the heat is rendered applicable to other purposes.

140. _Q._--Does air consist of oxygen?

_A._--Air consists of oxygen and nitrogen mixed together in the proportion
of 3.29 lbs. of nitrogen to 1 lb. of oxygen. Every pound of coal requires
about 2.66 lbs. of oxygen for its saturation, and therefore for every pound
of coal burned, 8.75 pounds of nitrogen must pass through the fire,
supposing all the oxygen to enter into combination. In practice, however,
this perfection of combination does not exist; from one-third to one-half
of the oxygen will pass through the fire without entering into combination
at all; so that from 16 to 18 lbs. of air are required for every pound of
coal burned. 18 lbs. of air are about 240 cubic feet, which may be taken as
the quantity of air required for the combustion of a pound of coal in
practice.

141. _Q._--What are the constituents of coal?

_A._--The chief constituent of coal is carbon or pure charcoal, which is
associated in various proportions with volatile and earthy matters. English
coal contains 80 to 90 per cent. of carbon, and from 8 to 18 per cent. of
volatile and earthy matters, but sometimes more than this. The volatile
matters are hydrogen, nitrogen, oxygen, and sulphur.

142. _Q._--What is the difference between anthracite and bituminous coal?

_A._--Anthracite consists almost entirely of carbon, having 91 per cent. of
carbon, with about 7 per cent. of volatile matter and 2 per cent. of ashes.
Newcastle coal contains about 83 per cent. of carbon, 14 per cent. of
volatile matter, and 3 per cent. of ashes.