174 
Wood, which is a vegetable substance, whose ultimate com- 
position is carbon 36, hydrogen 22, oxygen 22, is composed 
of three distinct constituents — first, woody fibre ; second, a 
compound of carbon, hydrogen, and oxygen, which forms 
the cellular tissue in which resides the sap; and, thirdly, 
water. The composition of dried wood may be taken (on 
the average of several analyses) as carbon 50, hydrogen 6, 
and oxygen 44. The woody fibre and the sap are sup- 
porters of combustion, which the remaining constituent, 
water, deteriorates. The quality of wood, therefore, as a 
combustible, depends on the quantity of water it contains ; 
but it cannot be said that those woods which contain the 
greatest amount of carbon are the best for combustion, as 
the lighter woods, whose sap is of a more resinous nature, 
usually burn more readily, and cause a greater elimination 
of heat. Wood is but little used as a fuel in this country, 
its rapid combustion and the small amount of heat and 
flame it affords, having prevented its adoption, except in 
cases where the prevalence of extensive forests, as in 
America, have rendered it the most economical fuel. In 
the lowlands of Lincolnshire, however, the remains of the 
ancient forests cut down by the Romans during their in- 
vasions of England, which have been preserved by the 
antiseptic properties of the peat water, rich in protoxide 
of iron, form a local magazine of fuel which is sufficient, 
not only for fencing in the lands, but also very serviceable, 
from its easy combustion and the brilliancy of its flame, 
for the fires of the adjacent dwellings. The practical 
experiments recently made on the evaporative power of 
wood, give results as low as 3.097 and 3.086. But the 
difference between these and the figures given by other 
chemists arose, probably, from the smallness and dampness 
of the wood employed, and the necessity of a quick draft 
to carry off the large volume of smoke evolved. 
