PERMEABILITY OF SOILS TO LIQUID WATEK. 177 



port it. When a body has pores so fine (surfaces so near 

 each other) that tlieir surface attraction is greater than 

 the gravitating tendency of water, then the body will im- 

 bibe and hohl water — will exhibit capillarity; a lump of 

 salt or sugar, a lamp-wirk, are familiar examples. When 

 the pores of a body are so large (the surfaces so distant) 

 that they cannot fill themselves or keep themselves full, 

 the body allows the water to run through or to percolate. 



Sand is most easily permeable to water, and to a higher 

 degree the coarser its particles. Clay, on the other hand, 

 is the least penetrable, and the less so the purer and more 

 plastic it is. 



When a soil is too coarsely porous, it is said to be leachy 

 or hungry. The rains that fdl upon it quickly soak 

 through, and it shortly becomes dry. On such a soil, the 

 manures that may be applied in the spring are to some de- 

 gree washed down below the reach of vegetation, and in 

 the droughts of summer, plants siiflfer or perish from want 

 of moisture. 



When the texture of a soil is too fine, — its pores too 

 small, — as happens in a heavy clay, the rains ]>enetrate it 

 too slowly ; they flow oif the surface, if the latter be in- 

 clined, or remain as pools for days and even weeks in the 

 hollows. 



In a soil of proper texture the rains neither soak off into 

 the under-earth nor stagnate on the surface, but the soil 

 always (except in excessive 'wet or drought) maintains 

 the moistness which is salutary to most of our cultivated 

 plants. 



Movements of Hater in the Soil.— If a wick be put 

 into a lamp containing oil, the oil, by capillary action, 

 gradually permeates its whole length, that which is above 

 as well as that below the surface of tlie liquid. Wlien the 

 lamp is set burning, the oil at the flame is consumed, and 

 as each particle disappears its place is supplied by a new 

 one, until the lamp is empty or the flame extinguished, 

 8* 



