94 CALORIFIC POWER OF FUELS. 



The calculation is as follows : 



City of Manchester gas, as analyzed by Bunsen, gives, 

 then, with complete combustion, 6099 calories per cubic 

 metre (685 B. T. U. per cubic foot). 



If, however, only the actual ultimate composition of the 

 gas is known or the total percentage of carbon, hydrogen, 

 oxygen and nitrogen, then the calculated result will differ from 

 the experimental one. This is because the heat units of the 

 elements added together do not make those of the compound, 

 as the heat of combination of the different constituent gases 

 is not allowed for. If this factor is known, then it can be 

 used as a correction and the correct heat determined. 



This heat of combination of the elements to form the 

 component gases will be seen in comparing the calculated and 

 the actual heat of combustion of the following gases : 



It will also be seen, that although two gases may have the 

 same percentage composition of the elements, yet the heat of 

 combustion may be different owing to the action of the various 

 physical forces at work in molecular condensation, etc. 



