ospheric air than for the other elements of the blood. Consequently,
when they are brought into contact with the air in the lungs, the
carbonic acid and water leave the other constituents of the blood, and
unite with the air. In this way the bluish, or impure blood is
relieved of its impurities, and becomes the red, or pure blood,
which contains the principles so essential to life. (Appendix K.)

489. The formation of carbonic acid and water, eliminated from the
system through the lungs and skin, is explained by the following
theory: In the lungs and upon the skin the oxygen separates from the
nitrogen and unites with the blood in the capillary vessels of these
organs. The oxygen is conveyed with the blood to the capillary
arteries and veins of the different tissues of the system. In these
membranes there is a chemical union of the oxygen with the carbon and
hydrogen contained in the blood and waste atoms of the system. This
combustion, or union of oxygen with carbon and hydrogen, is attended
with the disengagement of heat, and the formation of carbonic acid and
water. (Appendix L.)

490. The following experiment will illustrate the passage of fluids
through membranes, and the different affinity of gases for each other.
Put a mixture of water and alcohol into a phial and leave it uncorked.
Both the water and alcohol have a greater affinity for air than for
each other. Alcohol has a greater affinity for the air, and will be
diffused through it more readily than the water, when there is no
intervening obstacle. But tie a piece of bladder over the mouth of the
phial, and let it stand a few days,--the water will leave the
alcohol, and pass through the membrane. By the aid of this experiment,
we shall endeavor to explain the interchange of fluids in the lungs.

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488. What is formed when oxygen unites with carbon or hydrogen? 489.
Give the theory for the formation of carbonic acid and watery vapor
thrown out of the system. 490. Illustrate the passage of fluids
through membranes, and the different affinities of gases.

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491. The walls of the air-vesicles, and coats of the blood-vessels,
are similar, in their mechanical arrangement, to the membranous
bladder in the before described experiment. As the oxygen of the air
has greater affinity for blood than for nitrogen, so it permeates the
membranes that intervene between the air and blood more readily than
the nitrogen.