kind of oxidation, but it will only take place in the presence of
certain minute forms of life known as bacteria. Just how these assist in
the oxidation is not known. By this process the dead products of animal
and vegetable life which collect on the surface of the earth are slowly
oxidized and so converted into harmless substances. In this way oxygen
acts as a great purifying agent.

3. Oxygen is also used in the treatment of certain diseases in which the
patient is unable to inhale sufficient air to supply the necessary
amount of oxygen.

OZONE

~Preparation.~ When electric sparks are passed through oxygen or air a
small percentage of the oxygen is converted into a substance called
_ozone_, which differs greatly from oxygen in its properties. The same
change can also be brought about by certain chemical processes. Thus, if
some pieces of phosphorus are placed in a bottle and partially covered
with water, the presence of ozone may soon be detected in the air
contained in the bottle. The conversion of oxygen into ozone is attended
by a change in volume, 3 volumes of oxygen forming 2 volumes of ozone.
If the resulting ozone is heated to about 300 deg., the reverse change
takes place, the 2 volumes of ozone being changed back into 3 volumes of
oxygen. It is possible that traces of ozone exist in the atmosphere,
although its presence there has not been definitely proved, the tests
formerly used for its detection having been shown to be unreliable.

~Properties.~ As commonly prepared, ozone is mixed with a large excess of
oxygen. It is possible, however, to separate the ozone and thus obtain
it in pure form. The gas so obtained has the characteristic odor noticed
about electrical machines when in operation. By subjecting it to great
pressure and a low temperature, the gas condenses to a bluish liquid,
boiling at -119 deg.. When unmixed with other gases ozone is very explosive,
changing back into oxygen with the liberation of heat. Its chemical
properties are similar to those of oxygen except that it is far more
active. Air or oxygen containing a small amount of ozone is now used in
place of oxygen in certain manufacturing processes.

~The difference between oxygen and ozone.~ Experiments show that in
changing oxygen into ozone no other kind of matter is either added to
the oxygen or withdrawn from it. The question arises then, How can we
account for the difference in their properties? It must be remembered
that in