The Story of a Piece of Coal: What It Is, Whence It Comes, and Whither It Goes

by Edward A. Martin

Benzene, which is only a compound of carbon and hydrogen, was first discovered by Faraday in 1825; it is now obtained in large quantities from coal-tar, not so much for use as benzene; is for its conversion, in the first place, by the action of nitric acid, into nitro-benzole, a liquid having an odour like the oil of bitter almonds, and which is much used by perfumers under the name of essence de mirbane; and, in the second place, for the production from this nitro-benzole of the far-famed aniline. After the distillation of benzene from the crude coal-naphtha is completed, the chief impurities in the residue are charred and deposited by the action of strong sulphuric acid. By further distillation a lighter oil is given off, often known as artificial turpentine oil, which is used as a solvent for varnishes and lackers. This is very familiar to the costermonger fraternity as the oil which is burned in the flaring lamps which illuminate the New Cut or the Elephant and Castle on Saturday and other market nights.

By distillation of the heavy oils, carbolic acid and commercial anthracene are produced, and by a treatment of the residue, a white and crystalline substance known as naphthalin (C_{10}H_{8}) is finally obtained.

Thus, by the continued operation of the chemical process known as fractional distillation of the immediate products of coal-tar, these various series of useful oils are prepared.

The treatment is much the same which has resulted in the production of paraffin oil, to which we have previously referred, and an account of the production of coal-oils would be very far from satisfactory, which made no mention of the production of similar commodities by the direct distillation of shale. Oil-shales, or bituminous shales, exist in all parts of the world, and may be regarded as mineral matter largely impregnated by the products of decaying vegetation. They therefore greatly resemble some coals, and really only differ therefrom in degree, in the quantity of vegetable matter which they contain. Into the subject of the various native petroleums which have been found—for these rock-oils are better known as petroleums—in South America, in Burmah (Rangoon Oil), at Baku, and the shores of the Caspian, or in the United States of America, we need not enter, except to note that in all probability the action of heat on underground bituminous strata of enormous extent has been the cause of their production, just as on a smaller scale the action of artificial heat has forced the reluctant shale to give up its own burden of mineral oil. However, previous to 1847, although native mineral oil had been for some years a recognised article of commerce, the causes which gave rise to the oil-wells, and the source, probably a deep-seated one, of the supply of oil, does not appear to have been well known, or at least was not enquired after. But in that year Mr Young, a chemist at Manchester, discovered that by distilling some petroleum, which he obtained from a spring at Riddings in Derbyshire, he was able to procure a light oil, which he used for burning in lamps, whilst the heavier product which he also obtained proved a most useful lubricant for machinery. This naturally distilled oil was soon found to be similar to that oil which was noticed dripping from the roof of a coal-mine. Judging that the coal, being under the influence of heat, was the cause of the production of the oil, Mr Young tested this conclusion by distilling the coal itself. Success attended his endeavour thus to procure the oil, and indelibly Young stamped his name upon the roll of famous men, whose industrial inventions have done so much towards the accomplishment of the marvellous progress of the present century. From the distillation he obtained the well-known Young's Paraffin Oil, and the astonishing developments of the process which have taken place since he obtained his patent in 1850, for the manufacture of oils and solid paraffin, must have been a source of great satisfaction to him before his death, which occurred in 1883.

Cannel coal, Boghead or Bathgate coal, and bituminous shales of various qualities, have all been requisitioned for the production of oils, and from these various sources the crude naphthas, which bear a variety of names according to some peculiarity in their origin, or place of occurrence, are obtained. Boghead coal, also known as "Torebanehill mineral," gives Boghead naphtha, while the crude naphtha obtained from shales is often quoted as shale-oil. In chemical composition these naphthas are closely related to one another, and by fractional distillation of them similar series of products are obtained as those we have already seen as obtained from the crude coal-naphtha of coal-tar.

In the direct distillation of cannel-coal for the production of paraffin, it is necessary that the perpendicular tubes or retorts into which the coal is placed be heated only to a certain temperature, which is considerably lower than that applied when the object is the production of coal-gas. By this means nearly all the volatile matters pass over in the form of condensible vapours, and the crude oils are at once formed, from whence are obtained at different temperatures various volatile ethers, benzene, and artificial turpentine oil or petroleum spirit. After these, the well-known safety-burning paraffin oil follows, but it is essential that the previous three volatile products be completely cleared first, since, mixed with air, they form highly dangerous explosives. To the fact that the operation is carried on in the manufactories with great care and accuracy can only be attributed the comparative rareness of explosions of the oil used in households.

After paraffin, the heavy lubricating oils are next given off, still increasing the temperature, and, the residue being in turn subjected to a very low temperature, the white solid substance known as paraffin, so much used for making candles, is the result. By a different treatment of the same residue is produced that wonderful salve for tender skins, cuts, and burns, known popularly as vaseline. Probably no such widely-advertised remedial substance has so deserved its success as this universally-used waste product of petroleum.

We have noticed the fact that in order to procure safety-burning oils, it is absolutely necessary that the more volatile portions be completely distilled over first. By Act of Parliament a test is applied to all oils which are intended for purposes of illumination, and the test used consists of what is known as the flashing-point. Many of the more volatile ethers, which are highly inflammable, are given off even at ordinary temperatures, and the application of a light to the oil will cause the volatile portion to "flash," as it is called. A safety-burning oil, according to the Act, must not flash under 100° Fahrenheit open test, and all those portions which flash at a less temperature must be volatilised off before the residue can be deemed a safe oil. It seems probable that the flashing-point will sooner or later be raised.

One instance may be cited to show how necessary it is that the native mineral oils which have been discovered should have this effectual test applied to them.

When the oil-wells were first discovered in America, the oil was obtained simply by a process of boring, and the fountain of oil which was bored into at times was so prolific, that it rushed out with a force which carried all obstacles before it, and defied all control. In one instance a column of oil shot into the air to a height of forty feet, and defied all attempts to keep it under. In order to prevent further accident, all lights in the immediate neighbourhood were extinguished, the nearest remaining being at a distance of four hundred feet. But in this crude naphtha there was, as usual, a quantity of volatile spirit which was being given off even at the temperature of the surrounding atmosphere. This soon became ignited, and with an explosion the column of oil was suddenly converted into a roaring column of fire. The owner of the property was thrown a distance of twenty feet by the explosion, and soon afterwards died from the burns which he had received from it. Such an accident could not now, however, happen. The tapping, stopping, and regulating of gushing wells can now be more effectually dealt with, and in the process of refining; the most inflammable portions are separated, with a result that, as no oil is used in the country which flashes under 100° F. open test, and as our normal temperature is considerably less than this, there is little to be feared in the way of explosion if the Act be complied with.