A. Because air absorbs the carbonic acid, which produced the effervescence.
Q. Why is boiled water flat and insipid?
A. Because the whole of the carbonic acid is expelled by boiling, and absorbed by the air.
Q. Why does water become flat and insipid, after it has been drawn some time?
A. Because air absorbs its carbonic acid; and when its carbonic acid is absorbed, the water is flat and insipid.
Q. Why should spring water (used for washing) be exposed to the air?
A. Spring water contains carbonic acid; but (by being exposed to the air) this carbonic acid is absorbed, and the water becomes more soft.
Q. Why does yeast make bread light?
A. Flour contains a small portion of saccharine matter (or sugar); and the yeast (mixing with this) produces fermentation, as it does in brewing.