e noxious air is removed.

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500. What did the experiments of Allen and Pepy show? How can the
presence of carbonic gas in the expired air be demonstrated? State
observation 1st. Observation 2d.

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2d. It is the action of carbonic acid upon the respiratory organs,
that gives rise to a phenomenon frequently seen in mines and caves. A
man may enter these subterranean rooms, and feel no inconvenience in
breathing; but the dog that follows him, falls apparently dead, and
soon dies if not speedily removed to pure air. This arises from the
fact that this gas is heavier than air, and sinks to the bottom of the
room or cave.

3d. While it is true that carbonic acid possesses properties that
render it unfit to be breathed, it is, notwithstanding, productive of
very agreeable effects, when conveyed into the stomach. It forms the
sparkling property of mineral waters, and fills the bubbles that rise
when beer or cider is fermenting.

501. _Pure atmospheric air is best adapted to a healthy action of the
system._ As the air cannot be maintained pure under all circumstances,
the question may be asked, To what degree may the air be vitiated and
still sustain life? and what is the smallest quantity of pure air a
person needs each minute to maintain good health? Birnan says, that
air which contains more than three and a half per cent. of carbonic
acid is unfit for respiration, and, as air once respired contains
eight and a half per cent. of carbonic acid, it clearly shows that it
is not fitted to be breathed again.

502. No physiologist pretends that less than seven cubic feet of air
are adequate for a man to breathe each minute, while Dr. Reid allows
ten feet. The necessity of fifteen or twenty times the amount of air
actually taken into the lungs, arises from the circumstance, that the
expired air mixes with and vitiates the surrounding element that has
not been inhaled.

503. _The quantity of air which different persons actually need,
varies._ The demand is modified by the size, age, habits, and
condition of the body. A person of great size who has a large quantity
of blood, requires more air than a small man with a less amount of
circulating fluid. Individuals whose labor is active, require more air
than sedentary or idle persons, because the waste of the system is
greater. On the same principle, the gormandizer needs more of this
element than the person of abstemious hab