76 ELEMENTARY FORESTRY. 



"Count Rumfprd first propounded the theory that the 

 value of equal weights of wood for fuel was the same, without 

 reference to specific distinctions: that is that a pound of wood, 

 whatever the variety, would always produce the same amount 

 of heat. Marcus Bull, experimenting in 1826 upon the fuel 

 value of different woods, found a variation of only 11 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 vol- 

 umes of wood of different resinous and non-resinous species. 

 In burning wood, however, various circumstances 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, therefore, 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, burn- 

 ing 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 reducing the 

 temperature below the point of ignition. Green wood may 

 often contain as much as 50 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, 

 1731.5 units remain available for generating heat in wood con- 

 taining 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 con- 

 sidered of little value as fuel, on account of the rapidity with 

 which it decays when cut and the consequent loss of carbon 

 by decomposition." 



tFrom a given weight. 



