?6 CALORIFIC POWER OF FUELS. 



followed by many others, seems very logical, as it facilitates 

 comparison of different fuels by reducing them to the same 

 basis. Enormous errors due to comparison of values not 

 comparable are thus obviated. Coal having 5 per cent im- 

 purity has been compared with coal having only I per cent, 

 no account being made for the difference, and of course very 

 erroneous and misleading deductions obtained. 



It is a simple task for the engineer or the workman even, to 

 determine approximately the proportions of moisture and ash 

 as given on the grate. Knowing these proportions and the 

 heat of combustion of the pure coal, they can render a state- 

 ment of the practical working. If, on the contrary, the ex- 

 perimenter is limited in such way that he neglects the com- 

 position of the coal, it is impossible to make a conjecture as 

 to its intrinsic or comparative value; still less can he judge of 

 it as a steam generator. 



In 1879 Bunte made some experiments at Munich, using a 

 special apparatus devised by him for the occasion, which 

 was part calorimeter and part boiler. The tests were pub- 

 lished in Dingier 's Polytechnisches Journal. Some of the 

 results are included in the tables of this book. 



Since then numerous tests have been made on nearly all 

 the known coals. A collection of all available ones from 

 which the desired data could be obtained will be found far- 

 ther on. 



The question as to the actual evaporative effect of each 

 coal can be settled only by actual tests made on the boiler 

 intended for use, as the same coal will give slightly different 

 results with different kinds of boilers ; also, and in a more 

 marked degree, with different methods of firing and handling. 

 The results in the tables cannot be taken, then, as absolute 

 for all boilers under all circumstances, but they can be 

 depended on for comparison of the different fuels with the 

 same boiler and under proper conditions. 



The manner in which a coal acts under heat in a closed 



