22 THE FIRST BOOK OF FARMING 



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 ten- 

 sion 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 after- 

 noon feel of the surface of exposed rocks, bricks, 

 boards, or buildings on which the sun has been shin- 

 ing. 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 



