59 8 AGRICULTURE. 



simple body, out of which we can get nothing different. Ele- 

 ments are seldom found in this simple state in nature, but are 

 generally mixed with other things, making compound bodies. 

 Thus we have seen that air is a compound of two elements, oxy- 

 gen and nitrogen, which are both gases. Water also is a com- 

 pound body, made of two gases, hydrogen and oxygen. There 

 are, however, many other elements that 'are not gases, but solid 

 substances. Some of these solid elements we know as metals ; 

 such as iron, silver, gold, copper, etc. Others are solid, but not 

 metals ; as sulphur, carbon, phosphorus, chlorine, silicon, etc. 



Chemists have so far discovered in the air, and water, and soil, 

 63 single elements. Of these, 48 are metals, and 1 5 are either 

 gases or solid elements other than metals. 



Many elements are very common ; other metallic elements 

 are rare. When these elements unite and form compounds, 

 we find that those which are most unlike as free elements unite 

 more readily than those which in general resemble each other. 



Of all the metallic elements, iron is the most plentiful and 

 important. It is found in various combinations in all our cul- 

 tivated soils. In fact, without iron in the soil, no plant could 

 grow, because the formation of leaf-green in plants is dependent 

 upon iron in the soil. We always find some form of iron in the 

 ashes of plants, and all cultivatable soils have it in inexhaustible 

 quantities. 



The various things we find in the ashes of a plant exist in 

 certain combinations, called Acids ; Alkalies, and Salts. 



An Acid is a compound which is sour and corrosive. 



An Alkali is a compound which is the opposite of an acid, 

 and which, by uniting with an acid, destroys it and forms a 

 neutral body. 



A Salt is this neutral body, formed by the combination of an 

 acid with an alkali. 



Acids and alkalies are very unlike, but we find that they read- 

 ily unite to form a very different thing from either. 



Thus carbon dioxide, an acid gas, unites with lime, a caustic 

 alkali, and forms limestone or chalk. We find, everywhere in 

 nature, that acids and alkalies tend to combine with each other. 



We find four alkalies in the ashes of plants, magnesia, soda, 

 lime, and potash. 



