The profit however, to be gained from the sale of coke, must be both certain and considerable, to induce a preference for the former course; because the decomposition of coal tar, besides superseding a proportionate quantity of coal, is attended with several other very tempting advantages.
From experiments lately made in the metropolis on this subject, in which I have been engaged, it appears that in all large gas light establishments, where the quantity of coal tar rapidly accumulates, and must be got rid of, and in all places where the tar cannot be sold for more than four shillings the hundred weight, it will be certainly advantageous to the manufacturer to decompose the tar for the production of carburetted hydrogen gas.
The price of coal cannot effect the operation, because where coal bears a high price, the manufacturer of the tar gas, will diminish the quantity of coal which he would otherwise be called upon to employ for the production of the requisite quantity of gas. And in places where coal is cheap, the decomposition of the tar will be attended with less expence.
The carburetted hydrogen gas produced from coal tar, possesses a greater illuminating power than the gas obtained from coal.[53] It consists chiefly of supercarburetted hydrogen or olifiant gas, and a less quantity of it is of course sufficient.
[53] Vegetable tar, also affords carburetted hydrogen gas in abundance, and this no doubt might be employed to great advantage for the production of artificial light in places where it is cheap. 212 pounds of the most viscid Swedish tar, produce 1484 cubic feet of carburetted hydrogen, (or seven cubic feet to one pound of tar,) the illuminating power of this gas is equal to the gas obtained from pit coal.
The gas thus obtained, is purified likewise with far greater facility, taking only one hundred and twentieth part of the quantity of quicklime which is required for the purification of carburetted hydrogen obtained from pit coal. The apparatus for the production of carburetted hydrogen from coal tar, is moreover less bulky, less expensive, and less complicated; and it can be managed by fewer workmen. And as the combined result of these several advantages, it is obvious, that by the substitution of coal tar, the new mode of lighting by gas can be pursued on a smaller scale; which it can never be with any profit, where coal itself is immediately employed for the production of the gas.
The apparatus employed by Mr. Clegg, for the distillation of tar, is extremely simple. It consists of two hollow cast iron cylinders, twelve inches in diameter, and nine feet long, furnished with moveable lids or mouth pieces, and joined together at the extremity opposite to the mouth piece. These cylinders are fixed in a brick furnace, so that each inclines eleven degrees, one above and the other below the horizontal base of the furnace.
When the apparatus has acquired a dull red heat, the coal tar is suffered to flow into the upper cylinder, by small portions at a time.
The tar is contained in a closed vessel, situated at any convenient place above the apparatus. It has a small aperture for the admission of air. But as a sufficient small quantity of viscid tar does not flow freely in a thin stream, a larger portion than is wanted, is made to flow first into a a small box, upon the apex of a pyramid which divides the stream, so that the excess runs off by a waste pipe, whilst a due quantity only is conveyed into the retort where it is decomposed.
This apparatus[54] therefore differs only from the apparatus described in the Journal of Science and the Arts, 1816, No. II., p. 282; that the cylinders may be detached, for cleaning them out more conveniently.