It yields 42 per cent. of volatile matter. The mineral jet is a bituminous lignite. When the bituminous constituents of these rocks occur in sufficient quantity, naphtha and petroleum springs, or deposits of solid asphalt, are formed. At Baku in the Caspian Sea, and in Pennsylvania, mineral oils are obtained in enormous quantities by means of deep bore-holes. The oil regions of Pennsylvania alone have a productive area of 364 square miles, and the total output from 1859 to 1884 was 260,990,035 barrels, each of 42 gallons.

Petroleum contains a very high proportion of hydrogen, which in the case of the solid bituminous materials may reach 15 per cent. On distillation, from crude petroleum a product is obtained containing 79.82 to 88.5 per cent. of carbon and 11.5 to 20.18 per cent. of hydrogen. The boiling-point of petroleum varies from 110° to 280°, and the calorific power of the crude oil is 10,000 calories, a calorific power which is greater than that of refined oil.

The oil is employed in practice by burning it in a trough, by effecting its combustion in a spray or finely divided form by injecting it with a jet of steam or air, or by converting it into gas before combustion.*

7. Natural Gas. In the oil regions of Pennsylvania and of the adjoining States, natural gas issues from the strata at a depth of 500 to 2000 feet below the surface; and when bore-holes are sunk to the accumulated gas, it rises under a pressure of some 200 lbs. per square inch. When first reached, the gas is sometimes evolved at the enormous pressure of 1000 lbs. per square inch. Compared with air the gas has a density of 0.45 to 0.55, and varies in volumetric composition within the following limits:

H.

N.

C2H6.

C2H4

CH4. 60 to 80 5 to 20 I to 12 I to 8 o to 2

...

...

...

...

It has a calorific power of 14,000 to 15,600 calories, and a calorific intensity of 2745° to 2765°. Natural gas has long been used in Pennsylvania to a limited extent for heating purposes. Since 1883, however, it has attained a remarkably rapid development for industrial purposes, and is now largely used in Pittsburgh in the blast furnace, and for other metallurgical purposes. The territory containing the source of natural gas includes a section of country extending from Western New York, through Pennsylvania, into West Virginia and Ohio. It is estimated that

* On petroleum, consult Crew, Practical Treatise on Petroleum, Philadelphia, 1887.

in 1884 as much as £292,000 worth of coal was supplanted by natural gas, about two-thirds of this amount representing the consumption of the Pittsburgh district. The freedom of the gas from sulphur has been an important element in its metallurgical value.*

II. PREPARED FUELS.

1. Compressed Fuels.-Numerous attempts have been made to prepare a good fuel by mixing some binding material with small coal in proportions sufficient to enable the particles to cohere so as to be pressed into a block or briquette. Potatomeal, soluble glass, asphalt, and turpentine have been used as binding materials, but abandoned, whilst coal-tar, pitch, and even treacle have been successfully used for the purpose.†

2. Dried Fuels.-The advantages derived from the expulsion of the water and a certain proportion of the more volatile constituents of wood, peat, and lignite have already been pointed out.

3. Carbonised Fuels.-On heating fuels without access of air, their constituents re-arrange themselves in the form of solid, liquid, and gaseous compounds. For metallurgical purposes, the object of this operation is often only to obtain the solid constituent, charcoal or coke, which consists of carbon with subordinate amounts of hydrogen, oxygen, and ash-giving constituents, and which has a high calorific intensity. At the same time, the byproducts obtained during the carbonisation are frequently utilised. The carbonised fuels to be considered are—(a) charcoal and (b) coke.

(a) Charcoal.-This name is given to the carbonised residue remaining after the dry distillation of wood. When wood is heated to 200° without access of air, it remains unaltered; at 220° it becomes brown; and at 270° to 300° it suffers decomposition, torrefied wood (Rothholz) being formed. At 350° it is resolved into a fixed residue, or charcoal, and volatile products. Good charcoal prepared at a temperature of 350° to 400° retains the structure of the wood from which it was derived, the volume being less. It is black, porous, and burns without smoke, and, in separate pieces, without flame. The specific gravity of porous charcoal varies from 0.28 to 0.54 according to the nature of the original wood, and the temperature at which it was made.

* On natural gas, consult Phillips, Rep. Second Geol. Surv. of Pennsylvania; Carnegie, Journ. Iron and Steel Inst., 1885, p. 168; Ashburner, Trans. Amer. Inst. M.E., vol. xiv. p. 428.

+ Compare Preissig, Die Presskohlenindustrie, Freiberg, 1888.

Charcoal prepared at 350° is considerably lighter than that prepared at the melting-point of platinum. Hard woods give a dense and heavy charcoal, whilst soft woods give soft and friable charcoal. The chemical composition also varies considerably, the percentage composition of charcoal prepared at 340° (I.), and at the melting-point of platinum (II.), being as follows:

The ash of the first charcoal was 0.48, and of the second 1.94. per cent. On an average, dry charcoal contains 90 per cent. of carbon, 3 per cent. of hydrogen, and 7 per cent. of oxygen and nitrogen.

