9O AGRICULTURAL CHEMISTRY 



completely soluble in water, from which it is easily liber- 

 ated by heat. The gas can be reduced to liquid form by 

 cold and pressure. Liquefied ammonia passes back to the 

 gaseous condition with removal of the pressure, and in so 

 doing, heat is absorbed from surrounding bodies. If this 

 heat is absorbed from water, the temperature of the water 

 is lowered sufficiently to produce ice. This property of 

 liquefied ammonia is taken advantage of for the artificial 

 production of ice, and for refrigerating purposes. The 

 transportation of perishable food materials has been ren- 

 dered possible by this method of refrigeration. 



99. Uses. In the laboratory, ammonium hydroxid is 

 used extensively as a reagent for neutralizing acid solu- 

 tions and precipitating insoluble hydroxids. Ammo- 

 nium salts, when present in any appreciable amounts, are 

 destructive to plants, (NH 4 ) 2 SO 4 being less injurious than 

 either NH 4 C1 or (NH 4 ) 2 CO 3 . Hence (NHJ 2 SO 4 can be 

 used in limited amounts as a fertilizer. Because of its 

 being a volatile alkali, ammonia^is valuable as a reagent 

 for softening water. 



In small amounts, so as to form very dilute solutions, 

 the ammonium compounds serve as food for plants, sup- 

 plying them with nitrogen which is used for producing, 

 within the plant cells, complex nitrogenous compounds, 

 as proteids. Ammonium compounds supply only one 

 form of nitrogenous plant food. 



100. Oxids of Nitrogen. Nitrogen forms five com- 

 pounds with oxygen : 



N 2 O nitrogen monoxid or nitrous oxid. 

 N 2 O 2 nitrogen dioxid or nitric oxid. 



