A LESSON IN CHEMISTRY 7 



Plaster of Paris is derived from gypsum by burning. The rock 

 called apatite contains large amounts of calcium phosphate. 

 Calcium is one of the elements essential to plant life. It is 

 posed to neutralize the acids that would otherwise injure the 

 plant, as well as to play an important part in the production 

 of new tissues. Some plants, such as clover, beans, and peas, 

 are often called lime plants because they require so much of 

 this element for their proper development. 



Aluminum (Al) is another of the white metals that is never 

 found native, though it is the principal constituent of every 

 clay bank and forms one twelfth of the earth's crust. It is 

 malleable and ductile and does not oxidize in the air. Clays 

 and feldspars are silicates of aluminum, and the ruby, emerald, 

 oriental amethyst, and sapphire are crystalline forms of the 

 same metal combined with oxygen. Corundum, or emery, is 

 an impure crystalline form. Though the metal itself is soft, 

 the crystalline forms are exceeded in hardness by the diamond 

 only. Alum is a combination of sulphur and potassium with 

 aluminum. Aluminum is usually found in plants, though it 

 forms no part of the plant food. 



Iron (Fe) is too well known to need description. It is abun- 

 dant and widely distributed, occurring usually as carbonates 

 and oxides. It is an ingredient in practically all soils and 

 forms about one fifteenth of the earth's crust. Iron rust 

 and the ochers are oxides of iron, and it is these substances 

 which give^^e red and yellow colors to certain soils. Iron 

 is essentiaMBfcplants. Its presence is necessary for the 

 formation of chlorophyll, the green color of plants, though, 

 so far as known, it does not enter into the composition of 

 the color. 



Manganese (Mn) is a hard, grayish-white metal that is fused 

 with difficulty, but readily oxidizes. While often found in 

 plants, it has been proved that it is not necessary to their 

 growth. 



