The air which we expel from the lungs at every breath has a large proportion of carbonic acid. Let a man be shut up in an air-tight room for a day, and he will have changed nearly all the oxygen in it into this carbonic acid, and rendered it unfit for animal life. Dogs, cats, and birds would die in it. But, poisonous as it is to man and other animals, it is a feast to plants. They want it all day and every day; not in the night,--at that time they have a taste for oxygen. This effete air, which men and animals exhale, so charged with carbonic acid, the plants drink in through every pore. They take it from the mouth of man, appropriate it to their daily uses, and in time render it back to him mingled with other ingredients in wholesome fruit. Carbonic acid is death when it combines with the blood,--as it does when we inhale it; but not so when it enters the stomach in small quantities. One inspiration of it is enough to make us dizzy,--as when we enter an old well or stoop over a charcoal fire; but a draught of water fully charged with it is exhilarating and refreshing, as we know by repeated experiences at marble fountains that meet us on so many city-corners. If plants had souls, they would be pure ones, since they can bear such contamination and not be harmed,--nay, since even from such foul food as we give them they can evolve results so beautiful. We give them our cast-off and worn-out materials, and they return us the most beautiful flowers and the most luscious fruits.