In kindling a coal fire in a furnace the phosphorus of a match inflames
at so low a temperature (150 degrees Fahr.) that mere friction ignites
it, and in burning (combining with oxygen of the air) it gives out heat
enough to raise the sulphur of the match to the temperature of ignition
(500 degrees Fahr.), which, combining in its turn with the oxygen of
the atmosphere, gives out sufficient heat to raise the temperature
of the wood to the point of ignition (800 degrees Fahr.), and at
this temperature the wood combines with oxygen supplied by the air,
giving out a temperature sufficient to raise the coal to the point
of ignition (1000 degrees Fahr.), and the coal then combines with the
free oxygen of the air, the ensuing temperature in the furnace varying,
according to circumstances, from 3000 degrees to 4000 degrees Fahr.
Thus we see that the ignition of the coal is the last of a series of
progressive steps, each increasing in temperature.
And in each step it will be noted that a combination of oxygen is the
essential connecting link and _that the oxygen is supplied in each
instance at the same average temperature_—this fact contains a “point”
relating to supplying furnaces with so called “hot air.”