68 AGRICULTURE. [CH. 



away from the soil, so air is drawn into the soil to 

 take its place, otherwise the water could not escape 

 from the land. If you nearly fill a bucket with stones, 

 and pour water so as to cover them, the air has no 

 opportunity of gaining access to these stones ; but if 

 you make an opening in the bottom of the bucket, the 

 water runs out, and as the surface of the water lowers 

 so the air follows, and gains access to the stones. 

 This simple illustration will show how it is that the 

 construction of drains for carrying away the water, of 

 necessity brings the atmospheric air into the soil. 



136. As soon as the air is admitted to a newly 

 drained soil, a great change takes place. The un- 

 healthy decomposition which had been going on in 

 the stagnant water, had caused an accumulation of 

 organic acids which made the soil unsuitable for the 

 growth of any of our cultivated crops. The entrance 

 of the air, bringing with it the pure oxygen of the 

 atmosphere, soon converts these organic acids into 

 more useful forms. It also enables an action to be 

 commenced upon the inorganic matter present, 

 whereby some of its dormant elements are rendered 

 active and useful for plant growth. Thus the first 

 good result of drainage is to remove from the land the 

 stagnant water it contained, and draw in the purifying 

 atmospheric air to increase the fertility of the land. 



137. The drainage of land also gives an outlet from 

 the soil for any soluble matter which is injurious to 

 vegetation. The passage of water through the soil 

 gradually dissolves this out and practically washes it 

 from the land. 



138. A very marked difference is observable 

 in the temperature, or warmth of drained, and 

 undrained lands. You are no doubt aware that 

 evaporation necessitates the employment of heat. 

 If a vessel of water be placed upon a fire the heat 

 it receives first causes the water to boil. If the heat 

 be continued the water does not get any hotter, but 



