22 THE ELEMENTS OF PLANT FOOD 



Nitric acid, HNO 3 . --This is one of the most impor- 

 tant and one of the most powerful acids. It was 

 formerly called aqua fortis, meaning strong water, on 

 account of its caustic action. Care should be taken 

 not to get any of this acid on the hands or on the 

 clothing. When pure, nitric acid is colorless, but it 

 is usually slightly tinted with brownish oxides. Put 

 one or two drops in a glass of water, place a drop of 

 the mixture on the tongue and note the sour taste. 



It is a powerful oxidizing agent and one of the most 

 corrosive substances known. It colors animal and 

 vegetable tissues containing N (such as silk and skin) 

 yellow and changes some substances that contain no 

 N into highly explosive substances, such as gun cotton 

 and nitroglycerin. 



Nitric acid is formed in considerable quantities by 

 the electric discharges in thunder storms. Nitric acid 

 is also formed in the soil under favorable conditions, 

 certain bacterial ferments being active in producing 

 it. This acid combines so readily with minerals in the 

 soil that it exists as acid but a short time. 



Note. HNO 8 is prepared by heating a mixture of sodium ni- 

 trate (NaNO 3 ) and sulphuric acid (H L ,SO 4 ). The fumes that arise are 

 conducted into a retort and there condensed by cooling. 



NaNO 3 + H 2 SO 4 = NaHSO 4 + HNO 3 . 



Nature of Acids. --The description given of nitric 

 acid will apply largely to all acids, though they are not 

 all equally powerful, and not all are liquids. All acids 

 contain H. Not every compound, however, that 

 contains H is an acid. Most acids have a sour taste, 

 and everything that has a sour taste is an acid or con- 

 tains acid. The sour taste of a lemon is due to the 



