h
diameter opening and over a 5/8-inch diameter opening.

(H) Screenings; that which passes through a 1-1/4-inch diameter opening.

As the variation in character of bituminous coals is much greater than
in the anthracites, any rules set down for their handling must be the
more general. The difficulties in burning bituminous coals with economy
and with little or no smoke increases as the content of fixed carbon in
the coal decreases. It is their volatile content which causes the
difficulties and it is essential that the furnaces be designed to
properly handle this portion of the coal. The fixed carbon will take
care of itself, provided the volatile matter is properly burned.

Mr. Kent, in his "Steam Boiler Economy", described the action of
bituminous coal after it is fired as follows: "The first thing that the
fine fresh coal does is to choke the air spaces existing through the bed
of coke, thus shutting off the air supply which is needed to burn the
gases produced from the fresh coal. The next thing is a very rapid
evaporation of moisture from the coal, a chilling process, which robs
the furnace of heat. Next is the formation of water-gas by the chemical
reaction, C + H_{2}O = CO + 2H, the steam being decomposed, its oxygen
burning the carbon of the coal to carbonic oxide, and the hydrogen being
liberated. This reaction takes place when steam is brought in contact
with highly heated carbon. This also is a chilling process, absorbing
heat from the furnaces. The two valuable fuel gases thus generated would
give back all the heat absorbed in their formation if they could be
burned, but there is not enough air in the furnace to burn them.
Admitting extra air through the fire door at this time will be of no
service, for the gases being comparatively cool cannot be burned unless
the air is highly heated. After all the moisture has been driven off
from the coal, the distillation of hydrocarbons begins, and a
considerable portion of them escapes unburned, owing to the deficiency
of hot air, and to their being chilled by the relatively cool heating
surfaces of the boiler. During all this time great volumes of smoke are
escaping from the chimney, together with unburned hydrogen,
hydrocarbons, and carbonic oxide, all fuel gases, while at the same time
soot is being deposited on the heating surface, diminishing its
efficiency in transmitting heat to the water."

To burn these gases distilled from the coal, it is necessary