ioxide absorbed. The amount present in the open
normal air is from 3 to 4 parts by volume in 10,000 volumes of air, or
about 0.04%.

4. _Determination of water vapor._ The water vapor present in a given
volume of air may be determined by passing the air over calcium chloride
(or some other compound which has a strong affinity for water), and
noting the increase in the weight of the chloride. The amount present
varies not only with the locality, but there is a wide variation from
day to day in the same locality because of the winds and changes in
temperature.

~Processes affecting the composition of the air.~ The most important of
these processes are the following.

1. _Respiration._ In the process of respiration some of the oxygen in
the inhaled air is absorbed by the blood and carried to all parts of the
body, where it combines with the carbon of the worn-out tissues. The
products of oxidation are carried back to the lungs and exhaled in the
form of carbon dioxide. The amount exhaled by an adult averages about 20
l. per hour. Hence in a poorly ventilated room occupied by a number of
people the amount of carbon dioxide rapidly increases. While this gas is
not poisonous unless present in large amounts, nevertheless air
containing more than 15 parts in 10,000 is not fit for respiration.

2. _Combustion._ All of the ordinary forms of fuel contain large
percentages of carbon. On burning, this carbon combines with oxygen in
the air, forming carbon dioxide. Combustion and respiration, therefore,
tend to diminish the amount of oxygen in the air and to increase the
amount of carbon dioxide.

3. _Action of plants._ Plants have the power, when in the sunlight, of
absorbing carbon dioxide from the air, retaining the carbon and
returning at least a portion of the oxygen to the air. It will be
observed that these changes are just the opposite of those brought about
by the processes of respiration and combustion.

~Poisonous effect of exhaled air.~ The differences in the
percentages of oxygen, carbon dioxide, and moisture present in
inhaled air and exhaled air are shown in the following
analyses.

INHALED AIR EXHALED AIR
Oxygen 21.00% 16.00%
Carbon dioxide 0.04 4.38
Moisture variable saturated

The foul odor of respired air is due to the presence of a
certain amount of organic matter. It is possib