90 THE NEW AGKICULTUKE. 



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, hap- 

 pens 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 oxy- 

 gen. 



" 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 



