Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/92

Rh 72 C A L [MINING. prevent the air from taking a short cut back to the up cast, while preserving free access between the different districts without following the whole round of the air ways. The ventilation of ends is effected by means of brattices or temporary partitions of thin boards placed midway in the drift, and extending to within a few feet of the face. The air passes along one side of the brattice, courses round the free end, and returns on the other side. In many cases a light but air-proof cloth, specially made for the purpose, is used instead of wood for brattices, as being more handy and more easily removed. In large mines where the air- ways are numerous and complicated, it often happens that currents travelling in opposite directions are brought together at one point. In these cases it is neces- Crossings. sary to cross them in the manner shown in fig. 2, Plate III. The return air is usually made to pass over the intake by a curved drift carried some distance above in the solid measures, both ways being arched in brickwork, or even in some cases lined with sheet-iron so as to ensure a separation not likely to be destroyed in case of an ex plosion. The relation of the ventilation to the workings under the different systems is indicated on the several plates by arrows and other signs, from which the general character of the arrangements adopted can be made out without further description. Lighting. The lighting of underground workings in collieries is closely connected with the subject of ventilation. In many of the smaller pits in the Midland districts, and generally in South Staffordshire, the coals are sufficiently free from gas, or rather the gases are not liable to become explosive when mixed with air, to allow the use of naked lights, candles being generally used. Oil lamps are em ployed in many of the Scotch collieries, and are almost universally used in Belgium and other Continental coun tries. The buildings near the pit bottom, such as the stables and lamp cabin, and even the main roads for some distance, are often in large collieries lighted with gas brought from the surface, or in some cases the gas given off by the coal is used for the same purpose. Where the gases are fiery, the use of protected lights or safety lamps becomes a necessity. Composi- The nature of the gases evolved by coal when freshly tion of gas exposed to the atmosphere has been investigated by several y chemists, more particularly by Playfair and Meyer. The latter observer found the gases given off by coal from the district of Newcastle and Durham to contain carbonic acid (anhydride), marsh gas or light carburet ted hydrogen (the fire-damp of the miner), oxygen, and nitrogen. A newer investigation, by Mr J. W. Thomas, of the gases dissolved or occluded in coals from South Wales basin shows them to vary considerably with the class of coal. The results given below, which are selected from a much larger series published in the Journal of the Chemical Society, were obtained by heating samples of the different coals in vacuo for several hours at the temperature of boiling water.

Composition in Volumes per cent. olunie L Quality. Colliery. per ton in cubic feet. Car bonic Acid Oxvgen. Marsh Gas. Nitro gen. Bituminous. Cwm Clydach. 1972 5-41 1-05 6&quot;- 76 2970

Lnntwlt. 14-34 9-4:i 2-25 31-95 5G 34 Steam. Navigation. 89-62 13-21 0-49 81-64 4-C6 Anthracite. -J I 19894 2-C2 ... 93-13 4-25 In one instance, about 1 per cent, of bydride of ethyl was found in the gas from a blower in a pit in the Rhondda dis trict, which was collected in a tube and brought to the surface to be used in lighting the engine-room and pit-bank. The gases from the bituminous house coals of South Wales are comparatively free from marsh gas, as compared with thosa from the steam coal and anthracite pits. The latter class of coal contains the largest proportion of this danger ous gas, but holds it more tenaciously than do the steam coals, thus rendering the workings comparatively safer. It was found that, of the entire volume of occluded gas iu an anthracite, only one-third could be expelled at the tem perature of boiling water, and that the whole quantity, amounting to 650 cubic feet per ton, was only to be driven out by a heat of 300 C. Steam coals being softer and more porous give off enormous volumes of gas from the working face in most of the deep pits, many of which have been the scene of disastrous explosions. The gases evolved from the sudden outbursts or blowers in coal, which are often given off at a considerable tension, are the most dangerous enemy that the collier has to con tend with. They consist almost entirely of marsh gas, with only a small quantity of carbonic acid, usually under 1 per cent., and from 1 to 4 per cent, of nitrogen. Fire-damp when mixed with from four to twelve times its volume of atmospheric air is explosive ; but when the proportion is above or below these limits, it is inflam mable, burning quietly with a pale blue flame. When a lighted candle is exposed in a non-explosive mixture of this gas, the flame gradually elongates, forming a conical cap, floating above the wick, which may be extinguished by cautious withdrawal without communicating the fire to the surrounding atmosphere. This method of testing for gas in the working places and wastes, which is obviously only to be trusted in skilled hands, used to be commonly practised, but since the introduction of safety lamps it has fallen into disuse. The principle involved in the construction of safety- Safe lamps consists in surrounding the flame of a lamp by lami a protecting metal case, perforated with numerous small holes, through which the air for feeding the flame may freely enter, and the products of combustion pass out, while the passage of flame, or gases sufficiently heated to cause the ignition of the external air when laden with explosive gases, is prevented. In 1816 Sir Humphrey Davy made the great discovery that these conditions are fulfilled by the use of tubes reduced to a mere section, such as the apertures in wire gauze, when the substance of the wire is rightly proportioned to the size of the aperture. The standard adopted as the limit for safety at that time was a gauze of 28 iron wires to the linear inch, having 784 apertures per square inch, which has been used ever since. The common safety or Davy lamp consists of a small cylindrical oil lamp, covered with a cylinder of wire gauze about 6 inches long and 1J inches in diameter, with a flat gauze top. The upper part of the gauze is doubled to prevent its being worn into holes by the products ot combustion, and the air for feeding the flame enters round the wick. The gauze is mounted in a cage, consisting of three upright wires, screwed into a flat brass ring at each end. A handle is attached to the upper ring, while the lower one screws on to a collar on the oil-vessel of the lamp. When the two parts are screwed together the lamp is locked by a bolt passing through both parts, which is screwed down flush with or below the surface of the outer ring, so that the gauze cannot be removed without the use of a key. In Stephenson s safety-lamp, generally known as the &quot; Geordie,&quot; from the inventor George Stephenson, the light is covered by a glass chimney, surrounded by an outer casing and top of wire gauze. The feed air is admitted through numerous small holes in a copper ring a little below the level of the wick. This is one of the safest forms of lamp, but requires considerable care in use, espe cially in keeping the small feed holes clear from dust and oil ; the glass protects the gauze from becoming overheated,