THE NEW AGRICULTURE. 91 



action takes place with each particle of water, as in the case of the 

 water in the burst water-pipe or the cracked jar. It does not mat- 

 ter whether the water is collected into some hole or crevice, or is 

 diffused between the grains of the rocks and the soil. When it 

 freezes it expands, and in so doing tries to push asunder the walls 

 between which it is confined. 



" Water freezes not only between the component grains, but in 

 the numerous crevices or joints, as they are called, by which rocks 

 are traversed. You have, perhaps noticed, that on the face of a 

 cliff, or in a quarry, the rock is cut through by lines running more 

 or less in an upright direction, and that by means of these lines 

 the rock is split up by nature, and can be divided by the quarry- 

 men into large four-sided blocks or pillars. These lines, or joints, 

 have been already referred to as passages for water in descending 

 from the surface. You can understand that only a very little water 

 may be admitted at a time into a joint. But by degrees the joint 

 widens a little, and allows more water to enter. Every time the 

 water freezes it tries hard to push asunder the two sides of the 

 joint. After many winters, it is at last able to separate them a 

 little ; then more water enters, and more force is exerted in freez- 

 ing, until at last the block of rock traversed by the joint is com- 

 pletely split up. When this takes place along the face of a cliff, 

 one of the loosened parts may fall and actually roll down to the 

 bottom of the precipice. 



"In addition to carbonic acid, oxygen and frost, there are still 

 other influences at work by which the surface of the earth is made 

 to crumble. For example, when, during the day, rocks are highly 

 heated by strong sunshine, and then during night are rapidly 

 cooled by radiation, the alternate expansion and contraction caused 

 by the extremes of temperature loosen the particles of the stone, 



