More about the clay 17 



can often find that water has soaked through a brick 

 wall and made the wall paper quite damp. A pretty 

 experiment can be made with the piece of apparatus 

 shown in Fig. 9 : bore in a brick a hole about an inch 

 deep and a quarter of an inch wide, put into the hole 

 the piece of bent glass tubing, and fix it in with some 

 clay or putty, then pour some water blackened with ink 

 into the tube, marking its position with a label. Stand 

 the brick in a vessel so full of water that the brick 



Fig. 9. 

 A brick standing in water. The air 

 in the brick is driven inwards by 

 the water and forces the liquid 

 up the tube in order to escape 



is entirely covered. Water soaks into the brick and 

 presses the air out : the air tries to escape through the 

 tube and forces up the black liquid. 



One more experiment may be tried. Can a brick be 

 changed back into clay? Grind up the brick and it 

 forms a gritty powder. Moisten it, work it with your 

 fingers how you please, but it still remains a gritty 

 powder and never takes on the greasy, sticky feeling of 



R. 2 



