dye must contain two species of molecular groups, one of which is called the chromophore Ocolor-bearee), while the other serves to make the compound acid or basic, so that it can form salts. The chromophores are very numerous. They have a distinct effect upon the color of the dyes formed with their aid. The groups NO4 .N :N., and .0.0. are examples of c.hromophores. Compounds that contain chromophores but are neither acid nor basic are called chromogens Ocolor-gener ators0). They are not dyes, but may be trans forrned into dyes by the introduction of a salt forming group such as OH or NH.. Azoben zene, C4FLN :N.C.113, is a colored substance, but it is not a dye. It is a chromogen, however, because it contains the chromophore-group .N :N.; and if one of its hydrogen atoms is re placed by the group OH, the compound C.114.N :N.C.H4.0H, known as oxyazobenzene, is obtained, which is a true dye. Again if one of the atoms of hydrogen in azobenzene is replaced by NH., the compound Cal..N:N.C.H..NH$ is obtained; this is also a true dye, and is known to chemists as amidoazobenzene. (Consult Hjelt, 'Principles of General Organic Chentis try,' from which these examples are taken). Coloring matters that are basic in nature are always used, in dyeing, in the form of salts; that is, it is the compounds of these substances with acids that are used, and not the free bases themselves. °Substantive') coloring matters are those that are directly absorbed from solution by the fibre to be dyed. °Adjective° coloring matters are those that are not directly absorbed in this manner, but which require the fibre to be first umordanted,° or charged with certain metallic salts, or °animalized" with albumen, or treated in some other manner, before the dyeing 'can be done.