For hundreds of years it has been known that when coal is covered or enclosed, to keep out the air, and then heated for a certain length of time, instead of burning to ash it is converted into a porous grayish-black substance called “coke.” This material, which burns without smoke or flame, is a valuable fuel for many purposes; especially for use in blast-furnaces for the smelting of ore. Nowadays coke is made on a vast scale from certain grades of bituminous and semi-bituminous coal. The coal is heated in “coking-ovens,” of which there are several kinds. The most common form of oven in this country is the “bee-hive oven,” which produces coke only. Another type of coking-oven, more generally used in Europe than in America, is the “flue-oven,” which produces, besides coke, a number of valuable by-products. When coal is converted into coke it gives off combustible gases. The idea of saving these gases and using them for illuminating purposes was first practically applied in the latter part of the eighteenth century. “Coal-gas” is made by heating coal in a closed vessel, called a “retort.” It is a mixture of hydrogen and methane (a compound of hydrogen and carbon), with small amounts of several other gases. Most of the carbon in the coal remains in the retort as coke, which is, therefore, a by-product in the process of making coal-gas. After the gas is given off from the coal it passes through a series of vessels, where, by chemical and other methods, it is freed from ingredients which would impair its value as an illuminant, but which are saved and used for other purposes; the most important of these are “coal-tar” and “ammoniacal liquor.” The purified coal-gas is finally conveyed to a gas-holder or “gasometer,” from which it is distributed to the consumers. In recent times other methods of gas-making have come into use. In one of these nearly all the carbon in the fuel is turned into a combustible gas by passing air through the hot coal. The product is known as “producer-gas,” and is very valuable for use as fuel and as a motive power in gas-engines, but it is not an illuminant. A modification of this process, in which steam is passed over the heated fuel, gives a mixture of hydrogen and carbon monoxide, known as “water-gas.” This is also a valuable source of heat and power; but for use as an illuminant it must be mixed with a gas made from oil. It is then known as “carburetted water-gas,” and is very extensively used for lighting purposes; either by itself or mixed with ordinary coal-gas.