l have absorbed sufficient moisture to dissolve it. Such substances
are said to be _deliquescent_. A deliquescent body serves as a good
drying or _desiccating_ agent. We have already employed calcium chloride
as an agent for absorbing the moisture from hydrogen. Many substances,
as for example quartz, form crystals which contain no water of
crystallization.

~Mechanically inclosed water.~ Water of crystallization must be
carefully distinguished from water which is mechanically
inclosed in a crystal and which can be removed by powdering the
crystal and drying. Thus, when crystals of common salt are
heated, the water inclosed in the crystal is changed into steam
and bursts the crystal with a crackling sound. Such crystals
are said to _decrepitate_. That this water is not combined is
proved by the fact that the amount present varies and that it
has all the properties of water.

~Uses of water.~ The importance of water in its relation to life and
commerce is too well known to require comment. Its importance to the
chemist has also been pointed out. It remains to call attention to the
fact that it is used as a standard in many physical measurements. Thus
0 deg. and 100 deg. on the centigrade scale are respectively the freezing and
the boiling points of water under normal pressure. The weight of 1 cc.
of water at its point of greatest density is the unit of weight in the
metric system, namely, the gram. It is also taken as the unit for the
determination of the density of liquids and solids as well as for the
measurement of amounts of heat.

HYDROGEN DIOXIDE

~Composition.~ As has been shown, 1 part by weight of hydrogen combines
with 7.94 parts by weight of oxygen to form water. It is possible,
however, to obtain a second compound of hydrogen and oxygen differing
from water in composition in that 1 part by weight of hydrogen is
combined with 2 x 7.94, or 15.88 parts, of oxygen. This compound is
called _hydrogen dioxide_ or _hydrogen peroxide_, the prefixes _di-_ and
_per-_ signifying that it contains more oxygen than hydrogen oxide,
which is the chemical name for water.

~Preparation.~ Hydrogen dioxide cannot be prepared cheaply by the direct
union of hydrogen and oxygen, and indirect methods must therefore be
used. It is commonly prepared by the action of a solution of sulphuric
acid on barium dioxide. The change which takes place may be indicated as
follows:

sulphuric