Q. Where does the carbonic acid of close rooms and cities come from?
A. From the lungs of the inhabitants, the sewers, drains, and so on: besides, trees and gardens are not numerous enough to absorb the noxious gas as fast as it is generated.
Q. What becomes of the carbonic acid of crowded cities?
A. Some of it is absorbed by vegetables, and the rest is blown away by the wind, and diffused through the whole volume of the air.
Q. Does not this constant diffusion of carbonic acid affect the purity of the whole air?
A. No; because after it is thus diffused, it is carried to various lands, and absorbed in its passage by the vegetable world.
Q. Why do persons who ascend in balloons feel intense pain in their eyes and ears?
A. Because the air of the upper regions is more rarefied than the air on the earth; and the air inside their bodies (seeking to become of the same rarity) bursts through their eyes and ears, producing an intense pain.