s 1.4285 g., while under
similar conditions one liter of air weighs 1.2923 g. It is but slightly
soluble in water. Oxygen, like other gases, may be liquefied by applying
very great pressure to the highly cooled gas. When the pressure is
removed the liquid oxygen passes again into the gaseous state, since its
boiling point under ordinary atmospheric pressure is -182.5 deg..

~Chemical properties.~ At ordinary temperatures oxygen is not very active
chemically. Most substances are either not at all affected by it, or the
action is so slow as to escape notice. At higher temperatures, however,
it is very active, and unites directly with most of the elements. This
activity may be shown by heating various substances until just ignited
and then bringing them into vessels of the gas, when they will burn with
great brilliancy. Thus a glowing splint introduced into a jar of oxygen
bursts into flame. Sulphur burns in the air with a very weak flame and
feeble light; in oxygen, however, the flame is increased in size and
brightness. Substances which readily burn in air, such as phosphorus,
burn in oxygen with dazzling brilliancy. Even substances which burn in
air with great difficulty, such as iron, readily burn in oxygen.

The burning of a substance in oxygen is due to the rapid combination of
the substance or of the elements composing it with the oxygen. Thus,
when sulphur burns both the oxygen and sulphur disappear as such and
there is formed a compound of the two, which is an invisible gas, having
the characteristic odor of burning sulphur. Similarly, phosphorus on
burning forms a white solid compound of phosphorus and oxygen, while
iron forms a reddish-black compound of iron and oxygen.

~Oxidation.~ The term _oxidation_ is applied to the chemical change which
takes place when a substance, or one of its constituent parts, combines
with oxygen. This process may take place rapidly, as in the burning of
phosphorus, or slowly, as in the oxidation (or rusting) of iron when
exposed to the air. It is always accompanied by the liberation of heat.
The amount of heat liberated by the oxidation of a definite weight of
any given substance is always the same, being entirely independent of
the rapidity of the process. If the oxidation takes place slowly, the
heat is generated so slowly that it is difficult to detect it. If the
oxidation takes place rapidly, however, the heat is generated in such a
short interval of time that the substance ma