COKE is coal divested of its gaseous and more volatile constituents by partial combus tion in close chambers, or in heaps from which the free access of air is excluded. The sim plest mode of coking coal, which is still occa sionally followed, is to lay the coal in large flattened heaps, often containing thirty or forty tons each, in the open air ; and to cover it with ashes and earth to confine the heat, so that the mass of coal may be slowly burnt in a smothered manner : men being employed during the process to renew the covering wherever the fire may begin to burn too fiercely. But the best mode of making coke is by means of ovens. These are of different forms ; but in principle ttley consist of an in closed chamber, in which the coal is burned without access of air : the volatile ingredients are driven off, but the carbon remains. Coke is also made, in the process of making gas, in all gas-works : the coal which is put into the cylinders to make the gas being removed as coke after the volatile matter has been carried off to the gasometer. Coke is much prefer able to coal where pure fuel is required in many smelting operations, and where smoke is wished to be avoided. An immense quan tity is now used for locomotives ; and the principal railway companies have established coke-ovens at their depots. From a given quantity of coal, the produce of coke is about 20 per cent. less in respect to weight, hut 20 per cent. more in respect to bulk.
There has recently been established a com pany at Alba, for supplying coke to the rail way companies of Scotland. Coke ovens have been built at a spot which lies near the Forth, and also near the Alloa branch of the Scottish Midland Railway ; so that there are great fa cilities both for receiving coal and sending out coke.
Mr. Church's method of making coke, in troduced in 1846, has many peculiarities. The coke oven is much more complicated than those usually made, and the process altogether more carefully conducted. The coal is thrown into the oven so as to form a layer about two feet thick over the whole area. A slight de pression is made in the middle of the surface of the layer ; and in this depression is thrown a little burning coal. All apertures aro then
closed, except a few for admitting air for com bustion, and these are gradually more and more closed as the coking advances. When the cessation of flame shows that the coal is wholly converted into coke, the last aperture is closed. In ordinary coke-making, the coke is cooled by cold water being applied to it ; but by Mr. Church's method, two air passages are opened, which allow a stream of cold air to enter a series of passages which run under and around the oven, without communicating with the interior ; these passages end in a chimney or flue ; so that the air cools the oven as it passes, becomes itself heated, and finally escapes. Not until the oven and the contents are thoroughly cold is the coke re moved. The professed object of this method (which, regarded in a scientific point of view is much more complete than the ordinary me thod) is to produce coke which shall be purer, denser, harder, of more powerful heating qua lity, and more abundant in quantity, than was before producible from a given amount of coal. Of course a long and strict trial could alone ' test the existence of these superior qualities.
Mr. Fisher patented a new form of coke oven in 1819, intended to produce a better in gress and egress of air to the burning mass beneath the coke retorts, than in the ordinary construction.
It has been recently discovered, that al though coke is apparently a loose spongy sub. stance, the particles of which it is formed are intensely hard—so hard indeed as to cut glass like a diamond, which no other mineral will effect so well. This is deemed a striking con.. firmation of the well known chemical fact, I that the diamond is nothing more than pure carbon in a crystalline state. It is expected that coke, carefully prepared for the purpose, will gradually come into use as a substitute for the diamond in the ordinary cutting of plate and window glass. Mr. Nasmyth stated at the Swansea meeting of the British Associ ation, that the eminent firm of Messrs. Chance at Birmingham, by whom all the glass for the Crystal Palace has been since made, looked forward to a saving of 4001. a year, in respect to this apparently simple discovery.