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.