THE SOIL. 21 



water will sink through it very rapidly; but if it is 

 a stiff clay, it will be retained for a very long time, 

 generally till the next season's growth of plants 

 can use it. 



Dry ground which is wet, say four inches 

 deep, by a rain of one day will be found to be 

 wet six or eight inches by the next day. The 

 grains of earth had so much water clinging to them 

 that they could not hold it all firmly; so gravity 

 and the surface tension of the grains below pulled 

 some of it down still further. A limit, however, is 

 soon reached, and that same water starts on an 

 upward course, due to evaporation at the surface. 

 The top layer of soil grains gives up its moisture 

 to the air, while at the same time more is 

 drawn by capillarity from the ones below, till all 

 the water has passed back to the air whence it 

 came. 



Not all of the water held by soil escapes into the 

 air if plants are growing there. They take it up 

 with their roots wonderfully fast. If their roots 

 do not go down to where all the water is, the surface 

 tension of the soil-grains pulls it up to them, and 

 this is the useful office of that wonderful force. A 

 good example of surface tension can be seen along 

 the margins of streams and other bodies of water. 

 When the banks are dry, it will be noticed that the 

 earth is wet several inches above the surface of 

 the water. Posts and trees standing in the water 

 are also moist above the water surface. The 



