all changes we have to take into account _energy_ as well as
_matter_. By changing the amount of energy in a substance we change its
properties. That oxygen and ozone contain different amounts of energy
may be shown in a number of ways; for example, by the fact that the
conversion of ozone into oxygen is attended by the liberation of heat.
The passage of the electric sparks through oxygen has in some way
changed the energy content of the element and thus it has acquired new
properties. _Oxygen and ozone must, therefore, be regarded as identical
so far as the kind of matter of which they are composed is concerned.
Their different properties are due to their different energy contents._

~Allotropic states or forms of matter.~ Other elements besides oxygen may
exist in more than one form. These different forms of the same element
are called _allotropic states_ or _forms_ of the element. These forms
differ not only in physical properties but also in their energy
contents. Elements often exist in a variety of forms which look quite
different. These differences may be due to accidental causes, such as
the size or shape of the particles or the way in which the element was
prepared. Only such forms, however, as have different energy contents
are properly called allotropic forms.

MEASUREMENT OF GAS VOLUMES

~Standard conditions.~ It is a well-known fact that the volume occupied by
a definite weight of any gas can be altered by changing the temperature
of the gas or the pressure to which it is subjected. In measuring the
volume of gases it is therefore necessary, for the sake of accuracy, to
adopt some standard conditions of temperature and pressure. The
conditions agreed upon are (1) a temperature of 0 deg., and (2) a pressure
equal to the average pressure exerted by the atmosphere at the sea
level, that is, 1033.3 g. per square centimeter. These conditions of
temperature and pressure are known as the _standard conditions_, and
when the volume of a gas is given it is understood that the measurement
was made under these conditions, unless it is expressly stated
otherwise. For example, the weight of a liter of oxygen has been given
as 1.4285 g. This means that one liter of oxygen, measured at a
temperature of 0 deg. and under a pressure of 1033.3 g. per square
centimeter, weighs 1.4285 g.

The conditions which prevail in the laboratory are never the standard
conditions. It becomes necessary, therefore, to find a way to calcu