WOOD. 91 



It must not be forgotten, however, that age, climate, and soil ex- 

 ert a marked influence upon the specific gravity of the same species 

 of wood. 



Wood, according to the use for which it is intended, is distinguished 

 into fiie-vvood, building timber, and dye-wood. When first cut down, 

 all timber contains a considerable quantity of water ; 100 parts of 

 walnut-tree dried at 212" Fahr. lost 37.5 parts by weight ; of white 

 oak, 41 parts ; of maple, 48. On an average, the quantity of water 

 contained in green wood may be estimated at about 40 per cent. ; 

 and drying or seasoning for eight or ten months will not cause the 

 loss of more than about 25 per cent of water. The wood which is 

 used for burning almost always contains about a quarter of its weight 

 of moisture, which not only does not assist in producing heat, but 

 actually dissipates a great deal during its conversion into vapor. It 

 is, therefore, highly advantageous in all operations where wood is 

 the fuel, only to employ that which is thoroughly dry. So well is 

 this fact ascertained, that in some manufictories the wood is previ- 

 ously dried in stoves before being consumed in the furnace. 



The composition of woody matter may be represented by carbon 

 and water : of carbon the mean may be stated at 52, of hydrogen 

 and oxygen, in the proportions which form water, at 48. The defi- 

 nitive products of its combustion ought consequently to be carbonic 

 acid and water. The heat disengaged during this combustion, neces- 

 sarily proceeds from the union of the combustible elements of the 

 wood with the oxygen of the atmosphere. But in this particular 

 case, the hydrogen being already present with the proportion of oxy- 

 gen required for its combustion, it may be regarded as already 

 burned, the state of condensation in which the oxygen exists being 

 considered. The heat produced by the wood, therefore, depends 

 solely on the quantity of carbon which it contains. 



Natural philosophers in France agree in designating as unity, in 

 reference to caloric, the quantity of heat necessary to raise a kilo- 

 gramme, or 2.2 lbs. avoirdupois of water, one degree of the centi- 

 grade thermometer, (r.8 F.) The following table, by Rumford, 

 is intended to show the different calorific or heating powers of dif- 

 ferent kinds of wood, and its interpretation is this : since 1 kilo- 

 gramme or 2.2 lbs. avoird. of lime-tree gave out 3460 units of heat, 

 it follows that this quantity of the combustible would suffice to raise 

 by 1 degree centigrade, (r.8F.,) for example, from 10° to ITcenl. 

 3460 kilogrammes, or 7612 lbs. avoirdupois of water. 



