an element.~ Like water, air was at first
regarded as elementary in character. Near the close of the eighteenth
century Scheele, Priestley, and Lavoisier showed by their experiments
that it is a mixture of at least two gases,--those which we now call
oxygen and nitrogen. By burning substances in an inclosed volume of air
and noting the contraction in volume due to the removal of the oxygen,
they were able to determine with some accuracy the relative volumes of
oxygen and nitrogen present in the air.

~The constituents of the atmosphere.~ The constituents of the atmosphere
may be divided into two general groups: those which are essential to
life and those which are not essential.

1. _Constituents essential to life._ In addition to oxygen and nitrogen
at least two other substances, namely, carbon dioxide and water vapor,
must be present in the atmosphere in order that life may exist. The
former of these is a gaseous compound of carbon and oxygen having the
formula CO_{2}. Its properties will be discussed in detail in the
chapter on the compounds of carbon. Its presence in the air may be shown
by causing the air to bubble through a solution of calcium hydroxide
(Ca(OH)_{2}), commonly called lime water. The carbon dioxide combines
with the calcium hydroxide in accordance with the following equation:

Ca(OH)_{2} + CO_{2} = CaCO_{3} + H_{2}O.

The resulting calcium carbonate (CaCO_{3}) is insoluble in water and
separates in the form of a white powder, which causes the solution to
appear milky.

The presence of water vapor is readily shown by its condensation on cold
objects as well as by the fact that a bit of calcium chloride when
exposed to the air becomes moist, and may even dissolve in the water
absorbed from the air.

2. _Constituents not essential to life._ In addition to the essential
constituents, the air contains small percentages of various other gases,
the presence of which so far as is known is not essential to life. This
list includes the rare elements, argon, helium, neon, krypton, and
xenon; also hydrogen, ammonia, hydrogen dioxide, and probably ozone.
Certain minute forms of life (germs) are also present, the decay of
organic matter being due to their presence.

~Function of each of the essential constituents.~ (1) The oxygen
directly supports life through respiration. (2) The nitrogen,
on account of its inactivity, serves to dilute the oxygen, and
while contrary to the older v