ide
through the body* (S.D. Magers). Each moves from a place of relatively
high to a place of relatively low pressure. (See text.)

*Passage of Carbon Dioxide through the Blood.*--Part of the carbon dioxide
is dissolved in the plasma of the blood, and part of it is in weak
chemical combination with substances found in the plasma and in the
corpuscles. Its passage through the blood is accounted for in the same way
as the passage of the oxygen. Its ability to dissolve in liquids and to
enter into chemical combination varies as the _carbon dioxide
pressure_(47) This in turn varies with the amount of the carbon dioxide,
which is greatest at the cells (where it is formed), less in the blood,
and still less in the lungs. Because of these differences, the blood is
able to take it up at the cells and release it at the lungs (Fig. 55).

[Fig. 56]

Fig. 56--*Soap bubble* floating in a vessel of carbon dioxide, illustrating
the difference in weight between air and carbon dioxide gas.

*Properties of Carbon Dioxide.*--Carbon dioxide is a colorless gas with
little or no odor. It is classed as a heavy gas, being about one third
heavier than air(48) (Fig. 56). It does not support combustion, but on the
contrary is used to some extent to extinguish fires. It is formed by the
oxidation of carbon in the body, and by the combustion of carbon outside
of the body. It is also formed by the decay of animal and vegetable
matter. From these sources it is continually finding its way into the
atmosphere. Although not a poisonous gas, carbon dioxide may, if it
surround the body, shut out the supply of oxygen and cause death.(49)

*Final Disposition of Carbon Dioxide.*--It is readily seen that the union
of carbon and oxygen, which is continually removing oxygen from the air
and replacing it with carbon dioxide, tends to make the whole atmosphere
deficient in the one and to have an excess of the other. This tendency is
counteracted through the agency of vegetation. Green plants absorb the
carbon dioxide from the air, decompose it, build the carbon into compounds
(starch, etc.) that become a part of the plant, and return the free oxygen
to the air (Fig. 57). In doing this, they not only preserve the necessary
proportion of oxygen and carbon dioxide in the atmosphere, but also put
the carbon and oxygen in such a condition that they can again unite. The
force which enables the plant cells to de