[Illustration: =THE WATERS UNDER THE EARTH=
The underground lake in its magnificent setting of dazzling stone
columns and stalactites in the Cheddar Caves, England. All these
wonderful natural halls, chasms and snowy incrustations were formed by
the age-long action of the water on the limestone rocks through which it
filtered.]
Water is found in Nature in three states or conditions--as ice, vapor or
steam, and as simple water. These three forms have the same chemical
composition--the substance being a compound of oxygen and hydrogen,
represented by the formula H₂O; but the physical condition depends
entirely on its temperature. Under ordinary atmospheric conditions water
is a _solid_ below 32 degrees Fahrenheit; a _gas_ above 212 degrees
Fahrenheit, and a _liquid_ between these temperatures.
The purest form of water which exists in nature is rain water, though
this always contains a little oxygen and carbon dioxide dissolved from
the air. To obtain pure water artificially, any ordinary water is
distilled, when all the solids dissolved in it are left behind. River
water and spring water always contain a small quantity of solid matter,
the amount and nature of the dissolved solids depending on the nature of
the rocks over which the water has flowed.
Geographically it may be considered under the four heads of _springs_,
_rivers_, _lakes_, and the _ocean_, which taken together forms the
_hydrosphere_ of the earth.