Dr Fyfe on the Evaporative Power of Coal. 261 



temperature, will be inflamed ; but if air be not present, or if 

 there be a deficiency of it, then the greater part of the gas will 

 escape without undergoing combustion, and hence the waste 

 which, it may be said, occurs in every furnace constructed in 

 the usual way. It has been supposed by some, that the com- 

 bustion is perfect where there is no smoke, but this is by no 

 means a proof that the whole of the inflammable is con- 

 sumed ; a part of the gaseous matter may be escaping in the 

 state of hydro-carbon, or of carbonic oxide, without undergo- 

 ing any action, and if so, it is just so much fuel wasted. It is 

 evident, then, that the practical evaporative power must de- 

 pend very much on the manner in which the combustion is 

 effected, and also on the peculiar constitution of the coal ; 

 even when the combustion is as perfect as we can expect, still 

 there may be a loss of heat from the generation of the gaseous 

 materials ; for when the carbon and hydrogen are evolved as 

 hydro-carbons, they must absorb caloric to enable them to as- 

 sume the gaseous form. Though we should naturally expect, 

 therefore, that the heat evolved by those coals which contain 

 much hydrogen should be greater than when little of it is pre- 

 sent, this is not always the case ; indeed we shall find that the 

 practical evaporative power is greatest when the fuel contains 

 a great deal of fixed carbon ; for when the carbon is in that 

 state, it must, before it can escape, combine with oxygen and 

 thus be consumed, whereas, as already mentioned, the hydro- 

 carbons may partly fly off without being burned. 



From what has been said it is evident that the method 

 proposed by Eerthier is well adapted for ascertaining with 

 ease the amount of heat that ought to be evolved by the com- 

 bustion of a fuel ; yet it does not indicate the available heat, 

 in other words, what may be called the evaporative power in 

 practice. The only method, I conceive, by which this can be 

 done, is. by actual combustion in properly constructed fur- 

 naces, and by measuring the quantity of water that is evapo- 

 rated by the use of a given weight of the fuel, and we can 

 then compare this with the quantity which carbon will evapo- 

 rate, and which, as already stated, is, according to Desprctz, 

 12.3 times its own weight from the temperature of 32. 



I am aware that to this method the objection may be urged, 



