CHAMBERS'S INFORMATION FOR THE PEOPLE. 



matter of great importance, although it has been 

 somewhat simplified since the Act of 1863, which 

 requires that there must be two shafts to every 

 colliery. Fresh air not only requires to be sup- 

 plied to all the workings, but it must be forced 

 through the passages with a sufficient current to 

 sweep away all noxious gases. Of these, light 

 carburetted hydrogen or fire-damp, and carbonic 

 acid or choke-damp, are the most dangerous. The 

 former forms an explosive mixture in certain pro- 

 portions with air, and is the great cause of ex- 

 plosions in coal-mines. Wherever it occurs in 

 any quantity for it is not present in all coal-seams 

 safety-lamps must be used by the miners ; of 

 these there are several kinds, but it will be suffi- 

 cient to notice the one invented by Sir Humphry 

 Davy in 1816. This is simply an oil lamp 

 inclosed in a cylinder of wire-gauze containing 

 from six hundred to eight hundred 

 holes to the square inch seven hun- 

 dred and eighty-four being fixed upon 

 as the standard. Fire-damp passes 

 freely through the apertures, so that 

 when the lamp is held in a fiery 

 atmosphere, the gas inside the cylin- 

 der takes fire, but, on account of the 

 cooling action of the wire-gauze, the 

 flame cannot kindle the fire-damp 

 outside, unless it is blown through 

 with considerable force. Great care 

 is required to keep the Davy lamp in 

 good repair, because the slightest in- 

 jury may destroy its safety ; yet the 

 experience of half a century has 

 proved that, when used with proper 

 caution, it is practically safe in the 

 most explosive atmospheres. Its 

 feeble light, however, often tempts the 

 more reckless miners to remove the 

 wire-gauze, or to work with other 

 naked lights, even in the presence of fire-damp, 

 and hence the cause of many a dreadful accident. 

 The duration of our coal-supplies depends 

 greatly on the depth to which it is possible to 

 carry mining operations. As already stated, two 

 thousand feet has been reached in the north of 

 England ; and in Belgium, coal is being worked 

 at the depth of two thousand eight hundred feet. 

 Indeed, in that country, one shaft has been sunk 

 to over three thousand feet. The expense of 

 sinking shafts to these depths, however, is so 

 great that a considerable rise in the price of fuel 

 will probably take place before they are sunk 

 much deeper. At three thousand feet below the 

 surface, the temperature is known to be at 98 F., 

 or blood heat, and at this high temperature, 

 labour for more than short intervals is not practi- 

 cable, where, as in a coal-mine, the air is always 

 more or less moist. Still, many practical men 

 consider that by more effective ventilation, and 

 possibly also by other means of artificially cooling 

 the air, a depth of four thousand feet will ulti- 

 mately be reached. Of course, if it should ever 

 be possible to penetrate to still greater depths, 

 the areas of our available coal-seams will be 

 greatly extended. 



Within the last twenty years, some costly litiga- 

 tions have taken place in Edinburgh, London, 

 Prussia, and Canada about two minerals, in re- 

 gard to which the question was whether they 

 were or were not coal One of the substances in 



388 



Safety- 

 Lamp. 



dispute was the Boghead cannel coal or Torbane- 

 hill mineral, found at Bathgate, and now nearly 

 exhausted, which yields so much gas or mineral) 

 oil on distillation, that a few years ago it sold as. 

 high as four pounds sterling per ton. The other 

 was the mineral called Albert coal or Albertitc, , 

 found in New Brunswick, a beautiful, glossy, jet- 

 like substance, also very rich in the hydrocarbons 

 which form gas or burning oil. With regard to> 

 the first, the one side asserted that it was a true- 

 coal ; the other, that it was a bituminous shale,, 

 because, although it yielded products like coal, 

 yet, unlike most coals, it had a dull earthy look,, 

 and a very high percentage of ash. As respects 

 the Albertite, the issue was whether it was coal or 

 asphaltum, those who held the latter view found- 

 ing chiefly on the fact, that its geological position i 

 was that of a vein, and not that of a bed or 

 stratum. In these trials, which were very pro- 

 tracted and enormously expensive, great numbers 

 of scientific and practical men were engaged on 

 both sides. Their object, however, was quite as 

 much to decide with reference to the nature of a 

 bargain, as to make out a precise definition of 

 coal. Sometimes the verdict was one way, some- 

 times the other ; and on the whole, as has beem 

 well remarked, these trials have only succeeded; 

 in shewing that no suitable definition of coal 

 exists, and that there will always be differences of 

 opinion when the substance to be determined 

 approaches the boundary of the shales and of the- 

 bitumens. 



Bituminous Shale. In several geological for- 

 mations, but chiefly in the Carboniferous, there 

 exist extensive beds of shale so highly bituminous 

 that mineral oil for burning and lubricating, as 

 well as paraffine, can now be profitably distilled 

 from them, indeed more profitably than from even 

 the richest cannel coal. It is, remarkable to say,, 

 not more than fifteen or sixteen years since these 

 very shales were looked upon as among the most 

 worthless of sedimentary strata. Nothing, in fact, 

 in the history of mineral industries, is more 

 striking than the suddenness with which a new 

 process, chemical or mechanical, will sometimes 

 convert a waste or useless material into one of 

 great value, by extracting from it products of 

 much utility or beauty. These oil shales, as they 

 are now called, can sometimes be advantageously 

 distilled when they do not yield more than twenty- 

 five gallons of crude oil per ton ; but they not 

 unfrequently yield as much as fifty gallons, and 

 between these there are all degrees of richness. 

 In Great Britain there are now about twelve 

 million gallons of burning oil annually produced, 

 for which about seven hundred and fifty thousand 

 tons of shale are required. As those who have 

 most experience in the matter believe that the cost 

 of the distilling apparatus at present in use will 

 eventually be greatly lessened, it is highly prob- 

 able that much poorer shales will sooner or later 

 come into use. But even the richer shales extend 

 over an area of very many square miles ; and as- 

 the workable seams vary in thickness from three- 

 to six feet, the quantity of light and heat giving 

 material, still stored up in our coal-measures,, 

 apart altogether from coal itself, amounts to 

 millions of tons. The intrinsic value of it, too, 

 is, roundly speaking, about one-half that of coal. 



Naphtha Petroleum Asphalt. Bitumen, al- 

 though not a very exact term, is convenient as a. 



