Now these products—dimethylaniline, diethylaniline, and diphenylamine—are derived from aniline, and they are all sources of colouring-matters. Methyl-violet is obtained by the oxidation of dimethylaniline by means of a gentle oxidizer; a mixture of bases is not necessary as in the case of the magenta formation. Then in 1866 diphenylamine was shown by Girard and De Laire to be capable of yielding a fine blue by heating it with oxalic acid, and this blue, on account of the purity of its shade, is still an article of commerce. It can be made soluble by the action of sulphuric acid in just the same way as the other aniline blue. Furthermore, by acting with excess of methyl chloride on methyl violet, a brilliant green colouring-matter was manufactured in 1878, which was obviously analogous to the iodine green already mentioned, and which for some years held its own as the only good coal-tar green. These are the dyes—methyl violet and green, and diphenylamine blue—which were in commerce before the discovery of the Fischers, and which this discovery enabled chemists to class with magenta, aniline blue, and Hofmann violet in the triphenylmethane group.

Later developments bring us into contact with other dyes of the same class, and with the industrial evolution of the purely scientific idea concerning the constitution of the colouring-matters of this group. Benzene and toluene again form the points of departure. By the action of chlorine upon the vapour of boiling toluene there are obtained, according to the extent of the action of the chlorine, three liquids of use to the colour manufacturer. The first of these is benzyl chloride, the second benzal chloride, and the third benzotrichloride or phenyl chloroform. Benzyl chloride, it may be remarked in passing, plays the same part in organic chemistry as methyl chloride, and enables certain compounds to be benzylated, just in the same way that they can be methylated. The bluer shade of methyl violet, introduced in 1868, and still manufactured, is a benzylated derivative. By the action of benzotrichloride on dimethylaniline in the presence of dry zinc chloride, Oscar Doebner obtained in 1878 a brilliant green colouring-matter which was manufactured under the name of “malachite green.” It will be remembered that this was about the time when the Fischers were engaged with their investigations. These last chemists, by virtue of their scientific results, were enabled to show that Doebner’s green was a member of the triphenylmethane group, and they prepared the same compound by another method which has enabled the manufacturer to dispense with the use of the somewhat expensive and disagreeable benzotrichloride. The Fischers’ method consists in heating dimethylaniline with bitter-almond oil and oxidizing the product thus formed, when the green colouring-matter is at once produced. This method brings the technologist into competition with Nature, and we shall see the result.

Benzoic aldehyde or bitter-almond oil is one of the oldest known products of the vegetable kingdom, and has from time to time been made the subject of investigation by chemists since the beginning of the century. It arises from the fermentation of a nitrogenous compound found in the almond, and known as amygdalin, the nature of the fermentative change undergone by this substance having been brought to light by Wöhler and Liebig. The discovery of a green dye, requiring for its preparation a vegetable product which was very costly, compelled the manufacturer to seek another source of the oil. Pure chemistry again steps in, and solves the problem. In 1863 it was known to Cahours that benzal chloride, on being heated with water or alkali, gave benzoic aldehyde, and in 1867 Lauth and Grimaux showed that the same compound could be formed by oxidizing benzyl chloride in the presence of water. It was but a step from the laboratory into the factory in this case, and at the present time the aldehyde is made on a large scale by chlorinating boiling toluene beyond the stage of benzyl chloride, and heating the mixture of benzal chloride and benzotrichloride with lime and water under pressure. By this means the first compound is transformed into benzoic aldehyde, and the second into benzoic acid. This last substance is also required by the colour-maker, as it is used in the manufacture of blue by the action of aniline on rosaniline; without some such organic acid the transformation of rosaniline into the blue is very imperfect.

Benzoic acid, like the aldehyde, is a natural product which has long been known. It was obtained from gum benzoïn at the beginning of the seventeenth century, and its preparation from this source was described by Scheele in 1755. The same chemist afterwards found it in urine, and from these two sources, the one vegetable and the other animal, the acid was formerly prepared. Its relationship to benzene has already been alluded to in connection with the history of that hydrocarbon. It will be remembered that by heating this acid with lime Mitscherlich obtained benzene in 1834. In one operation, therefore, setting out from toluene, we make these two natural products, the aldehyde and acid, which are easily separable by technical processes. The wants of the technologist have been met, and he has been enabled to compete successfully with Nature, for he can manufacture these products much more cheaply than when he had to depend upon bitter almonds or gum benzoïn. The synthetical bitter-almond oil is chemically identical with that from the plant. Besides its use for the manufacture of colouring-matters, it is employed for flavouring purposes and in perfumery, this being the first instance of a coal-tar perfume which we have had occasion to mention. The odour in this case, it must be remembered, is that of the actual compound which imparts the characteristic taste and smell to the almond; it is not the result of substituting a substance which has a particular odour for another having a similar odour, as is the case with nitrobenzene, which, as already mentioned, is used in large quantities under the name of “essence of mirbane,” for imparting an almond-like smell to soap.

The introduction of malachite green marks another epoch in the history of the technology of the triphenylmethane colours. The action between benzoic aldehyde and other bases analogous to dimethylaniline was found to be quite general, and the principle was extended to diethylaniline and similarly constituted bases. Various green dyes—some of them acids formed by the action of sulphuric acid on the colour base—are now manufactured, and many other colouring-matters of the same group are synthesised by the benzoic aldehyde process.

One other development of this branch of manufacture has yet to be recorded. The new departure was made in 1883 by Caro and Kern, who patented a process for the synthesis of colouring-matters of this group. In this synthesis a gas called phosgene is used, the said gas having been discovered by John Davy in 1811, who gave it its name because it is formed by the direct union of chlorine and carbon monoxide under the influence of sunlight. Caro and Kern’s process is the first technical application of Davy’s compound. By the action of phosgene on dimethylaniline and analogous bases in the presence of certain compounds which promote the chemical interaction, a number of basic colouring-matters of brilliant shades of violet (“crystal violet”) and blue (“Victoria blue,” “night blue”) are produced, these being all members of the triphenylmethane group. One of these dyes is a fine basic yellow known as “auramine,” which is a derivative of diphenylmethane.

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From benzene and toluene alone about forty distinct colouring-matters of the rosaniline group are sent into commerce. The relationship of those compounds to each other and to their generating substances is not easy to grasp by those to whom the facts are presented for the first time. The scheme on page 107 shows these relationships at a glance.