156 LECTURES ON 



way that these narrow (capillary*) tubes support it. When a body has 

 pores so fine (surfaces so near each other) that their surface attrac- 

 tion is greater than the gravitating tendency of water, then the body 

 will suck up 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 them- 

 selves 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 pene- 

 trable, 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 ap- 

 plied in the spring are to some degree washed down below the 

 reach of vegetation, and in the droughts of summer plants suffer and 

 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 latter be inclined, 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. 



The part which the capillarity of the soil plays in the nutrition of 

 the plant deserves a moment's notice. 



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 disap- 

 pears its place is supplied by a new one, until the lamp is empty or 

 the flame extinquished. 



Something quite analogous occurs in the soil, by which the plant 

 (corresponding to the flame in our illustration) is fed. The soil is at 

 once lamp and wick, and the water of the soil represents the oil. Let 

 evaporation of water from the surface of the soil or of the plant take 

 place of the combustion of the oil from a wick and the matter stands 

 thus: Let us suppose dew or rain to have saturated the ground with 

 moisture for some depth. On recurrence of a dry atmosphere with 

 sunshine and wind, the surface of the soil rapidly dries; but as each 

 particle of water escapes (by evaporation) into the atmosphere, its 

 place is supplied (by capillarity) from the stores below. The ascending- 

 water brings along with it the soluble matters of the soil, and thus the 

 roots of plants are situated in a stream of their appropriate food. The 

 movement proceeds in this way so long as the surface is dryer than 

 the deeper soil. When, by rain or otherwise, the surface is saturated, 

 it is like letting a thin stream of oil run upon the apex of the lamp- 



* From capillus the Latin word for hair, because as fine as hair; (hut a hair is no tube, as 

 is often supposed.) 



