FUEL VALUE OF WOODS. 171 
the variety, would always produce the same amount of heat. 
Marcus Bull, experimenting in 1826 upon the fuel value of dif- 
ferent woods, found a variation of only eleven per cent between 
the different species tested. Kumford’s theory must be regarded 
as nearly correct, if woods are separated into resinous and non- 
resinous classes. The specific gravity gives a direct means of 
comparing heat values of equal volumes of wood of different 
resinous and non-resinous species. In burning wood, however, 
various circumstances affect its value; few fire places are con- 
structed to fully utilize the fuel value of resinous woods, and 
carbon escapes unconsumed in the form of smoke. Pine, there- 
fore, which although capable of yielding more heatt than Oak 
or Hickory, may in practice yield considerably less, the Pine 
losing both carbon and hydrogen in the form of smoke, while 
Hickory or Oak, burning with a smokeless flame, is practically 
entirely consumed. The ash in a wood, being non-combustible, 
influences its fuel value in proportion to its amount. The state 
of dryness of wood also has much influence upon its fuel value, 
though to a less degree than is generally supposed. The water 
in green wood prevents its rapid combustion, evaporation reduc- 
ing the temperature below the point of ignition. Green wood 
may often contain as much as fifty per cent of water, and this 
water must evaporate during combustion; but as half a kilogram 
of ordinary wood will give 2,000 units of heat, while half a kilo- 
gram of water requires only 268.5 units to evaporate it, 1,731.5 
units remain available for generating heat in wood containing 
even a maximum amount of water. 
“A factor in the general value of wood as fuel is the ease with 
which it can be seasoned; Beech, for example, a very dense wood 
of high fuel value when dried, is generally considered of little 
value as fuel, on account of the rapidity with which it decays 
when cut and the consequent loss of carbon by decomposition.” 
y7From a given weight. 
