191 
No. of lbs. of peat charcoal (foreign) 21.1 
Ditto brown coal 19*9 
Ditto mineral coal, as given by Berthier.. 26.8 
Ditto mineral coal, as by Mr. Phillips ... 29.8 
Ditto coke 26.2 
Ditto anthracite 29.6 
The preceding table will enable us to form a tolerably 
accurate idea of the relative heating powers of ditferent 
fuels. I must, however, reiterate the caution that these 
figures exhibit only an approximation to their absolute 
calorific values. 
We have now to consider the second head into which 
I divided the relative value of fuels, namely, the influence 
of the time required by fuel to emit its heat. 
The general formula of woody fibre is, carbon 36, hydro- 
gen 22, and oxygen 22, and hence the ultimate analysis of 
all woods invariably indicates the hydrogen and oxygen as 
present in the proportion to form water without an excess 
of either element. It is found, however, practically, that all 
woods contain an excess of hydrogen, varying in quantity 
in different woods. Thus, in oak there is an excess of 5.08 
of hydrogen, in lime of 13, in deal of 9.50, and in larch of 
8.60. This excess of hydrogen, on the application of heat, 
gives rise to hydro- carbons, the evolution of which diminishes 
the carbon in the wood, and consequently decreases the resi- 
due of charcoal. The first and more rapid stage of com- 
bustion, therefore, — the combustion with flame, — is aug- 
mented at the expense of the second and more permanent, 
viz., the incandescence of the charcoal. The quantity of 
heat evolved may be the same, but the rapidity of its evolu- 
tion diminishes the value of the fuel as a heating agent, for 
our arrangements for heating are generally of such a nature 
as to require time for the heat given off by the fuel to 
communicate itself to the part to be heated. The harder 
