143. _Q._--Will you recapitulate the steps by which you determine the
quantity of air required for the combustion of coal?

_A._--Looking to the quantity of oxygen required to unite chemically with
the various constituents of the coal, we find for example that in 100 lbs.
of anthracite coal, consisting of 91.44 lbs. of carbon, and 3.46 lbs. of
hydrogen, we shall for the 91.44 lbs. of carbon require 243.84 lbs. of
oxygen--since to saturate a pound of carbon by the formation of carbonic
acid, requires 2-2/3 lbs. of oxygen. To saturate a pound of hydrogen in the
formation of water, requires 8 lbs. of oxygen; hence 3.46 Fibs. of hydrogen
will take 27.68 lbs. of oxygen for its saturation. If then we add 243.84
lbs. to 27.68 lbs. we have 271.52 lbs. of oxygen required for the
combustion of 100 lbs. of coal. A given weight of air contains nearly 23.32
per cent of oxygen; hence to obtain 271.52 lbs. of oxygen, we must have
about four times that quantity of atmospheric air, or more accurately, 1164
lbs. of air for the combustion of 100 lbs. of coal. A cubic foot of air at
ordinary temperature weighs about .075 lbs.; so that 100 lbs. of coal
require 15,524 cubic feet of air, or 1 lb. of coal requires about 155 cubic
feet of air, supposing every atom of the oxygen to enter into combination.
If, then, from one-third to one-half of the air passes unconsumed through
the fire, an allowance of 240 cubic feet of air for each pound of coal will
be a small enough allowance to answer the requirements of practice, and in
some cases as much as 300 cubic feet will be required,--the difference
depending mainly on the peculiar configuration of the furnace.

144. _Q._--Can you state the evaporative efficacy of a pound of coal?

_A._--The evaporative efficacy of a pound of carbon has been found
experimentally to be equivalent to that necessary to raise 14,000 lbs. of