The charcoal used in metallurgical operations is prepared by various methods, which may be divided into two groups-viz., methods of coking (1) in the open air and (2) in closed chambers. For charcoal burning in the open air, a suitable site is necessary. This should be dry, and sheltered from any prevailing wind.. Water should be at hand for quenching the charcoal when made.. The wood employed must be mature, cut while free from sap, barked, and air dried for some months.*

When the charring is effected in circular piles, or meilers, the bed is made to slope from the circumference to the centre at an inclination of 1 in 15. Three stakes, 10 to 15 feet high, are driven in near the centre, so as to form a central triangular chimney. Around this, the timber, cut into suitable lengths, is stacked horizontally and radially. The mass is then covered with a mixture of fine charcoal and clay, and then with sods with the grassy side inwards. To keep this covering up, wedges are driven in, and props put in so as to form hoops around the lower part of the pile. Brushwood is then thrown down the chimney, and ignited, vents being made near the top of the pile. This causes a cone, with the apex downwards, of the pile to be charred, and, by opening vents lower down, the angle of the cone is enlarged. This process is continued until the base is reached. When the smoke issuing from the pile is seen, by its blue colour, to be free from aqueous vapour, the charring is complete. The charcoal is then drawn from the bottom of the pile and in small quantities. quenched with water or dust. Small piles are carbonised in six to

* On charcoal burning, consult G. Svedelius' Handbook for Charcoal Burners, translated from the Swedish by R. B. Anderson and W. J. L.. Nicodemus (New York, 1875).

fourteen days; but if the diameter be more than 30 feet, the process occupies a month.

Logs as much as 24 feet in length may be charred in rectangular piles. They are laid together in the form of a wedge, of which the breadth is limited by the length of the logs. The heap is 20 to 30 feet long, and 7 to 9 feet high at the upper end, and only 3 feet high at the lower end. It is surrounded on all sides with a layer of sods and charcoal dust, and by a wooden covering supported by vertical stakes. On the top is placed a roof of twigs, leaves, and charcoal dust. At the lower end, a horizontal chimney is left. Vents are opened at the opposite end so as to give planes of charring. Rectangular piles are used in Sweden and in Austria. In China the carbonisation is effected in pits provided with a chimney communicating with the bottom.

Experiments, on a large scale, on the amount of charcoal yielded gave the following results:-In France, with circular piles of 2120 to 3180 cubic feet, the yield by weight was 17 to 21.3 per cent.; in Belgium, on charring wood fifteen to twenty years old, half hard, half soft, the yield at the ordinary rate was 15 to 17 per cent., but when charred more slowly, 20 to 22 per cent.; in Sweden, from pine wood, the yield was 20 to 28 per cent. By volume, the yield of charcoal varies from 50 to 75 per cent.

By using closed ovens or kilns, the yield of charcoal may be somewhat increased, but as about 5 per cent. is required for heating the oven or retort, the advantages presented are very slight. Peat may be charred, like wood, in heaps or in kilns. Peat-charcoal, however, on account of its lightness, friability, and its high percentage of ash, is not adapted for metallurgical purposes, and its application has not advanced beyond the experimental stage.

(b) Coke.-Coke is the carbonaceous residue left on the dry distillation of coal. Good coke should possess sufficient strength to withstand the pressure in a blast furnace without crushing. For this reason, only those coals which give on dry distillation a coherent residue can advantageously be used. The coals of the second, third, and fourth groups of Gruner's classification are suitable for coking. A high percentage of ash has a detrimental influence on the coke produced, and must therefore be removed by subjecting the coal to a preliminary washing.

Coke varies considerably in its external characters. It may be porous and light, or compact and heavy; black and dull, or light-grey and bright, with a semi-metallic lustre. The porosity of coke induces a tendency to absorb gas: for example, Belgian locomotive coke contains-

Like charcoal, coke is hygroscopic, absorbing 1 to 2.5 per cent. of water from air at the ordinary temperature. When dipped in water, coke will absorb 20 to 50 per cent. The calorific power of coke, free from ash, is 8000 calories, or almost equal to that of pure carbon.

The earliest method of preparing coke was by carbonising coal in the open air in heaps, without any external covering. A more economical method is by carbonising the coal in mounds, piled round a central octagonal chimney. The mound is ignited at the top, and the operation proceeds exactly as in charcoal burning. The mounds are usually 12 to 21 feet in diameter, and 9 to 15 feet high. The yield after five days' coking is 65 per cent. of the theoretical quantity.

In Upper Silesia, rectangular kilns have been used for coking for many years. They are cheap, easy to work, and give a large output, but, like the mounds, they do not yield a uniformly coked product. They are chiefly used for coking coal slack. These kilns, or Schaumburg coke-ovens, Fig. 46, have two fixed parallel

walls 18 yards long and 5 feet high. The walls are 8 feet apart, and are provided with a series of square openings 2 feet apart and I foot above the sole, the apertures, c, in one wall being opposite those in the other. From each aperture ascends a vertical flue, d, which may be closed with a tile. In order to charge the kiln one of the open ends is bricked up, and moistened coal slack is stamped-in up to the level of the apertures. From the apertures in one wall wood stakes are placed across, the ends passing into the corresponding apertures on the opposite side, and the whole kiln is filled with moistened slack, and covered with loam. On withdrawing the stakes, the passages formed communicate with the horizontal apertures and with the vertical flues in the side