The chief constituents of coal-gas are, therefore, briefly as follows:—
/ (1) Hydrogen, | (2) Marsh-gas (carburetted hydrogen or fire-damp), | (3) Carbon monoxide, | (4) Olefiant gas (ethylene, or heavy carburetted hydrogen), with \ other olefines, / (5) Nitrogen, | (6) Carbonic acid gas, | (7) Sulphuretted hydrogen, \ (8) Carbon bisulphide (vapour),
the last four being regarded as impurities, which are removed as far as possible in the manufacture.
In the process of distillation of the coal, we have seen that various other important substances are brought into existence. The final residue of coke, which is impregnated with the sulphur which has not been volatilised in the form of sulphurous gases, we need scarcely more than mention here. But the gas-tar and the ammoniacal liquor are two important products which demand something more than our casual attention. At one time regarded by gas engineers as unfortunately necessary nuisances in the manufacture of gas, they have both become so valuable on account of materials which can be obtained from them, that they enable gas itself to be sold now at less than half its original price. The waste of former generations is being utilised in this, and an instance is recorded in which tar, which was known to have been lying useless at the bottom of a canal for years, has been purchased by a gas engineer for distilling purposes. It has been estimated that about 590,000 tons of coal-tar are distilled annually.
Tar in its primitive condition has been used, as every one is aware, for painting or tarring a variety of objects, such as barges and palings, in fact, as a kind of protection to the object covered from the ravages of insects or worms, or to prevent corrosion when applied to metal piers. But it is worthy of a better purpose, and is capable of yielding far more useful and interesting substances than even the most imaginative individual could have dreamed of fifty years ago.
In the process of distillation, the tar, after standing in tanks for some time, in order that any ammoniacal liquor which may be present may rise to the surface and be drawn off, is pumped into large stills, where a moderate amount of heat is applied to it. The result is that some of the more volatile products pass over and are collected in a receiver. These first products are known as first light oils, or crude coal-naphtha, and to this naphtha all the numerous natural naphthas which have been discovered in various portions of the world, and to which have been applied numerous local names, bear a very close resemblance. Such an one, for instance, was that small but famous spring at Biddings, in Derbyshire, from which the late Mr Young—Paraffin Young—obtained his well-known paraffin oil, which gave the initial impetus to what has since developed into a trade of immense proportions in every quarter of the globe.
After a time the crude coal-naphtha ceases to flow over, and the heat is increased. The result is that a fresh series of products, known as medium oils, passes over, and these oils are again collected and kept separate from the previous series. These in turn cease to flow, when, by a further increase of heat, what are known as the heavy oils finally pass over, and when the last of these, green grease, as it is called, distils over, pitch alone is left in the still. Pitch is used to a large extent in the preparation of artificial asphalte, and also of a fuel known as "briquettes."
The products thus obtained at the various stages of the process are themselves subjected to further distillation, and by the exercise of great care, requiring the most delicate and accurate treatment, a large variety of oils is obtained, and these are retailed under many and various fanciful names.
One of the most important and best known products of the fractional distillation of crude coal-naphtha is that known as benzene, or benzole, (C_{6}H_{6}). This, in its unrefined condition, is a light spirit which distils over at a point somewhat below the boiling point of water, but a delicate process of rectification is necessary to produce the pure spirit. Other products of the same light oils are toluene and xylene.
Benzene of a certain quality is of course a very familiar and useful household supplement. It is sometimes known and sold as benzene collas, and is used for removing grease from clothing, cleaning kid gloves, &c. If pure it is in reality a most dangerous spirit, being very inflammable; it is also extremely volatile, so much so that, if an uncorked bottle be left in a warm room where there is a fire or other light near, its vapour will probably ignite. Should the vapour become mixed with air before ignition, it becomes a most dangerous explosive, and it will thus be seen how necessary it is to handle the article in household use in a most cautious manner. Being highly volatile, a considerable degree of cold is experienced if a drop be placed on the hand and allowed to evaporate.