This property of oxygen is used
in oxy-acetylene cutting of steel.
The combination of oxygen with other substances does not necessarily cause
great heat, in fact the combination may be so slow and gradual that the
change of temperature can not be noticed. An example of this slow
combustion, or oxidation, is found in the conversion of iron into rust as
the metal combines with the active gas. The respiration of human beings
and animals is a form of slow combustion and is the source of animal heat.
It is a general rule that the process of oxidation takes place with
increasing rapidity as the temperature of the body being acted upon rises.
Iron and steel at a red heat oxidize rapidly with the formation of a scale
and possible damage to the metal.
_Air._--Atmospheric air is a mixture of oxygen and nitrogen with
traces of carbonic acid gas and water vapor. Twenty-one per cent of the
air, by volume, is oxygen and the remaining seventy-nine per cent is the
inactive gas, nitrogen. But for the presence of the nitrogen, which deadens
the action of the other gas, combustion would take place at a destructive
rate and be beyond human control in almost all cases. These two gases exist
simply as a mixture to form the air and are not chemically combined. It is
therefore a comparatively simple matter to separate them with the processes
now available.
_Water._--Water is a combination of oxygen and hydrogen, being
composed of exactly two volumes of hydrogen to one volume of oxygen.
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