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. Water freezes not only between the component grains, but in the numerous crevices or joints, as they are called, by which rocks are traversed. You have, perhaps, noticed that on the face of a cliff, or in a quarry, the rock is cut through by lines running more or less in an upright direction, and that by means of these lines the rock is split up by nature, and can be divided by the quarrymen into large four-sided blocks or pillars. These lines, or joints, have been already referred to as passages for water in descending from the surface. You can understand that only a very little water may be admitted at a time into a joint. But by degrees the joint widens a little, and allows more water to enter. Every time the water freezes it tries hard to push asunder the two sides of the joint. After many winters, it is at last able to separate them a little; then more water enters, and more force is exerted in freezing, until at last the block of rock traversed by the joint is completely split up. When this takes place along the face of a cliff, one of the loosened parts may fall and actually roll down to the bottom of the precipice. In addition to carbonic acid, oxygen, and frost, there are still other influences at work by which the surface of the earth is made to crumble. For example, when, during the day, rocks are highly heated by strong sunshine, and then during night are rapidly cooled by radiation, the alternate expansion and contraction caused by the extremes of temperature loosen the particles of the stone, causing them to crumble away, or even making successive crusts of the stone fall off. Again, rocks which are at one time well soaked with rain, and at another time are liable to be dried by the sun’s rays and by wind, are apt to crumble away. If then it be true, as it is, that a general wasting of the surface of the land goes on, you may naturally ask why this should be. Out of the crumbled stones all soil is made, and on the formation and renewal of the soil we depend for our daily food.