185 
which causes the evolution of heat, and the illuminating 
powers of the fuel. There is, however, no heat lost by 
the previous distillatory process, as, although there is an 
immense amount of latent heat absorbed in rendering these 
products volatile and gaseous, it is exactly counterbalanced 
by the quantity of heat eliminated by their subsequent com- 
bustion. 
An augmented proportion of hydrogen in a fuel, accom- 
panied, as it usually is, by a corresponding deficiency of 
oxygen, increases the number and quantity of the hydro- 
carbons evolved, and has frequently the effect of causing 
the fuel to cake into solid lumps, choking up the bars, 
and preventing a due supply of air to the fire ; while, on 
the other hand, anthracite being, as we have seen, almost 
pure carbon, evolves so few hydrogenised products, as not 
to soil the whitewashed fireplaces and chimneys in which 
its combustion is effected. 
Let us now pass on to consider the relative value of fuel. 
And first, let us endeavour to discover the principles govern- 
ing the maximum theoretical value of each. To accomplish 
this we must take into consideration, first, the quantity of 
heat which a certain amount of fuel is capable of pro- 
ducing ; and secondly, the time which is required for that 
object. 
The experiments of Lavoisier and Laplace on this sub- 
ject have warned us that it is the relative, and not the 
absolute, quantity of heat produced by different fuels which 
we must seek to ascertain. Attempts have been made to 
establish this by the aid of calorimeters, which, by register- 
ing the quantity of ice melted, or the difference in the 
temperature of water occasioned by the combustion of a 
given quantity of each fuel, rendered the calculation of 
their respective heating values easy. There are, however, 
other more convenient methods grounded on the law enun- 
