The

an angle of 45° to 60°, with the grate at the bottom. grate thus resembles the step-grate largely used on the Continent. The fuel is charged in at the top of the incline, and falls in a thick bed upon the grate, where air is admitted. Passing from the top of a brick shaft or up-take, 8 to 10 feet high, placed above the producer at the back, there is a cooling tube, having not less than 60 square feet of surface per producer. Its object is to cool the gases issuing from the producer, thus giving them increased density, causing an onward movement towards the furnace, and rendering it unnecessary to place the producer at a much lower level than that of the furnace. This cooling, however, results in a condensation of tar, and, to overcome this annoyance, modifications have been adopted in the producer and its working. In the new type of Siemens producer the volatile products of distillation are obliged to descend through highly heated fuel, thus causing the tar to undergo decomposition.

The principal forms of gas-producers are described by Rowan.* Since the introduction of the regenerative principle, many designs for furnaces have been proposed, without separate gas-producers. One of the most successful of these is the Boëtius furnace, largely used for zinc-smelting.

The Wilson gas-producer (Fig. 52), working under a slight pressure, presents the advantage of burning fine slack coal. Fuel is introduced in the chamber A, through a hopper, h. The ashes are drawn by two small doors, c, at the bottom every twenty-four hours, the operation occupying about twenty minutes. During this time the production of gas is stopped. The producer represented in the figure has a diameter of 8 feet, and is constructed to burn 4 cwt. of small coal per hour. Air is injected into the chamber by two steam jets, b, each having a diameter of inch. Analyses of gas from the Wilson producer gave the following results :

I. is from a producer using Durham coal. II. is an average of six samples of gas taken over a time of one hour from a producer working on fine slack from a Yorkshire colliery. III. is from a producer working with coal of the Jemmapes district, near Mons, Belgium.

In 1814 Aubertot first used the waste gases of blast furnaces for roasting ore, burning lime, and similar purposes, and these

gases are now largely used when a very high and uniform temperature is not required. The composition by weight of the waste gases, according to the fuel used in the blast furnace, is as follows:

Water-gas.-The gaseous fuel known as water-gas is made in the following manner :-An iron cylinder is lined with fire-brick, and provided with the necessary apparatus for introducing the coke. When this has been lighted, a current of air is forced in until the mass is brought to a high temperature. The blast is then stopped, the charging aperture is closed, and a jet of steam is passed through the incandescent carbon. The steam is decomposed; its oxygen burns the carbon into carbonic oxide, setting free the hydrogen. The resulting mixture is known as water-gas. It consists of one volume of hydrogen and one volume of carbonic oxide, the weights being in the proportion of 2 to 28. If, by burning one unit of carbon, it were possible to generate one unit of hydrogen, the exchange effected in the water-gas apparatus might be a very profitable one. Such a condition of things is, however, shown by Sir Lowthian Bell* to be directly opposed to the known facts of the case, for 25 per cent. only of the carbon used is burnt to the condition of water-gas, whilst the other 75 per cent. is converted into producer-gas, containing 68 per cent. of inert nitrogen. From 25 parts by weight of carbon there will be generated 62.5 parts of water-gas, containing 4.16 of hydrogen and 58.34 of carbonic oxide. The producer-gas from the remainder (75 parts) of the carbon will weigh 551.19 parts, of which 376.19 will be incombustible nitrogen and 175 carbonic oxide. The following estimate gives the quantity of heat generated by the combustion of the two gases :

Had the 100 parts of carbon been burnt direct, the heat generated would have been 800,000 calories. The loss is thus 14.71 per cent. Besides this, as coke was used, there is the loss of combustible matter which is incurred at the coke-oven, and the labour in conducting the process of coking.

Sir Lowthian Bell calculates that the relative values of coal, producer-gas, and water-gas are as follow:

Bibliography.-For fuller information on fuel the student is referred to the following standard treatises :-Percy, Metallurgy, vol. i. London, 1861; second edition, 1875; Galloway, Treatise on Fuel, London, 1880; Schwackhöfer and Browne, Fuel and Water, London, 1885; Phillips and Bauerman, Elements of Metallurgy, London, 1887, pp. 16-107; Mills and Rowan, Fuel and its Applications, London, 1889; Gruner, Traité de Métallurgie, Paris, 1875, vol. i. pp. 36-161; Kerl, Grundriss der allgemeinen Hüttenkunde, Leipzig, 1879, pp. 36-198; Muck, Grundzüge der Steinkohlen-Chemie, Bonn, 1881; Fischer, Chemische Technologie der Brennstoffe, Leipzig, 1887. Schnabel, Lehrbuch der Allgemeinen Hültenkunde, Berlin, 1890.

CHAPTER VI.

MATERIALS AND PRODUCTS OF METALLURGICAL PROCESSES.

Ores. This term is applied by the metallurgist only to those minerals from which, on a large scale, metals may be obtained with profit. The ores must be supplied to the works in a suitable condition for smelting, the preliminary washing and dressing operations being carried out at the mine. Gangue, vein-stuff, or matrix is the extraneous earthy matter associated with the ore.

Ores contain the metals—(1) in the native or metallic state (examples gold, silver, copper, mercury); (2) in combination with oxygen as oxides (for example, ferric oxide, tin oxide); (3) as oxides in combination with water (limonite, Fe,H,O,); (4) in combination with halogens (silver chloride); (5) in combination with sulphur, arsenic, and antimony (galena, PbS); (6) in combination with acids as salts (anglesite, PbSO); ores also occur in nature in a state of mixture; (7) as various combinations of the same metal (for example, azurite, 2CuCO, + Cu(HO),); (8) as various combinations of more than one metal in one mineral species (for example, pyrargyrite, 3Ag,S,Sb,S,); lastly, (9) as several mineral species occurring together in the ore-deposit, galena and blende, spathic iron ore and iron pyrites.

The value of an ore depends upon the nature of the metal it contains and the difficulty with which its extraction is attended. Thus, iron ores containing less than 30 per cent. of metal are rarely smelted. Ores of iron, lead, or zinc are not considered rich unless they contain 50 per cent. of metal. Copper ores are rich when they contain 25 per cent. of metal, whilst ores yielding a few ounces of gold per ton are extremely valuable.

Fluxes. In order to separate the extraneous matter usually contained in a furnace charge of ore and reducing agent, certain materials must be added to form slag. These materials are known as fluxes.

In the smelting processes earthy, alkaline, and metallic sub