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?
If it seem strange to you to be told that the surface of the earth is crumbling away, you should take every opportunity of verifying the statement. Examine your own district. You will find proofs that, in spite of their apparent steadfastness, even the hardest stones are really crumbling down. In short, wherever rocks are exposed to the air they are liable to decay. Now let us see how this change is brought about.
First of all we must return for a moment to the action of carbonic acid, which has been already described. You remember that rainwater abstracts a little carbonic acid from the air, and that, when it sinks under the earth, it is enabled by means of the acid to eat away some parts of the rocks beneath. The same action takes place with the rain, which rests upon or flows over the surface of the ground. The rainwater dissolves out little by little such portions of the rocks as it can remove. In the case of some rocks, such as limestone, the whole, or almost the whole, of the substance of the rock is carried away in solution. In other kinds, the portion dissolved is the cementing material whereby the mass of the rock was bound together; so that when it is taken away, the rock crumbles into mere earth or sand, which is readily washed away by the rain. Hence one of the causes of the mouldering of stone is the action of the carbonic acid taken up by the rain.
In the second place, the oxygen of the portion of air contained in rainwater helps to decompose rocks. When a piece of iron has been exposed for a time to the weather, in a damp climate, it rusts. This rust is a compound substance, formed by the union of oxygen with iron. What happens to an iron railing or a steel knife, happens also, though not so quickly nor so strongly, to many rocks. They, too, rust by absorbing oxygen. A crust of corroded rock forms on their surface, and, when it is knocked off by the rain, a fresh layer of rock is reached by the ever-present and active oxygen.
In the third place, the surface of many parts of the world is made to crumble down by means of frost. Sometimes during winter, when the cold gets very keen, pipes full of water burst, and jugs filled with water crack from top to bottom. The reason of this lies in the fact that water expands in freezing. Ice requires more space than the water would if it remained fluid. When ice forms within a confined space, it exerts a great pressure on the sides of the vessel, or cavity, which contains it. If these sides are not strong enough to bear the strain to which they are put, they must yield, and therefore they crack.
You have learned how easily rain finds its way through soil. Even the hardest rocks are more or less porous, and take in some water. Hence, when winter comes the ground is full of moisture; not in the soil merely, but in the rocks. And so, as frost sets in, this pervading moisture freezes. Now, precisely the same kind of action takes place with each particle of water, as in the case of the water in the burst water-pipe or the cracked jar. It does not matter whether the water is collected into some hole or crevice, or is diffused between the grains of the rocks and the soil. When it freezes it expands, and in so doing tries to push asunder the walls between which it is confined.