258 Dr Fyfe on the Evaporative Power of Coal. 



loric received by water during its conversion into steam, in 

 other words, of the latent heat of steam. If we suppose, as 

 has been stated by Lardner and others, that it requires 5^ 

 times as long to evaporate water that it does to raise it from 

 the freezing to the boiling point, then the latent heat will 

 amount to 990. But others have made it lower than this. Ac- 

 cording to Despretz it is only 955.8. Assuming this as cor- 

 rect, then in steam the total number of degrees of tempera- 

 ture beyond 32 is (180 + 955.8) 1135.8— say 113G. Hence 

 if lib. of carbon will raise 78.151b., as stated by Despretz, 

 from 32 to 212, it will evaporate 12.3 lb. from 32 — and this is 

 the quantity fixed on by him. 



It is well known that the different substances used as fuel 

 consist, in their original state, chiefly of carbon and of hydro- 

 gen, in addition to which there is generally a minute quantity 

 of oxygen and of nitrogen, and there is always a portion of 

 earthy and metallic matter, constituting the ashes. The only 

 one of these which, in addition to the carbon, will evolve heat 

 during the combustion, is the hydrogen. Now 1 of hydrogen 

 combines with 8 of oxygen, or exactly three times as much as 

 carbon requires. 1 lb. of hydrogen will therefore evaporate 

 37 lb. of water from 32. 



It is evident from this, that if we know the composition of 

 the fuel, we can calculate the evaporative power by knowing 

 the quantity of oxygen necessary for converting the carbon 

 and hydrogen into carbonic acid and water. Of course, the 

 greater the proportion of hydrogen, the greater ought the eva- 

 porative power to he. If the fuel contain nitrogen, a part of 

 the hydrogen must be deducted from the whole quantity, be- 

 cause the nitrogen will unite with it to form ammonia ; and, 

 again, if oxygen exist in the fuel, the hydrogen which is re- 

 quisite to convert that oxygen into water must also be de- 

 ducted, and, accordingly, in addition to the carbon, it is only 

 the hydrogen over and above what is required for uniting with 

 the nitrogen and oxygen, that are to evolve heat by the com- 

 bustion. 



The most recent account of the analysis of various kinds 

 of coal, is that published by Mr Richardson in the Trans, of 

 the Nat. History Society of Newcastle, and also in the Lond. 



