68 CLASSIFICATION OF THE PUBLIC LANDS. 



lack one or more of three things — quality, thickness, accessibility — 

 that is, they are too poor, too thin, or too deep. Other things — such 

 as poor roof, gas, water, faults, pitch, and lack of timber — ^may 

 render mining difficult and temporarily unprofitable, but most or all 

 of them are subject to engineering control. They may depreciate 

 the value of the coal and defer its mining but may not make it 

 unworkable. 



QUALITY. 



Coal is essentially a fuel. The heat afforded by burning coal is 

 derived mainly from its carbon and the hydrogen that is free to 

 burn. Associated with these are oxygen, nitrogen, water, and ash. 

 A pound of the best coal, which contains about 90 per cent of carbon 

 and " available " hydrogen and 10 per cent of the other ingredients, 

 will yield from 14,000 to 15,500 British thermal units. A British 

 thermal unit ("B. t. u.") is the amount of heat required to raise the 

 temperature of 1 pound of water 1° F. under certain standard condi- 

 tions. Poorer coals contain larger percentages of noncombustible 

 constituents and correspondingly less carbon and available hydrogen, 

 and their heating value (in British thermal units or other units of 

 measurement) is reduced in much the same ratio. On analysis some 

 coals show a content of all impurities that is in excess of the average ; 

 others show an excess of ash or water only ; but whether the increase 

 is in ash or water, or both, its effect is to decrease the heating value 

 of the coal as expressed in B. t. u. Instead, therefore, of specifying 

 the maximum quantity of ash or water allowable in a commercial or 

 salable coal the regulations fix a minimum value in B. t. u. which will 

 cover either one, or any combination of these two or any other impuri- 

 ties. The limit of allowable impurity in a salable coal is affected by 

 the facts that very wet coals may be improved by air drying and that 

 coals which are very high in ash may usually be improved by washing 

 or may, perhaps, be used in a producer-gas plant. The possibility that 

 improvements may be devised in the utilization of coal — ^such as its 

 entire utilization at the mine for producing electric power — ^makes the 

 determination of the powest limit of usable quality of coal diffi- 

 cult and very uncertain. Again, in a study of the B. t. u. value 

 of low-grade coals that are now worked, difficulty is encountered in 

 the fact that many of the earlier samples taken were weathered coal. 

 On account of the possible improvement of the quality of the coal 

 by drying or by washing, the lowest limit of usable quality is fixed 

 by analysis of an air-dried sample, and if the coal as obtained in the 

 mine contains a very high percentage of ash the possibility of its 

 being washed is considered. Analyses of samples of coal cut in the 

 mine from unweathered coal, according to the practice of the Geo- 

 logical Survey and the Bureau of Mines, indicate that any coal mined 



