Coal tar is of complicated nature, containing a variety of substances, many of which are more or less volatile. When it is distilled by sending a current of steam through it, the steam collects the volatile parts, condenses them into naphtha; the first product is condensed steam or water with naphtha swimming on its surface, the next product is dead oil, and the remainder is pitch.
By the aid of the crude naphtha thus produced, Indian rubber is dissolved and waterproof clothes are made. When purified by sulphuric acid, it forms a substance like tar which is thrown away, and the remaining products when clarified are acid oils and neutral hydro-carbons. The carbolic and cressylic acids are the most important of these acid oils. The carbolic acid, which has the property of arresting the putrefaction and decay of organic matter, consists of 12 equivalents of carbon, 6 of hydrogen, and 2 of oxygen. The cressylic acid only differs from the preceding by having two more equivalents of hydrogen and two of oxygen in its chemical composition.
Creosote is a mixture of these two acids. Those vast beams of wood that are driven as piles into the sand or mud at the bottom of the sea, as well as the timbers that form marine superstructures, are saturated with it to a certain depth to preserve them from the attacks of marine insects, especially Limnoria terebrans, an isopod crustacean, which is so destructive in some of our harbours. The wood is deprived of its air by heat and the creosote easily enters.
Carbolic acid is liquid, but becomes solid when purified and dried; and as already mentioned the brilliant yellow dye, carbazotic acid, one of the coal tar colours, is a compound radical, in which the peroxide of nitrogen has replaced three equivalents of hydrogen. The other coal tar colours are obtained from the neutral hydro-carbons, that is to say, compounds of hydrogen and carbon, such as benzol, toluol, and other analogous substances.
Benzol, which consists of 12 equivalents of carbon and 6 of hydrogen, is very volatile, boiling at 117° Fahr., and when acted upon by nitric acid, it forms a compound radicle in which one equivalent of oxide of nitrogen takes the place of one of hydrogen. It smells strongly of bitter almonds, and may be used with safety instead of them. When water and iron are mixed with nitro-benzol, the iron combines with the oxygen and forms oxide of iron, and the result is rusted iron and aniline, which is the origin and foundation of the coal tar colours. Now aniline consists of 12 equivalents of carbon, 7 of hydrogen, and 1 of nitrogen. It is a compound radical: it is ammonia in which one equivalent of hydrogen has been replaced by the radical phenyle, consisting of 12 equivalents of carbon and 5 of hydrogen. It may be remarked that in all these chemical operations the quantity of carbon has remained the same.
Aniline is a colourless liquid, and, being an analogue of ammonia, it readily combines with the different acids to form the beautiful coal tar dyes, for which the world is indebted to the brilliant researches of Dr. Hofmann, professor of chemistry.
By combining a solution of the chloride of lime with the colourless liquid aniline, he obtained the beautiful colour mauve, but it could not be used as a dye till it was rendered permanent by his pupil, Mr. Perkins. His next discovery was the rich crimson crystalline dye magenta, which M. Verguin first introduced into trade at Lyons as a dyeing agent. It may be produced by mixing the anhydrous bichloride of tin with aniline and then driving off the excess of aniline by heat. Other metallic chlorides, nitrates, and many oxidizing agents, have the power of converting aniline into magenta; as for example when the two colourless liquids acetic acid and aniline are mixed and heated, a chemical combination takes place in which three atoms of ammonia have coalesced into one, a salt is formed which is the acetate of aniline or magenta. Here two liquids unite to form a solid and as in many other instances the resulting substance has the power of decomposing light which neither of its constituents can do. Magenta has a redder tint than mauve, and on that account it is sometimes called aniline red. Professor Hofmann has discovered quite recently that pure aniline has not the property of producing these colours, but that they originate in an impurity of the aniline called toluidine.
Rosaniline or roseine, a white substance, is the base of aniline. It is a powerful alkali, readily combining with acids to form highly coloured salts, many of which have a tendency to crystallize, like magenta. This base is most easily extracted from the acetate of aniline. The boiling solution of that salt decomposed by a large excess of ammonia, yields a crystalline precipitate of a reddish colour, and when the colourless liquid is separated by filtration from the precipitate, it deposits on cooling perfectly white needles and tablets of pure rosaniline. This substance unites to acids in three different proportions forming three kinds of salts. The salts that contain one equivalent of acid are extremely stable compounds; for the most part they have a green metallic reflection like some insects’ wings; by transmitted light they are red, and their solutions in alcohol have the magnificent crimson colour of magenta.
A bright purple dye is furnished by mixing equal weights of magenta and aniline. When this mixture is kept at the temperature of 329° for some hours and then mixed with water and hydrochloric acid to remove any excess of magenta or aniline, the result is an insoluble purple residuum or precipitate, but which when well washed with water becomes soluble in alcohol and boiling water slightly acidulated with acetic acid. When the insoluble purple residue is boiled several times with dilute hydrochloric acid, a fine blue dye is formed; azuline, the most beautiful of the blue dyes, which resists the action of the strongest acids, and which is produced by oxidizing aniline under high pressure. It was first prepared at Lyons from phenic acid, a product of the distillation of coal; when pure it appears under the form of copper bronze-coloured crystals soluble in alcohol, to which they communicate a magnificent blue colour tinged with red; but most of the blue dyes are derived from carbolic acid and from creosote. A blood-red colour is the direct result of mixing the muriatic and phenic acids. Aniline, the great source of the coal tar colours, yields also a fine yellow. A vast deal of talent has been employed in the research of colouring dyes both at home and abroad, in which the manufacturers themselves have shown great scientific knowledge.
Attempts have been unsuccessfully made to obtain a green dye from chlorophyll, the green colouring matter of plants. The want was for a short time supplied by Lo-hao, a Chinese dye, but being unstable it was given up. However the very same substance has been procured from the Rhamnus cathartica (Buckthorn), one of the commonest European trees. M. Charwin of Lyons, who made the discovery, has utilized a waste substance, and rendered it permanent as a dye. It is the only known substance which with proper reagents is capable of producing all the seven colours of the spectrum.[[14]]