(_c_) Aniline oil for safranine.
(_d_) _Liquid toluidine.
From the first of these, which is almost pure aniline, aniline black is derived, and a number of organic compounds which are further used for the production of dyes. The hydrochloride of aniline is important and is known commercially as "aniline salt."
The distillation and rectification of aniline oil is practised on a similar principle to the fractional distillation which we have noticed as being used for the distillation of the naphthas. First, light aniline oils pa.s.s over, followed by others, and finally by the heavy oils, or "aniline-tailings." It is a matter of great necessity to those engaged in colour manufacture to apply that quality oil which is best for the production of the colour required. This is not always an easy matter, and there is great divergence of opinion and in practice on these points.
The so-called aniline colours are not all derived from aniline, such colouring matters being in some cases derived from other coal-tar products, such as benzene and toluene, phenol, naphthalene, and anthracene, and it is remarkable that although the earlier dyes were produced from the lighter and more easily distilled products of coal-tar, yet now some of the heaviest and most stubborn of the distillates are brought under requisition for colouring matters, those which not many years ago were regarded as fit only to be used as lubricants or to be regarded as waste.
It is scarcely necessary or advisable in a work of this kind to pursue the many chemical reactions, which, from the various acids and bases, result ultimately in the many shades and gradations of colour which are to be seen in dress and other fabrics. Many of them, beautiful in the extreme, are the outcome of much careful and well-planned study, and to print here the complicated chemical formulae which show the great changes taking place in compounds of complex molecules, or to mention even the names of these many-syllabled compounds, would be to destroy the purpose of this little book. The Rosanilines, the Indulines, and Safranines; the Oxazines, the Thionines: the Phenol and Azo dyes are all substances which are of greater interest to the chemical students and to the colour manufacturer than to the ordinary reader. Many of the names of the bases of various dyes are unknown outside the chemical dyeworks, although each and all have complicated; reactions of their own. In the reds are rosanilines, toluidine xylidine, &c.; in the blues--phenyl-rosanilines, diphenylamine, toluidine, aldehyde, &c.; violets--rosaniline, mauve, phenyl, ethyl, methyl, &c.; greens--iodine, aniline, leucaniline, chrysotoluidine, aldehyde, toluidine, methyl-anilinine, &c.; yellows and orange--leucaniline, phenylamine, &c.; browns--chrysotoluidine, &c.; blacks--aniline, toluidine, &c.
To take the rosanilines as an instance of the rest.
Aniline red, magenta, azaleine, rubine, solferino, fuchsine, chryaline, roseine, erythrobenzine, and others, are colouring matters in this group which are salts of rosaniline, and which are all recognised in commerce.
The base rosaniline is known chemically by the formula C_{20}H_{l9}N_{3}, and is prepared by heating a mixture of magenta aniline, toluidine, and pseudotoluidine, with a.r.s.enic acid and other oxidising agents. It is important that water should be used in such quant.i.ties as to prevent the solution of a.r.s.enic acid from depositing crystals on cooling. Unless carefully crystallised rosaniline will contain a slight proportion of the a.r.s.eniate, and when articles of clothing are dyed with the salt, it is likely to produce an inflammatory condition of skin, when worn. Some years ago there was a great outcry against hose and other articles dyed with aniline dyes, owing to the bad effects which were produced, and this has no doubt proved very prejudicial to aniline dyes as a whole.
Again, the base known as mauve, or mauveine, has a composition shown by the formula C_{27}H_{24}N_{4}. It is produced from the sulphate of aniline by mixing it with a cold saturated solution of bichromate of potash, and allowing the mixture to stand for ten or twelve hours. A blue-black precipitate is then formed, which, after undergoing a process of purification, is dissolved in alcohol and evaporated to dryness. A metallic-looking powder is then obtained, which const.i.tutes this all-important base. Mauve forms with acids a series of well-defined salts and is capable of expelling ammonia from its combinations. Mauve was the first aniline dye which was produced on a large scale, this being accomplished by Perkin in 1856.
The substance known as carbolic acid is so useful a product of a piece of coal that a description of the method of its production must necessarily have a place here. It is one of the most powerful antiseptic agents with which we are acquainted, and has strong anaesthetic qualities. Some useful dyes are also obtained from it. It is obtained in quant.i.ties from coal-tar, that portion of the distillate known as the light oils being its immediate source. The tar oil is mixed with a solution of caustic soda, and the mixture is violently agitated. This results in the caustic soda dissolving out the carbolic acid, whilst the undissolved oils collect upon the surface, allowing the alkaline solution to be drawn from beneath. The soda in the solution is then neutralised by the addition of a suitable quant.i.ty of sulphuric acid, and the salt so formed sinks while the carbolic acid rises to the surface.
Purification of the product is afterwards carried out by a process of fractional distillation. There are various other methods of preparing carbolic acid.
Carbolic acid is known chemically as C_{6}H_{5}(HO). When pure it appears as colourless needle-like crystals, and is exceedingly poisonous. It has been used with marked success in staying the course of disease, such as cholera and cattle plague. It is of a very volatile nature, and its efficacy lies in its power of destroying germs as they float in the atmosphere. Modern science tells us that all diseases have their origin in certain germs which are everywhere present and which seek only a suitable _nidus_ in which to propagate and flourish. Unlike mere deodorisers which simply remove noxious gases or odours; unlike disinfectants which prevent the spread of infection, carbolic acid strikes at the very root and origin of disease by oxidising and consuming the germs which breed it. So powerful is it that one part in five thousand parts of flour paste, blood, &c., will for months prevent fermentation and putrefaction, whilst a little of its vapour in the atmosphere will preserve meat, as well as prevent it from becoming fly-blown. Although it has, in certain impure states, a slightly disagreeable odour, this is never such as to be in any way harmful, whilst on the other hand it is said to act as a tonic to those connected with its preparation and use.
The new artificial colouring matters which are continually being brought into the market, testify to the fact that, even with the many beautiful tints and hues which have been discovered, finality and perfection have not yet been reached. A good deal of popular prejudice has arisen against certain aniline dyes on account of their inferiority to many of the old dye-stuffs in respect to their fastness, but in recent years the manufacture of many which were under this disadvantage of looseness of dye, has entirely ceased, whilst others have been introduced which are quite as fast, and sometimes even faster than the natural dyes.
It is convenient to express the const.i.tuents of coal-tar, and the distillates of those const.i.tuents, in the form of a genealogical chart, and thus, by way of conclusion, summarise the results which we have noticed.