rying proportions. The factors of time, depth of beds, disturbance of
beds and the intrusion of mineral matter resulting from such
disturbances have produced the variation in the degree of evolution from
vegetable fiber to hard coal. This variation is shown chiefly in the
content of carbon, and Table 35 shows the steps of such variation.

TABLE 35

APPROXIMATE CHEMICAL CHANGES FROM WOOD
FIBER TO ANTHRACITE COAL

+----------------------+-------+--------+-------+
|Substance |Carbon |Hydrogen|Oxygen |
+----------------------+-------+--------+-------+
|Wood Fiber | 52.65 | 5.25 | 42.10 |
|Peat | 59.57 | 5.96 | 34.47 |
|Lignite | 66.04 | 5.27 | 28.69 |
|Earthy Brown Coal | 73.18 | 5.68 | 21.14 |
|Bituminous Coal | 75.06 | 5.84 | 19.10 |
|Semi-bituminous Coal | 89.29 | 5.05 | 5.66 |
|Anthracite Coal | 91.58 | 3.96 | 4.46 |
+----------------------+-------+--------+-------+

Composition of Coal--The uncombined carbon in coal is known as fixed
carbon. Some of the carbon constituent is combined with hydrogen and
this, together with other gaseous substances driven off by the
application of heat, form that portion of the coal known as volatile
matter. The fixed carbon and the volatile matter constitute the
combustible. The oxygen and nitrogen contained in the volatile matter
are not combustible, but custom has applied this term to that portion of
the coal which is dry and free from ash, thus including the oxygen and
nitrogen.

The other important substances entering into the composition of coal are
moisture and the refractory earths which form the ash. The ash varies in
different coals from 3 to 30 per cent and the moisture from 0.75 to 45
per cent of the total weight of the coal, depending upon the grade and
the locality in which it is mined. A large percentage of ash is
undesirable as it not only reduces the calorific value of the fuel, but
chokes up the air passages in the furnace and through the fuel bed, thus
preventing the rapid combustion necessary to high efficiency. If the
coal contains an excessive quantity of sulphur, trouble will result from
its harmful action on the metal of the boiler where moisture is present,
and because it unites with the ash to form a fusible slag or clinker
which will choke up the grate bars and form a solid mass in which large
quantities of unconsumed carbon may be