which is passed the vapor obtained
by boiling some liquid which has a boiling point above 100 deg.. In this way
it has been proved that 2 volumes of hydrogen and 1 volume of oxygen
combine to form exactly 2 volumes of water vapor, the volumes all being
measured under the same conditions of temperature and pressure. It will
be noted that the relation between these volumes may be expressed by
whole numbers. The significance of this very important fact will be
discussed in a subsequent chapter.

[Illustration: Fig. 23]

~Occurrence of water.~ Water not only covers about three fourths of the
surface of the earth, and is present in the atmosphere in the form of
moisture, but it is also a common constituent of the soil and rocks and
of almost every form of animal and vegetable organism. The human body is
nearly 70% water. This is derived not only from the water which we drink
but also from the food which we eat, most of which contains a large
percentage of water. Thus potatoes contain about 78% of water, milk 85%,
beef over 50%, apples 84%, tomatoes 94%.

~Impurities in water.~ Chemically pure water contains only hydrogen and
oxygen. Such a water never occurs in nature, however, for being a good
solvent, it takes up certain substances from the rocks and soil with
which it comes in contact. When such waters are evaporated these
substances are deposited in the form of a residue. Even rain water,
which is the purest form occurring in nature, contains dust particles
and gases dissolved from the atmosphere. The foreign matter in water is
of two kinds, namely, _mineral_, such as common salt and limestone, and
_organic_, that is the products of animal and vegetable life.

~Mineral matter in water.~ The amount and nature of the mineral
matter present in different waters vary greatly, depending on
the character of the rocks and soil with which the waters come
in contact. The more common of the substances present are
common salt and compounds of calcium, magnesium, and iron. One
liter of the average river water contains about 175 mg. of
mineral matter. Water from deep wells naturally contains more
mineral matter than river water, generally two or three times
as much, while sea water contains as much as 35,000 mg. to the
liter.

~Effect of impurities on health.~ The mineral matter in water does not,
save in very exceptional cases, render the water injurious to the human
system. In