CHAP. IX DRAINAGE 67 



the soil are finer and more closely packed together, and each Fine parti- 

 particle is surrounded by a thin layer of water in which are sun-ounded 

 dissolved substances useful for the nourishment of plants. f/J/Qf"^ 

 Capillary attraction may be easily demonstrated by taking water. 

 two small sheets of glass, and applying their faces so close 

 together as to make them almost touch. If they be then 

 dipped into water, it will run up for a distance on the inner 

 surfaces of the two glasses. This action is capillary attrac- 

 tion, and to it is due the fact that all the water does not run 

 out of a well-drained soil, but a portion remains to benefit 

 vegetation. 



When the rain, which falls on the earth, sinks down into 

 the soil, and does not leave it sodden or saturated with 

 moisture, the land is naturally drained ; but in some Natural 

 instances there is an obstacle to the water running off, it 

 remains in the soil, it keeps the soil cold, it prevents the 

 entry of the atmosphere, thus putting a stop to the change 

 of the dormant into the active constituents, it becomes after 

 a time impregnated with vegetable acids that are injurious Sour or acid 

 to plant life, and it causes the roots to damp or rot off. The 

 reason that a damp soil is a cold one, is explained in several 

 ways. In the first place the water is colder than the 

 surrounding air, and the presence of the water prevents the 

 air from entering in to warm the soil. Then again, the land Damp soils 

 is rendered cold by the evaporation of the water. Every one ^ ^^^^ ^° 

 knows that water left in a saucer, in the atmosphere, will 

 gradually pass o0" into vapour— in fact, it will dry up. But 

 the conversion of water into vapour carries off a good deal 

 of heat, and this is the principle made use of in water-coolers Evaporation 



' ^ . . , abstracts 



and goblets. These are made of porous earthenware heat. 

 through which the water gradually percolates and is then 

 evaporated from the outside surface. This causes a great 

 abstraction of heat, thereby making the water in the goblet, 

 and the goblet itself, much cooler than the surrounding air. 

 This is also what takes place in soils sodden with moisture, 

 and the object of drainage is to remove the surplus water 



F 2 



