102 NATURE TEACHING 



lime (chalk), and which also attacks the mineral known 

 as felspar, dissolving a portion of it and leaving a residue 

 which is clay. A little search amongst the stones in a 

 garden is almost sure to reveal that while some of the 

 stones are quite hard, others are relatively soft ; some 

 being found which may be crushed in the hand, or 

 crushed or broken by the spade. In these soft stones 

 the felspar has been attacked and partly converted 

 into clay. If the stones are of flint or chert (quartz), 

 these, being practically indestructible, never become 

 soft. 



Frost is, in temperate climates, an important agent 

 in breaking up rocks and stones to form soil. Many 

 rocks and stones are somewhat porous, absorbing 

 appreciable quantities of water. Now, water expands 

 in changing from the liquid to the solid state ; in other 

 words, a certain quantity of water increases in bulk 

 when it is frozen and changed into ice. This may easily 

 be shown by filling a bottle with water, tightly corking 

 it, and exposing it out-of-doors on a cold winter night, 

 when the bottle breaks, unless the glass is very strong, 

 in which case we usually find the cork forced out. The 

 ice must get extra room somehow, and it is merely a 

 question whether less force is required to break the 

 bottle or to force out the cork. The bursting of water- 

 pipes in winter is due to the same cause. 



To return to the consideration of a porous rock ; it 

 absorbs water, and if this water is subsequently frozen it 

 expands, and in doing so often exerts sufficient force to 

 crack the rock. The cracking due to any one freezing 

 may be very small, but when repeated over and over 



