unt of carbonic acid is increased. 3d. A certain proportion of
watery vapor is ejected from the lungs in the expired air. Of the
twenty-one parts of oxygen in the inspired air, only eighteen parts
are expired, while the carbonic acid and watery vapor are increased
about four per cent. The quantity of nitrogen is nearly the same in
the expired as in the inspired air.

_Observation._ It is now fully ascertained that while the chemical
composition of the blood is essentially changed, its weight remains
the same, as the carbon and hydrogen discharged are equal to the
united weight of the oxygen and nitrogen absorbed.

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495-546. _Give the hygiene of the respiratory organs._ 495. What is
necessary that man enjoy the highest degree of health? 496. How does
the volume Of expired air compare with that which was inspired? Does
this loss vary, and what is an average estimate? 497. How is the
purity of the air affected by respiration? How is the inhaled oxygen
affected? What effect on the carbonic acid and watery vapor? On the
nitrogen? What is said respecting the weight of the blood?

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498. If one fourth part of the volume of air received by the lungs at
one inspiration is decomposed at one "beat" of the heart, it might be
supposed that if the expired air be again received into the lungs, one
half of the oxygen would be consumed, and, in a similar ratio, if
re-breathed four times, all the oxygen would be consumed. But it does
not follow, if the air is thus re-breathed, that the same changes will
be effected in the lungs. For air that has been inspired does not part
with its remaining oxygen as freely as when it contains the proper
amount of this life-giving element, and thus the changes in the impure
blood are not so completely effected.

_Illustration._ In the process of dyeing, each successive article
immersed in the dye weakens it; but it does not follow that the dye
each time is affected in the same degree, or that the coloring matter
by repeated immersions can be wholly extracted. The same principle
applies to the exchange of oxygen and carbonic acid gas in the lungs.

499. _If the inspired air is free from moisture and carbonic acid,
these substances contained in the blood will be more readily imparted
to it._ When the air is loaded with vapor, they are removed more
slowly; but if it is saturated with moisture, no vapor will escape
from the blood through the agenc