_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 the noxious air is removed. -=-=-=-=-=-=-=-=-=-=-=-= 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. -=-=-=-=-=-=-=-=-=-=-=-= 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.