More about the clap 



11 



We can now understand why some of our model bricks 

 cracked. The cracks were caused by the shrinkage 

 just as happens with our model field. As soon as the 

 clay becomes wet it SAvells again. A very pretty experi- 

 ment can be made to show this. Fill a glass tube or an 

 egg-cup with dry powdered clay, scrape the surface 

 level with a ruler, and then stand in a glass jar full of 

 rain water so that the whole is completely covered. 

 After a short time the clay begins to swell and forces 



Before dryiug 



After drying 



Fig. 4. 



Clay was plastered over a square piece of board and completely 



covered it. After drying for a week the clay had 



shrunk and cracked 



its way out of the egg-cup as shown in Fig. 5, falling 

 over the side and making quite a little shower. In 

 exactly the same way the ground swells after heavy 

 rain and rises a little, then it falls again and cracks 

 when it becomes dry. Darwin records some careful 

 measurements in a book called Earthivornis and 

 Vegetable Mould — "a large flat stone laid on the 

 surface of a field sank 3*33 millimetres* whilst the 

 weather was dry between May 9th and June 13th, 



1 A little more than one-eighth of an inch. 



