16 Moy^e about the clojy 



full of clay kept moist to prevent it cracking, and at the 

 same time sow a few in some moist sand. The seeds 

 soon germinate in the sand but not in the clay. It is 

 known that seeds will not germinate unless they have 

 air and water and are warm enough. They had water 

 in both jars, and they were in both cases warm, but 

 they got no air through the clay and therefore could 

 not sprout. Pure clay would not be good for plants to 

 grow in. Air came through the sand, however, and gave 

 the seeds all they wanted for germination. 



This also explains something else that you may have 

 noticed. If you tried baking one of your model bricks 

 in the fire you probably found that the brick exploded 

 and shattered to pieces : the water still left in the brick 

 changed to steam when it was heated, but the steam 

 could not escape through the clay, and so it burst the 

 clay. In a brick works the heat is very gradually 

 applied and the steam only slowly forms, so that it has 

 time to leak away, then when it has all gone the brick 

 can be heated strongly. You should try this with one 

 of your model bricks ; leave it in a hot place near the 

 stove or on the radiator for a week or more and then 

 see if you can bake it without mishap. 



Let us now compare a piece of clay with a brick. 

 The difierences are so great that you would hardly think 

 the brick could have been made from clay. The brick 

 is neither soft nor sticky, and it has not the smooth 

 surface of a piece of clay, but is full of little holes or 

 pores, which look as if they were formed in letting the 

 steam out. A brick lets air through; some air gets 

 into our houses through the bricks even when the 

 windows are shut. AVater will get through bricks more 

 easily than it does through clay. After heavy rain you 



