rock. The chemists tell us water will also dissolve the limestone, but
very slowly. There are large areas of soil which are the refuse from
the dissolving of great masses of limestone.

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