A. The nitrogen is exhaled, and taken up by the leaves of trees and other vegetables. (see p.[35]).
Q. Why does the vitiated air (after the oxygen has been absorbed) come out of the mouth, and not sink into the stomach?
A. The vitiated air (being heated by the heat of the body) ascends naturally, and passes by the heavier fresh air (which we inhale) without obstruction or injury.
Q. If (both in combustion and in respiration) the oxygen of the air is consumed, and the nitrogen rejected—Why are not the proportions of the air destroyed?
A. Because the upper surface of vegetable leaves (during the day) gives out oxygen and absorbs nitrogen, and thus the proper balance is perpetually restored.
Q. Show how God has made animal and vegetable life dependent on each other?
A. Animals require oxygen to keep them alive, and draw it from the air by inspiration; the upper surface of leaves (all day long) gives out oxygen, and thus supplies the air with the very gas required by man and other animals.
Q. Do not animals exhale the very gas needed by vegetables?
A. Yes; animals reject the nitrogen of the air (as not suited to the use of animal life), but vegetables absorb it, as it is the food they live on; and thus the vegetable world restores the equilibrium of the air, disturbed by man and other animals.