260 Dr Fyfe on the Evaporative Power of Coal. 



metal is reduced, and the same also occurs when hydrogen is 

 passed over the oxide previously heated. Now when coal is 

 exposed to heat along with an oxide, both the carbon and 

 hydrogen unite with the oxygen, and metal is set free. If 

 therefore we previously knew the composition of the oxide, 

 and after heating it with the combustible, we can ascertain the 

 weight of the metal produced, we of course know the quan- 

 tity of oxygen which that metal has yielded to the combus- 

 tible. By far the best oxide for that purpose is litharge, be- 

 cause it parts with its oxygen easily, while both the metal 

 set free and the excess of oxide used are melted, and during 

 the subsequent cooling and congelation the heavy metal falls to 

 the bottom of the vessel, and when solid can be easily re- 

 moved from the oxide and weighed. For the necessary pre- 

 cautions in conducting this process, the reader is referred to 

 Berthier's Traite des essais par la voie seche. 



As carbon requires 2.66 of oxygen for complete combustion, 

 it will set free 34.66 of metallic lead from litharge. Now 1 of 

 carbon will boil off 12.3 of water ; accordingly a fuel which, 

 when heated with litharge, yields 34.66 times its weight of 

 lead, ought to evaporate 12.3 times its weight of water from 

 32, provided the combustion is perfect, and provided also the 

 whole of the heat evolved by the combustion is absorbed by 

 the water. Should the quantity of lead be greater or smaller, 

 then the evaporative power of the coal will be proportion ably 

 greater or less. It is well known, however, that in burning fuel 

 the combustion is rarely, if ever, perfect, and it never happens 

 that the whole of the heat dissolved is taken up by the water ; 

 as to the latter, there always is, indeed, in the common way of 

 consuming fuel, there must be, a waste, which is necessary to 

 keep up the draught ; but in addition to this, much of the heat 

 must also be lost by the ascending current of air, which in 

 some furnaces is excessive, and of course the waste is enor- 

 mous, frequently amounting to one-third, sometimes to one- 

 half of that evolved. With regard to the imperfect combus- 

 tion, much must depend on the nature of the fuel, and on the 

 particular construction of the furnaces. Thus, when the fuel 

 in common use is heated, it gives off gaseous inflammable mat- 

 ter, which, if it be brought in contact with air at the requisite 



