118 NATURE TEACHING 



some hours. When cold, compare it with a portion of 

 fresh, unburned clay. Note that, although it is still able 

 to absorb water by capillarity, it has lost its plastic 

 character and can no longer be moulded into shape by 

 the hand. Crush it to powder and moisten with water ; 

 the plasticity is not restored, it is permanently lost. 

 Note also the change in colour. 



Make small bricks of wet earth, sand and clay 

 respectively. Measure and record their length, breadth 

 and thickness. Place the bricks on a piece of board and 

 put them on one side for a day or two until quite dry. 

 Now measure them again, and note any changes in size. 

 Note also the looseness or hardness of the dried bricks. 



Take a small piece of clay, about the size of a hazel- 

 nut, and rub it down with water to the consistency of 

 thin cream. Pour this into a pint of rain water or dis- 

 tilled water and stir well. After standing a minute or two 

 pour off the muddy water from any sand which has settled 

 to the bottom. Now take two similar glass cylinders 

 or large tumblers, and fill each with the clay water, which 

 should again be stirred before being poured out. To 

 one cylinder or tumbler add a tablespoonful or two of 

 lime-water and stir. Set the two vessels aside for the 

 clay to settle. Note the difference in the way subsidence 

 takes place in each, and recognise that lime causes the 

 fine particles of clay to collect together into larger 

 masses or floccules which subside quickly. 



Vegetable Matter in Soils. 



A place should be set apart in the school garden 

 for a compost heap. All available refuse should be 



