FUEL VALUE OF WOODS. 173 



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. Rumford'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 heatf 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." 



fFrom a given weight. 



