densation, these entering the lower flue at the place where the hearth used to be, whilst air is forced in through an annular pipe. In the Simon-Carvès oven* the air introduced receives a previous heating to some 500° or 600° by its being brought into contact with the hot flues conveying away the spent firegases from the ovens. The two lower flues are thrown into one, and, at the part of the bottom flue where the greatest heat is sustained, the walls are lined with the best silica bricks. The heated air admitted into the bottom flue is purposely insufficient for complete combustion of the gas introduced there, the further supply of hot air being admitted into the side flues of the oven. The arrangement for thus admitting the air is completely controlled by dampers. At the Bear Park Colliery, Durham, fifty of these ovens have been erected. Each is charged with 4 tons of coal, the time for coking being a little over forty-eight hours. At Messrs. Pease's collieries, near Crook, Durham, there are twenty-five of these ovens, each 23 feet long, 6 feet 6 inches high, and 19 inches wide, with side and bottom flues, the capacity being 4 tons of coal. The average cost of an oven of this description, if a hundred were erected, is calculated to be £180.

The Pernolet coke-oven (Fig. 48) differs but little from the ordinary beehive, but it has a fireplace and grate, and the gas is carried into the upper collecting tube a, and returned to the bottom flue, b, where it is fired with solid fuel.

At the Almond Ironworks, Falkirk, Mr. H. Aitken† adapted means for the recovery of by-products to the beehive oven. The Aitken coke-oven, as shown in Fig. 49, is a beehive oven fitted with two pipes, a, a', for conveying the blast and gas from the condensers through small openings in the roof distributed equally round the circumference, whilst channels, b, b′, b', in the floor of the oven conduct the collected by-products to an external pipe, c, which leads them to the condensers. The main body of the oven measures 5 feet from the opening in the roof for filling in coal to the floor, and has a diameter of 9 feet.

The Jameson coke-oven ‡ is another improvement on the beehive oven. It is shown in cross-section and in sectional plan in Fig. 50. In the bottom of the ordinary beehive oven, a, channels are formed, covered with perforated tiles, b, b', b', connected out

* Journ. Iron and Steel Inst., 1883, p. 494; 1885, p. 108.

+ Trans. N. Eng. Inst. M.E., vol. xxix. (1879).

Journ. Soc. Chem. Ind., vol. ii. (1883), p. 114; Journ. Iron and Steel Inst., 1883, p. 504.

side the oven with pipes leading to apparatus, c, c', for producing a slight suction and for discharging the by-products when required. The cost of applying this process is small, and the results of a series of trials show that the average yield of coke is 65.49 per cent., the average yield of ammonium sulphate and of oil being 4.6 lbs. and 6.1 lbs. per ton respectively.

The Lürmann coke-oven (Fig. 51) works continuously, and yields a good compact coke. It consists of a large chamber, a, which may be either a huge tunnel or arched as shown in the figure. Opening into this are a number of coking chambers, b, b', into which fine coal is fed continuously by a piston-feed, worked by a crank, from hoppers. The gaseous products pass into a, and, if required to be collected, they are drawn off at an aperture at the top, and thence conducted into the spaces, c, c', under the

retorts b, b', where they are burnt by means of air admitted for the purpose. The coke as it falls from the ends of b, b', is received in a, and is removed at intervals. A great advantage presented by this coke-oven is that it does not need special fire-bricks.

4. Gaseous Fuel.-The use of gaseous fuel in metallurgical operations has effected a great saving in coal. Besides this advan

tage, there are two circumstances that must lead to its more general adoption; these are the possibility of employing inferior fuels and waste products, and the high temperatures attainable by the use of such fuel. The method of gas-firing is, however, still far short of a universal adoption, but it may be predicted that, as the sub

ject becomes more widely understood, the examples of any other method of treating fuel will become rare.

The use of gas as fuel is based on the principle that at one place all the fuel is converted into combustible gas, which is consumed by admixture with the necessary amount of air at another. These gases are produced by burning the fuel in a long column, whereby most of the carbon is burnt to carbonic oxide, whilst the hydrogen either remains free or is converted into carburetted hydrogen. Directly above the grate carbonic anhydride is formed, but this is converted into carbonic oxide on passing through the column of heated fuel above it. The reaction is shown by the following equation:-CO, + C = 2CO. As a rule, the finer the fuel, that is, the more compactly it lies, the lower is the column required for the reduction of the carbonic anhydride. Besides the products of combustion mentioned above, distillation products are formed in the upper layers of the fuel and mix with the combustible gases. Amongst these, heavy carburetted hydrogen (CH) is that which principally increases the value of the gas as fuel, and the drier the fuel used the more of this gas is produced.

The decomposition of fuel into combustible gas is effected in special apparatus, termed producers, from which it is conducted to the furnace, and burnt. For the formation of producer-gases a certain temperature is required, which must not be exceeded. This temperature is dependent on the amount of air introduced. If this is large, complete combustion is effected, and the desired object is only imperfectly attained. If, however, it is too small, the heat is not sufficient for the formation of carbonic oxide, and the evolution of gas ceases. The exact amount of air that should be introduced into a gas-producer can only be determined by experiment.

The composition of producer-gases is, by weight, as follows :—

The gas-producer first assumed importance in 1856, on the introduction of the Siemens regenerative system. In 1861 the well-known Siemens gas-producer was patented. It consists of a chamber lined with fire-brick, with one side sloping at