THE USE OF WOOD FOR’ FURL. rl 
to buy firewood for industrial use, except to keep a plant running 
when other fuel can not be had. 
EFFICIENCY OF WOOD FUEL. 
THEORETICAL HEATING VALUES OF WOOD. 
The heating power or fuel value of a given volume of dry wood 
is in direct ratio to its specific gravity. By specific gravity is meant 
the ratio of the weight of a given volume of wood to that of an 
equal volume of water. Water weighs a little over 62 pounds per 
cubic foot, and wood, which weighs 31 pounds per cubic foot when 
perfectly dry, is said to have a specific gravity of 0.50, and so on for 
other weights. 
In theory equal weights of wood substance will give the same 
amount of heat regardless of the species. In other words, a hundred 
pounds of absolutely dry cottonwood should furnish as much heat 
as a hundred pounds of hickory. In reality the varying forms of 
tissue found in the different species, the addition of resin, gums, 
tannin, oils, and pigments, as well as water present in varying 
amounts, cause different woods to have different heating values. 
The presence of rosin in wood increases the heating power mate- 
rially, the results of numerous tests showing a difference ranging 
up to 12 per cent or more. 
The composition of absolutely dry wood is approximately as 
follows: Carbon, 49 per cent; oxygen, 44 per cent; hydrogen, 6 per 
cent; ash, 1 per cent. 
This is fairly constant for all species, except as modified by infil- 
trations, such as gums, pigments, resins, tannin, etc., so that equal 
weights of dry nonresinous woods give off practically the same 
amount of heat in burning. A pound of thoroughly dry wood will 
furnish under good conditions between 7,000 and 9,000 British ther- 
mal units. A pound of good coal will furnish from 12,000 to 14,000 
units, making dry wood about 57 per cent as efficient as coal. 
When wood containing water is burned part of the heat the wood 
is capable of yielding is taken up in raising the water to the boiling 
point and converting it into steam. The steam must then be raised 
to the temperature of the flue gases. All this heat is lost, and the 
greater the amount of water present the more heat is carried off. 
The water in green wood often makes up half of the total weight, 
especially in sapwood. After such wood is thoroughly air-seasoned 
there would remain about 20 per cent of water. If the wood is kiln- 
dried, from 2 to 5 per cent of water remains, and if it is exposed to 
the air, this percentage is increased by absorption (hygroscopically) 
from 10 to 15 per cent, depending upon the humidity. 
A hundredweight of wood as sold on the market contains about 25 
pounds of water, 74 pounds of wood substance, and 1 pound of ash, 
