y of the lungs. This may be
illustrated by the following experiment: Take two and a half pounds of
water, add to it half a pound of common salt, (chloride of sodium,)
and it will readily mix with the water; and to this solution add the
same quantity of salt, and it will be dissolved more slowly. Again,
add more salt, and it will remain undissolved, as the water has become
saturated by the pound before dissolved.

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498. Does air that is re-breathed freely impart its oxygen? Why? 499.
What is the effect on the blood when the air is free from vapor and
carbonic acid? When loaded with vapor? When saturated? How is this
illustrated?

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500. The principle in this experiment is analogous to that of the
union between carbonic gas and atmospheric air. Allen and Pepy showed
by experiment, that air which had been once breathed, contained eight
and a half per cent. of carbonic acid. They likewise showed, that no
continuance of the respiration of the same air could make it take up
more than ten per cent. This is the point of saturation.

_Experiment._ Sink a glass jar that has a stop-cock, or one with a
glass stopper, into a pail of water, until the air is expelled from
the jar. Fill the lungs with air, and retain it in the chest a short
time, and then breathe into the jar, and instantly close the
stop-cock. Close the opening of the jar that is under the water with a
piece of paper laid on a plate of sufficient size to cover the
opening, invert the jar, and sink into it a lighted candle. The flame
will be extinguished as quickly as if put in water.[15] Remove the
carbonic acid by inverting the jar, and place a lighted candle in it,
and the flame will be as clear as when out of the jar.

[15] As a substitute for a jar with a stop-cock, take a piece of lead
pipe bent in the form of a siphon, and insert it in the mouth
of a reversed jar. This experiment is as conclusive whether the
air is inhaled once only or breathed many times.

_Observations._ 1st. It is familiarly known that a taper will not burn
where carbonic acid exists in any considerable quantity, or when there
is a marked deficiency of oxygen. From this originated the judicious
practice of sinking a lighted candle into a well or pit before
descending into it. If the flame is extinguished, respiration cannot
there be maintained, and life would be sacrificed should a person
venture in, until th