ia 
PERMEABILITY OF SOILS TO LIQUID WATER. 177 
port it. When a body has pores so fine (surfaces so near 
eich other) that their surface attraction is greater than 
the gravitating tendency of water, then the body will im- 
bibe and hold water—will exhibit capillarity; a lump of 
salt or sugar, a lamp-wick, 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 penetrab!e, 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 fall 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 suffer 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 penetrate it 
too slowly; they flow off the surface, if the latier 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 Water 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 the liquid. When 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, 
gs 
