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.

Work of Moving Water.—­Visit some neighboring beach or the banks of some rapid stream.  See how the waves are rolling the sand and pebbles up and down the beach, grinding them together, rounding their corners and edges, throwing them up into sand beds, and carrying off the finer particles to deposit elsewhere.  Now visit a quiet cove or inlet and see how the quiet water is laying down the fine particles, making a clay bed.  Notice also how the water plants along the border are helping.  They act as an immense strainer, collecting the suspended particles from the water, and with them and their bodies building beds of soil rich in organic matter or humus.

The sun, besides expanding and cracking the rocks by its heat, helps in another way to make soils.  It warms the water that has been grinding soil on the beach or along the river banks and causes some of it to evaporate.  This vapor rises, forms a cloud and floats away in the air.  By and by the vapor forms into rain drops which may fall on the top of some mountain.  These rain drops may wash loosened particles from the surface or crevices of exposed rocks.  These drops are joined by others until, by and by, they form a little stream which carries its small burden of rock dust down the slope, now dropping