16. Q. Why must air be heated before combining with coal?

A. Air, like coal and its gases, must be heated before they will unite
to form what is known as combustion and so as not to reduce the
temperature of the fire-box below the igniting point of the gases.

17. Q. Why is it necessary to provide for combustion a supply of air
through the fuel in the furnace?

A. In order to supply the oxygen necessary for combustion.

18. Q. What is the effect upon combustion if too little air is supplied?
If too much air is supplied?

A. If too little air is supplied, combustion is not complete, and only
one-third as much heat is obtained. If too much air is supplied,
combustion is complete; but the excess air must be heated, resulting in
a lower temperature. If twice the amount of air required for complete
combustion be supplied, the temperature of the fire-box will be about
one-half as high.

19. Q. Give a practical definition of the igniting temperature.

A. In all ordinary combustion there is a definite temperature, called
the ignition or kindling temperature, to which combustible substance
must be heated in order that it may unite with the gas in supporting the
combustion. The burning substance must not only be heated up to the
kindling temperature, but it must be kept as high as this temperature,
or combustion will cease.