le that this
organic matter rather than the carbon dioxide is responsible
for the injurious effects which follow the respiration of
impure air. The extent of such organic impurities present may
be judged, however, by the amount of carbon dioxide present,
since the two are exhaled together.

~The cycle of carbon in nature.~ Under the influence of sunlight,
the carbon dioxide absorbed from the air by plants reacts with
water and small amounts of other substances absorbed from the
soil to form complex compounds of carbon which constitute the
essential part of the plant tissue. This reaction is attended
by the evolution of oxygen, which is restored to the air. The
compounds resulting from these changes are much richer in their
energy content than are the substances from which they are
formed; hence a certain amount of energy must have been
absorbed in their formation. The source of this energy is the
sun's rays.

If the plant is burned, the changes which took place in the
formation of the compounds present are largely reversed. The
carbon and hydrogen present combine with oxygen taken from the
air to form carbon dioxide and water, while the energy absorbed
from the sun's rays is liberated in the form of energy of heat.
If, on the other hand, the plant is used as food, the compounds
present are used in building up the tissues of the body. When
this tissue breaks down, the changes which it undergoes are
very similar to those which take place when the plant is
burned. The carbon and hydrogen combine with the inhaled oxygen
to form carbon dioxide and water, which are exhaled. The energy
possessed by the complex substances is liberated partly in the
form of energy of heat, which maintains the heat of the body,
and partly in the various forms of muscular energy. The carbon
originally absorbed from the air by the plant in the form of
carbon dioxide is thus restored to the air and is ready to
repeat the cycle of changes.

~The composition of the air is constant.~ Notwithstanding the changes
constantly taking place which tend to alter the composition of the air,
the results of a great many analyses of air collected in the open fields
show that the percentages of oxygen and nitrogen as well as of carbon
dioxide are very nearly constant. Indeed, so constant are the
pe