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the volatile compound of those that are formed 

 is disengaged. It is this reaction which takes 

 place in the soil. The contrary behaviour is 

 exemplified, by pouring together solutions of 

 sulphate of lime and carbonate of ammonia ; 

 double decomposition ensues, and the in- 

 soluble carbonate of lime is formed and pre- 

 cipitates. 



Water. Water is indispensable to the ex- 

 istence of plants. It acts both in facilitating 

 the reactions occurring in the soil, and as an 

 organic and mineral manure. The rain which 

 falls during a storm contains, as Liebig has 

 proved, nitrate of lime and ammonia. Ordi- 

 nary rain contains but little more than traces 

 of common salt, though in both instances it 

 carries down all the organic matters which it 

 meets, as dusty particles, suspended in the 

 air. 



River water, and spring or fountain water, 

 are both used for irrigation, but the two latter 

 contain much larger quantities of saline mat- 

 ters. These salts, very variable in their na- 

 ture, are derived from the soil over whicl. 

 these waters flow. 



That water which is richest in alkaline salts 

 is most preferable for the irrigation of lands. 



