soot. That it is temporarily
present in a well-burning luminous flame may be demonstrated by holding
a cold object, such as a small evaporating dish, in the flame for a few
seconds. This cold object cools the carbon below its kindling
temperature, and it is deposited on the object as soot.

2. _Pressure._ A second factor in the luminosity of flames is the
pressure under which the gases are burning. Under increased pressure
there is more matter in a given volume of a gas, and the chemical action
is more energetic than when the gases are rarefied. Consequently there
is more heat and light. A candle burning on a high mountain gives less
light than when it burns at the sea level.

If the gas is diluted with a non-combustible gas, the effect is the same
as if it is rarefied, for under these conditions there is less
combustible gas in a given volume.

3. _Temperature._ The luminosity also depends upon the temperature
attained in the combustion. In general the hotter the flame the greater
the luminosity; hence cooling the gases before combustion diminishes the
luminosity of the flame they will make, because it diminishes the
temperature attained in the combustion. Thus the luminosity of the
Bunsen flame is largely diminished by the air drawn up with the gas.
This is due in part to the fact that the burning gas is diluted and
cooled by the air drawn in. The oxygen thus introduced into the flame
also causes the combustion of the hot particles of carbon which would
otherwise tend to make the flame luminous.

~Illuminating and fuel gases.~ A number of mixtures of combustible gases,
consisting largely of carbon compounds and hydrogen, find extensive use
for the production of light and heat. The three chief varieties are coal
gas, water gas, and natural gas. The use of acetylene gas has already
been referred to.

~Coal gas.~ Coal gas is made by heating bituminous coal in large retorts
out of contact with the air. Soft or bituminous coal contains, in
addition to large amounts of carbon, considerable quantities of
compounds of hydrogen, oxygen, nitrogen, and sulphur. When distilled the
nitrogen is liberated partly in the form of ammonia and cyanides and
partly as free nitrogen gas; the sulphur is converted into hydrogen
sulphide, carbon disulphide, and oxides of sulphur; the oxygen into
water and oxides of carbon. The remaining hydrogen is set free partly
as hydrogen and partly in combination with carbon in the form of
hydrocarb