240 Dr Fyfe on the Bluminating a?id Heating Power of 



cubic area of the apartments alluded to was in all 36,780 feet ; 

 now, for 3d. only about 25 feet of gas is purchased, the heat 

 from which would not be more than sufficient to boil off IJ 

 gallons of water. 



Before concluding, I have one observation to make in refe- 

 rence to the heat evolved by coal and by coal-gas, irrespective 

 of the economy. 



I have already stated, that it is generally allowed that 1 lb. 

 of coal will evaporate about 14 lb. of water, supposing all 

 the heat evolved were applied. From the experiments of 

 Despretz, it is concluded that, by the combustion of 1 lb. of 

 pure carbon, 12.3 lb. of water will be evaporated. The supe- 

 rior evaporative power of coal must therefore be owing to the 

 bituminous matter which it contains, which bitumen is the 

 source of the gaseous matter evolved when the coal is heated ; 

 hence the necessity, when using coal as fuel, of so burning it, 

 that the whole of the gaseous matter evolved during the 

 heating of the coal shall be consumed ; for, when any of it 

 escapes combustion, the loss is considerable. I have already 

 mentioned, that 1 gal. water may be boiled off by the con- 

 sumpt of about from 17 to 20, say, on an average, 18 feet of 

 gas. The quantity of gas which coal affords varies much ac- 

 cording to its quality. Of course, the quantity of coke or 

 carbon which coal will leave after being deprived of its gaseous 

 ingredients, must also vary. It is generally allowed that 1 lb. of 

 coal will, on an average, yield about 5 feet of gas, which, at the 

 rate above stated, will evaporate rather more than one-fourth 

 of a gallon, that is 2J lb. ; but, in addition to the gas which is 

 collected by the decomposition of coal, a large quantity of tar 

 and volatile oil is set free, all of which also give forth much 

 heat during their combustion ; and hence it is, that the more 

 perfect the combustion of the coal, the greater is its evapora- 

 tive power ; for, while the whole of the coke, which is a fixed 

 principle, is always consumed, with the exception of the cinder, 

 yet more or less of the volatile and gaseous matter may escape, 

 and as these generate a great deal of heat during their com- 

 bustion, the loss becomes great when much of them is allowed 

 to fly off unburnt. 



