226 PRINCIPLES OF AMERICAN FORESTRY. 



species. In burning wood, however, various circum- 

 stances affect its value; few fire places are constructed 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 j'ielding more heat* 

 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 smoke- 

 less 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 

 reducing 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 combus- 

 tion; but as half a kilogram of ordinary wood will give 

 2,000 units of heat, while half a kilogram of water requires 

 only 1'68.5 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 gener- 

 ally considered of little value as fuel, on account of the 

 rapidity with which it decays when cut and the conse- 

 (luent loss of carbon by decomposition." 



* From a given weight. 



