Rain is water nearly in a state of purity. After journeying up and down underground it comes out again in springs, always more or less mingled with other materials, which it gets from the rocks through which it travels. They are not visible to the eye, for they are held in what is called chemical solution. When you put a few grains of salt or sugar upon a plate, and pour water over them, they are dissolved in the water and disappear. They enter into union with the water. You can not see them, but you can still recognize their presence by the taste which they give to the water which holds them in solution. So water, sinking from the soil downward, dissolves a little of the substance of the subterranean rocks, and carries this dissolved material up to the surface of the ground. One of the important ingredients in the air is carbonic acid gas, and this substance is both abstracted from and supplied to the air by plants and animals. In descending through the atmosphere rain absorbs a little air. As ingredients of the air, a little carbonic acid gas, particles of dust and soot, noxious vapors, minute organisms, and other substances floating in the air, are caught up by the descending rain, which in this way washes the air, and tends to keep it much more wholesome than it would otherwise be. But rain not merely picks up impurities from the air, it gets a large addition when it reaches the soil. Armed with the carbonic acid which it gets from the air, and with the larger quantity which it abstracts from the soil, rainwater is prepared to attack rocks, and to eat into them in a way which pure water could not do. Water containing carbonic acid has a remarkable effect on many rocks, even on some of the very hardest. It dissolves more or less of their substance, and removes it. When it falls, for instance, on chalk or limestone, it almost entirely dissolves and carries away the rock in solution, though still remaining clear and limpid. In countries where chalk or limestone is an abundant rock, this action of water is sometimes singularly shown in the way in which the surface of the ground is worn into hollows. In such districts, too, the springs are always hard; that is, they contain much mineral matter in solution, whereas rainwater and springs which contain little impurity are termed soft. When a stone building has stood for a few hundred years, the smoothly-dressed face which its walls received from the mason is usually gone. Again, in the burying-ground surrounding a venerable church you see the tombstones more and more mouldered the older they are. This crumbling away of hard stone with the lapse of time is a common familiar fact to you. But have you ever wondered why it should be so? What makes the stone decay, and what purpose is served by the process?