THE ELEMENTS OF PLANT FOOD 23 



presence of citric acid, that of sour milk to lactic acid, 

 and that of vinegar to acetic acid. Acids will change 

 blue litmus paper to red. This is the most common 

 test for an acid. 



There are fourjvreiy common acids with which stu- 

 dents should become familiar : 



Sulphuric acid (H^SCX) 

 Nitric acid (HNO 3 ) 

 Hydrochloric acid (HC1) 

 Carbonic acid (H 2 CO 3 ) 



Note. Litmus paper is made by staining unsized paper with lit- 

 mus solution. Litmus is made from certain lichens. A substitute for 

 litmus paper may be made by boiling red cabbage leaves and soaking 

 unsized paper in the liquid. A strong acid will turn any dark-colored 

 vegetable matter red. (A little book of blue litmus may be obtained 

 at any drug store for a few cents.) 



Ammonia (NH 3 ). Ammonia ^xists m small quan- 

 tities in the air and is brought down to the earth in the 

 rain and snow. When we open a bottle of aqua 

 ammonia bought at the drug store, a gas escapes that 

 has a most pungent odor. This gas is ammonia (NH 3 ). 

 The liquid in the bottle is aqua ammonia. It is water 

 combined with ammonia, and thus charged, it is put 

 up as an article of commerce. 



Ammonia is formed in all decaying animal bodies, 

 in manure and urine, and in decaying vegetable matter 

 in the soil. NH 3 may be smelled about manure heaps, 

 in horse stables or cow barns. It escapes very easily 

 into the air and is lost. It is one of the chief fertilizing 

 substances in manure, ~hence its -escape into the air 

 should be prevented as far as possible. 



The ammonia of commerce is obtained as a by- 



