We find that the rocks about us differ in hardness: they are ground to powder when rubbed together, some are easily dissolved in water, others are dissolved by weak acids. Geologists tell us that the whole crust of the earth was at one time made up of rocks, part of which have been broken down into coarse and fine particles which form the gravel, sand and clay of our soils. The organic matter of our soils has been added by the decay of plants and animals. Several agencies have been active in this work of breaking down the rocks and making soils of them. If we look about we can perhaps see some of this work going on now. _Work of the Sun._--Examine a crockery plate or dish that has been many times in and out of a hot oven, noticing the little cracks all over its surface. Most substances expand when they are heated and contract when they are cooled. When the plate is placed in the oven the surface heats faster than the inner parts, and cools faster when taken out of the oven. The result is that there is unequal expansion and contraction in the plate and consequently tension or pulling of its parts against each other. The weaker part gives way and a crack appears. If hot water is put into a thick glass tumbler or bottle, the inner surface heats and expands faster than the outer parts and the result is tension and cracking. If cold water be poured on a warm bottle or piece of warm glass, it cracks, because there is unequal contraction. In the early part of a bright sunny afternoon feel of the surface of exposed rocks, bricks, boards, or buildings on which the sun has been shining. Examine them in the same way early the next morning. You will find that the rocks are heated by the sun just as the plate was heated when put into the oven, and when the sun goes down the rocks cool again. This causes tension in the rocks and little cracks and checks appear in them just as in the heated plate, only more slowly. This checking may also be brought about by a cool shower falling on the sun heated rocks just as the cool water cracked the warm glass. Many rocks if examined closely will be found to be composed of several materials. These materials do not expand and contract alike when heated and cooled and the tendency for them to check is greater even than that of the plate. This is the case with most rocks. [Illustration: FIG. 19.--COMPARING SOILS.] [Illustration: FIG. 20.--WATER TEST OF SOILS. Bottle _A_ contains sand and water, bottle _B_ clay and water. The sand settles quickly, the clay very slowly.] _Work of Rain._--Rain falling on the rocks may dissolve a part of them just as it dissolved the rock salt; or, working into the small cracks made by the sun, may wash out loosened particles; or, during cold weather it may freeze in the cracks and by its expansion chip off small pieces; or, getting into large cracks and freezing, may split the rock just as freezing water splits a water pitcher or the water pipes.