Dr Fyfe on the Evaporative Power of Coal. 279 



I have given, it bears a close approximation to it ; — then a 

 question arises, What becomes of the heat evolved by the 

 combustion of the volatile ingredients \ We know that the 4 

 greater part of these is consumed, and they must give out heat 

 by their combustion ; if so, how does it happen that the eva- 

 poration is not greater than what should be occasioned by the 

 consumption merely of the fixed carbon ? This may, to a cer- 

 tain extent, be explained by the volatilization of the elements 

 that assume the gaseous form. When heat is applied to coal, 

 by which it is made to unite with oxygen and itself give forth 

 heat, a part of that heat must be spent in enabling the 

 hydrogen, with the equivalent proportion of carbon, to assume 

 the elastic form. Now, if the whole of this was to escape 

 without undergoing combustion, while, at the same time, the 

 fixed carbon only is burned, we can easily conceive that the 

 evaporative power of a coal thus consumed, would be less than 

 in the ratio of the fixed carbon it contains ; but this is never 

 the case in properly constructed furnaces ; a great part of the 

 gaseous matters is consumed, and they, by their combustion, 

 again evolve the heat which was absorbed by them when as- 

 suming the elastic form. Perhaps in furnaces as now con- 

 structed, even on the best principles, the whole, or nearly the 

 whole, of the heat thus disengaged is so required ; of course, 

 the greater the proportion of volatile matter, the greater will 

 be the abstraction of heat while they become gaseous, and, 

 consequently, it will require the consumption of a proportional 

 part to supply the heat thus abstracted. While, therefore, 

 the heat evolved by the combustion of the fixed carbon is a 

 fixed quantity in each fuel, and as, when the gaseous matter 

 evolved is great, a proportionally smaller quantity of it is ge- 

 nerally consumed, hence the lower evaporative power of those 

 fuels containing much of the elements that yield the volatile 

 matter ; and, again, as it is much easier to effect the complete 

 combustion of fixed carbon than of all the ingredients, both 

 fixed and volatile, of a bituminous coal, hence the value of 

 those that contain much of the fixed element, when consumed 

 in furnaces as now usually constructed. I know that what I 

 now assert is at variance with the opinions entertained on this 

 point by practical engineers, who generally think that a great 



