Power of Coal and of Coke. 35 



the view of ascertaining the actual amount of evaporation 

 which could be obtained by the combustion of a given weight 

 of fuel, but the comparative evaporative power, or, in other 

 words, what can be done in practice ; consequently, if there 

 was a loss of heat in the one case, there would be the same, 

 or nearly the same, in the other. Considering the experiments, 

 the results of which have now been given, in this point of view, 

 viz., that 100 lb. of coal evaporated 566 of water, and that 

 this coal yielded 52.5 of coke, — then, had the evaporative 

 power of the coke been the same as that of the coal from 

 which it was got, 52.5 lb. of coke ought also to have given 

 the same amount of evaporation. But, in the trials, 100 of 

 the coke evaporated 733, and 52.5 would therefore have given 

 only 384, — making a deficiency of 182 below that which the 

 coal afforded. 



I may here remark, that the result of the experiment with 

 coal is another proof, in addition to those given, in confirma- 

 tion of the conclusion which I deduced from the results of the 

 trials stated in my former paper, viz., that the practical eva- 

 porative power of bituminous coal is in proportion to the fixed 

 carbon ; 1 lb. of carbon will evaporate 12.3 of water ; now, 

 the fixed carbon in the coal, used in the trials just stated, was 

 50 per cent. ; the quantity of water evaporated by the com- 

 bustion was 5.66 ; it ought from the proportion of fixed car- 

 bon to have been 6.16 ; the deficiency is only 0.5, thus mak- 

 ing a very near approximation. 



Supposing this position to be correct, it may very naturally 

 be asked. Why does not coke also evaporate according to the 

 fixed carbon which it contains ? Now, to this a satisfactory 

 answer can, I conceive, be given. In coal there is bituminous 

 matter, which, before it is inflamed, must assume the gaseous 

 form, and must, therefore, take up a part of the heat evolved 

 by the combustion of the fuel previously on the fire, part of 

 which it may again evolve by its own combustion, and thus 

 supply that necessary for the draught ; but, in coke there is 

 very little of the gaseous inflammable matter ; that in the 

 coke I used was only 6.5 per cent. Now, though this would 

 absorb very little heat during its volatilization, it would of 

 course give forth little by its combustion ; hence a part of that 



