AGENTS OF WEATHERING 181 



Extreme cold, where there is water present, is one of the 

 greatest forces in weathering. When water freezes, it expands 

 about one eleventh of its volume. Water is everywhere pres- 

 ent in crevices and even within many rocks. Therefore, 

 whenever cold weather comes, this water freezes, expands, and 

 breaks the rock into countless numbers of pieces, for the ex- 

 pansive power of freezing water is almost irresistible. When 

 warmer weather comes, the ice melts, runs out, carrying with it 

 the smaller pieces of rock and leaving larger gaps behind for 

 rain, the roots of trees, and burrowing animals, to enter. 

 Water pipes, in cold places, are burst in a similar fashion. 



Chemical changes are those which alter the make-up of 

 matter in its finest divisions. On the surface of the earth 

 oxygen is the most active in producing these chemical changes. 

 Oxygen combines very readily with a large number of ele- 

 ments which compose the surface of the earth, and by its com- 

 bination changes insoluble parts into other substances, which 

 are readily soluble in water. Thus oxygen prepares the way 

 for the action of the water, which is twofold : mere wearing 

 away, and solution. Oxygen is especially active on iron com- 

 pounds. 



In organic compounds, that is, different compounds of 

 carbon which have had life at some time, oxygen produces a 

 decay, or aids in the decay started by certain bacteria which 

 form various compounds with carbon and hydrogen. There- 

 fore oxygen, working in conjunction with bacteria, tends to 

 remove all dead vegetable and animal matter, and return its 

 constituents either to the ground or to the atmosphere. Con- 

 sequently the same material is used over and over again, in 

 plant life and animal life. 



