SOILS. I9 



their sour taste. Water absorbs about seven hundred 

 times its volume of this gas, and in this condition 

 it is commonly called hartshorn. Ammonia is 

 an alkali that is, it neutralizes acids and restores 

 vegetable colors, like litmus, that have been made red 

 by means of an acid. 



29. Ammonia is formed naturally by the decay 

 of animal and vegetable matter, as in manure-piles. 

 Its odor can be detected in stables, or where organic 

 substances containing nitrogen are undergoing de- 

 composition. As a gas it exists in minute quantities 

 in the atmosphere, .and finds its way into plants 

 through their roots. 



30. Nitrogen and oxygen unite to form several 

 oxides, one of which when combined with water is 

 known as nitric acid. This is a very strong acid of 

 intensely sour taste. It corrodes or destroys the 

 flesh, and acts upon nearly all the metals, forming 

 with them a class of compounds, called nitrates. 

 One of these, potassium nitrate, is known as nitre or 

 saltpetre ; and another, sodium nitrate, goes by the 

 name of Chili saltpetre. Both of these nitrates are 

 used as fertilizers (n). 



31. Carbon is an element that exists in three 

 distinct forms. Charcoal, coke, and lampblack are 

 varieties of the first; graphite, or plumbago, com- 

 monly known as black lead and used for mak- 

 ing lead-pencils, is the second ; and the diamond, 

 which is pure crystallized carbon, the third and most 

 valuable form. When wood is heated in a close ves- 



