88 AGRICULTURAL CHEMISTRY 



94. Properties. When pure, nitric acid is a colorless 

 liquid; the commercial acid has a yellow color because 

 of the presence of oxids of nitrogen. Nitric acid is an 

 active oxidizing reagent, and when metals, as copper and 

 iron, are dissolved in it, brown fumes of NO 2 are given 

 off because the hydrogen, as soon as liberated, is oxidized 

 by the excess of acid and NO 2 is formed. H -f HNO 3 

 H 2 O -j- NO 2 . Nitric acid imparts a permanent yellow 

 color to wool, silk, and all albuminous matter. 



95. Importance. In the laboratory, nitric acid is used 

 as an oxidizing agent. It is used commercially in the 

 dyeing of cloth, although it has a tendency to weaken the 

 wool fibers. Salts of nitric acid are important because 

 they are of so much value as plant food, and particularly 

 in the manufacture of commercial fertilizer, where it sup- 

 plies nitrogen. Potassium nitrate is used in the manu- 

 facture of gunpowder. Nitrates are of great importance 

 in agriculture. 



Ammonia 



96. Occurrence. Ammonium compounds are present 

 in small amounts in the air, rain water, and in the soil, 

 and are produced from decaying nitrogenous organic 

 matter. The chief source of the ammonia which serves 

 as the basis for the preparation of ammonium salts is 

 the ammonia water obtained in the process of purifying 

 illuminating gas made from soft coal. The nitrogen 

 compounds of the coal form ammonia gas, NH 3 , during 

 the destructive distillation process. 



97. Preparation. In the laboratory, ammonia is usu- 



