272 Dr Fyfe on the Evaporative Power of Coal. 



evaporative power of both it is only 4.9 per cent. This may 

 also in a great measure be accounted for. The pressure at 

 which my trials were made was 17 lb., that of Schafhaeutl was 

 13.3 ; and though, as is well known, the heat necessary to con- 

 vert water into steam is the same at all pressures, yet in prac- 

 tice it is allowed that the higher the pressure the greater is 

 the heat required, owing to a loss occasioned by different 

 circumstances. Making allowance for pressure, according to 

 Schafhaeutl, the quantity that would have been evaporated in 

 his trial, supposing the temperature at 212°, would be 11.11 ; 

 in mine it would have been, according to the same rule, 9.39 ; 

 which would make the loss of heat in the former amount to 

 9.6 per cent., in the latter to 12.3 per cent. ; thus giving a 

 difference of only 3.3 per cent. As to this greater deficiency, 

 it was most probably owing to the inferior quality of the fuel, 

 perhaps also owing to the defective draught of the furnace, 

 but more particularly to the former. When, for instance, a 

 coal contains much incombustible matter, a greater deduction 

 must be made from the heat that is likely to be available, 

 than in the ratio of the per-centage of the non-inflammable 

 materials, and especially if these contain metallic matter, for 

 they then form slag on the bars, and prevent the due trans- 

 mission of air through the furnace. Hence, which is a very 

 great impediment to the practical application of the heat, it 

 is necessary from time to time to remove the slag from the 

 bars, by which not only is the fire disturbed, but what is a 

 very great drawback, during the whole of the time that the 

 fireman is employed in doing this, cold air is rushing into the 

 furnace, and impinging on the boiler, lowers the temperature, 

 and consequently reduces the evaporation. This happens 

 with common coal, but in addition to this, when anthracite is 

 disturbed in this way, the combustion is materially affected 

 by the cold air. In my experiments this frequently occurred. 

 It was necessary to remove the slag, and as it adhered to the 

 bars, it required a considerable time to get it done. I have 

 no hesitation, therefore, in stating, that had an anthracite 

 with less volatile ingredients, and more especially with a 

 smaller quantity of incombustible matter been used, there 



