This air which has been used by us, and is no longer fit for our use, feeds the plants and trees, the grass, and all living things which are not animals; the plants, through tiny mouths at the edge of their leaves, breathe it in. They grow by it; and, wonder of wonders, all day long, if only the plant is where the sun can shine upon it, every green bit of it is busy making this same air fit for us to breathe again; using up what it wants, and what we do not want; every fragment, as it were, being gathered up, and nothing lost. I used to think, when I first learnt this beautiful lesson, that every part of a plant was useful in purifying the air, and also that plants are always busy at this purifying work, and so I liked to keep geraniums and fuchsias in my room at night, for I thought that while I was asleep they would keep the air fresh and sweet. But now I know--for as long as we live in this world we can always be learning--that it is only in the daytime, when there is light, that a plant can keep the air pure, by using up what we have spoilt for our own use, and giving away what is good for us to breathe; and also that, it is only the green part of it that has the power to take out of the air the carbonic acid which we are constantly breathing into it, using the carbon for its own food, and giving the oxygen back into the air for our use; the parts which are not green, such as the roots and flowers, breathe just as animals do, and spoil the air for us instead of making it more fit for us to breathe. You never thought, did you, that you help to feed the trees, and to keep them alive and green, and that the trees and grass in their turn help to keep you alive? We were saying the other day how a ray of light will come through a little round hole in the shutters when they are closed, or by any cranny through which it can force its way. As long as that one ray is shining into the darkened room you may watch the little grains of dust, like bright specks, dancing up and down in it. But someone opens the shutters, the room becomes all light, and you no longer see those tiny specks--and yet the dust is still there, not only where you saw it, but all over the room. Why could you see the dust just where the ray of light shone, and nowhere else? The light did not make the dusty specks, they were in the room already, but it showed them to you. Just so there are many wonderful things going on around us in earth and sky and sea--in what people call Nature--which we cannot see or hear or feel; for God is always working mightily and graciously, unseen and unheard by us, though He does allow us to know "parts of His ways," and to look with wonder upon many more which we cannot understand.