imbedded.

Moisture in coal may be more detrimental than ash in reducing the
temperature of a furnace, as it is non-combustible, absorbs heat both in
being evaporated and superheated to the temperature of the furnace
gases. In some instances, however, a certain amount of moisture in a
bituminous coal produces a mechanical action that assists in the
combustion and makes it possible to develop higher capacities than with
dry coal.

Classification of Coal--Custom has classified coals in accordance with
the varying content of carbon and volatile matter in the combustible.
Table 36 gives the approximate percentages of these constituents for the
general classes of coals with the corresponding heat values per pound of
combustible.

TABLE 36

APPROXIMATE COMPOSITION AND CALORIFIC VALUE
OF GENERAL GRADES OF COAL ON BASIS OF COMBUSTIBLE

+-------------------+----------------------------+--------------+
| Kind of Coal | Per Cent of Combustible | B. t. u. |
| +------------+---------------+ Per Pound of |
| |Fixed Carbon|Volatile Matter| Combustible |
+-------------------+------------+---------------+--------------+
|Anthracite |97.0 to 92.5| 3.0 to 7.5 |14600 to 14800|
|Semi-anthracite |92.5 to 87.5| 7.5 to 12.5 |14700 to 15500|
|Semi-bituminous |87.5 to 75.0| 12.5 to 25.0 |15500 to 16000|
|Bituminous--Eastern|75.0 to 60.0| 25.0 to 40.0 |14800 to 15300|
|Bituminous--Western|65.0 to 50.0| 35.0 to 50.0 |13500 to 14800|
|Lignite | Under 50 | Over 50 |11000 to 13500|
+-------------------+------------+---------------+--------------+

Anthracite--The name anthracite, or hard coal, is applied to those dry
coals containing from 3 to 7 per cent volatile matter and which do not
swell when burned. True anthracite is hard, compact, lustrous and
sometimes iridescent, and is characterized by few joints and clefts. Its
specific gravity varies from 1.4 to 1.8. In burning, it kindles slowly
and with difficulty, is hard to keep alight, and burns with a short,
almost colorless flame, without smoke.

Semi-anthracite coal has less density, hardness and luster than true
anthracite, and can be distinguished from it by the fact that when newly
fractured it will soot the hands. Its specific gravity is ordinarily
about 1.4. It kindles quite readily and burns more freely than the true
anthracites.

Semi-bit