pon a cold, dry
mirror for a few minutes, and it will be covered with moisture. This
is condensed vapor from the lungs. In warm weather, this watery vapor
is invisible in the expired air, but in a cold, dry morning in winter,
the successive jets of vapor issuing from the mouth and nose are
sufficiently obvious.

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Give the experiment showing that oxygen changes the dark-colored blood
to a bright red color. What is represented by fig. 99? 493. How can
the presence of carbonic acid in the lungs be proved?

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494. From the lungs are eliminated other impurities beside carbonic
acid, the perceptible quality of which is various in different
persons. The offensive breath of many persons may be caused by decayed
teeth, or the particles of food that may be retained between them, but
it often proceeds from the secretion, in the lungs, of certain
substances which previously existed in the system.

_Illustration._ When spirituous liquors are taken into the stomach,
they are absorbed by the veins and mixed with the dark-colored blood,
in which they are carried to the lungs to be expelled from the body.
This will explain the fact, which is familiar to most persons, that
the odor of different substances is perceptible in the breath, or
expired air, long after the mouth is free from these substances.

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How the watery vapor? 494. Are there other excretions from the lungs?
Give the illustration.

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_Note._ Let the anatomy and physiology of the respiratory organs be
reviewed from figs. 96, 97, and 99, or from anatomical outline plates
Nos. 5 and 7.

CHAPTER XXV.

HYGIENE OF THE RESPIRATORY ORGANS.

495. For man to enjoy the highest degree of health, it is necessary
that the impure "venous" blood be properly changed. As this is
effected in the lungs by the action of the air, it follows that this
element, when breathed, should be pure, or contain twenty-one per
cent. of oxygen to about seventy-nine per cent. of nitrogen.

496. The volume of air expelled from the lungs is somewhat less than
that which is inspired. The amount of loss varies under different
circumstances. An eightieth part of the volume taken into the lungs,
or half a cubic inch, may be considered an average estimate.

497. _The quality and purity of the air is affected by every
respiration._ 1st. The quantity of oxygen is diminished. 2d. The
amo