nature.

This gas is a colorless, odorless, tasteless element. It is sixteen times
as heavy as the gas hydrogen when measured by volume under the same
temperature and pressure. Under all ordinary conditions oxygen remains in
a gaseous form, although it turns to a liquid when compressed to 4,400
pounds to the square inch and at a temperature of 220 below zero.

Oxygen unites with almost every other element, this union often taking
place with great heat and much light, producing flame. Steel and iron will
burn rapidly when placed in this gas if the combustion is started with a
flame of high heat playing on the metal. If the end of a wire is heated
bright red and quickly plunged into a jar containing this gas, the wire
will burn away with a dazzling light and be entirely consumed except for
the molten drops that separate themselves. 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