THE SOUTHERN PLANTER. 



677 



minute pores. The cause of capillarity is 

 the same surface attraction which has been 

 already mentioned. 



When a narrow vial is partly filled with 

 water, it will be seen that the liquid ad- 

 heres to its sides, and if it be not more 

 than one-half inch in diameter, the sur- 

 face of the liquid will be curved or con- 

 cave. In a very narrow tube the liquid 

 will rise to a considerable height. In these 

 cases the surface attraction of the glass for 

 the water neutralizes or overcomes the 

 weight of (earth's attraction foi') the latter. 

 The pores ot a sponge raise and hold wa- 

 ter in them, in the same way that these 

 narrow (capillary*) tubes support it. When 

 a body has pores so fine (surfaces so near 

 each other,) that their surface attraction 

 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 ex- 

 amples. 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 nin 

 through or to percolate. 



Sand is most easily permeable to water, 

 and to a higher degree, the coarser its par- 

 ticles. Clay on the other hand is the least 

 penetrable, and the less so, the purer and 

 more plastic it is. In*an agricultural sense, 

 sand implies those coarse particles or 

 grains whose form can be defined by the 

 eye, while all the finer impalpable portions 

 of a soil, though consisting in part of very 

 fine grains of sand, may be called clay. 

 The chemist, however, understands by clay 

 a definite chemical compound. The dis- 

 tinction should be borne in mind. Sand, 

 i. e. grains of quartz, or undecomposed 

 rock, may be made so fine, that with the 

 admixture of a little true clay, it opposes 

 the passage of water to a considerable ex- 

 tent. 



When a soil is too coarsely porus, 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 a great degree washed down 

 below the reach of vegetation, and in the 

 droughts of summer, plants suffer and per- 

 ish from want of moisture. 



* From capillus the Latin word for hair, be- 

 cause as fine as a hair, (but a hair is no tube, as 

 is often supposed.) 



When the texture of a soil is too fine, 

 its pores too small, as happens in a heavj'' 

 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 al- 

 ways (except in excessive wet or drought) 

 maintains the moistness which is salutary 

 to most of our cultivated plants. 



What part the capillaj'ity of the soil 

 plays in the nutrition of the plant may now 

 be noticed in detail. 



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 sup- 

 plied by a new one, until the lamp is emp- 

 ty or the flame extinguished. 



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 on the soil repiesents 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 

 stieam 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 wick, 

 no more evaporation into the air can oc- 

 cur, and consequently there is no longer 

 any ascent of water ; on the contrary, the 

 water by its own weight penetrates the 

 soil, and if the underlying ground be not 

 saturated with moisture, as can happen 

 where the subterranean fountains yield a 

 meagre supply, then capillarity will aid 



