Coal-tar in its crude state is not of very great importance, its use being confined to such rough work as the water-proofing of boats and the painting of outhouses and the like. But in the hands of the chemist its applications cannot be lightly regarded, in fact its distillation is of sufficient importance to form a distinct branch of trade. In this process coal-tar is separated into three different products—naphtha (which in a rectified state is the benzol of commerce); heavy or creosote oil, which is used almost exclusively for the preservation of railway sleepers; and the residue pitch. The last is of great use to shipbuilders, and has more recently found employment in the preparation of asphalt roofing and paving. But naphtha is by far the most important of the three substances, if it were only for its use as a solvent for both india-rubber and gutta-percha. No doubt, failing this, other solvents for caoutchouc would have been found; but naphtha is a particularly cheap and effective menstruum for the purpose; and when we consider the varied uses to which india-rubber and gutta-percha are now applied—from elastic hosiery to submarine cables—we must acknowledge that naphtha is a valuable addition to our manufacturing resources. It is a significant circumstance that the date of the introduction of manufactured india-rubber (by Mr Mackintosh) follows the general adoption of gas-lighting by only a few years. Previous to this, india-rubber was imported merely as a curiosity, its first use being to obliterate pencil-marks, for which purpose it was once advertised in London at the modest price of six shillings per square inch.
Besides its use as a solvent, benzol is of particular importance in yielding, when treated with nitric acid, a substance called aniline. The discovery of aniline is one of the most remarkable triumphs of chemistry, as applied to the advancement of a manufacturing industry. (Before the date of coal-tar it was obtained from indigo, and the name it bears is the Portuguese for that colour.) The production of aniline caused quite a revolution in the various trades which are dependent in any way upon the colour-manufacturer; for lithographers, paper-stainers, calico-printers, and especially dyers, owe their most brilliant tints to its aid. The various dyes which are now commonly retailed for household use are also derived from the same source. Aniline is an almost colourless liquid, of a peculiar vinous odour, which after exposure to the air, changes to a dark resinous matter. The treatment which it undergoes in producing the various colours (and nearly every colour of the rainbow can now be obtained from it), is of too complicated a nature to be of any interest to the general reader. Magenta, the advent of which some years back many of our readers will remember, was the first aniline dye which appeared. The other colours have followed in quick succession, nearly all of them being the subjects of one or more patents. It is questionable whether all these colours are strictly permanent; but it is a pleasing thought that the hues which in one form or another existed at a period long before mankind had a place in nature, are now reproduced for man’s delight and benefit.