138 HOW CROPS GROW. 



When arsenic, in the form of Paris green or London purple, is applied 

 to land the arsenic soon becorn.es converted into highly insoluble iron 

 compounds and is not taken up by plants in appreciable quantity. 



The Metallic Elements which remain to be noticed, 

 viz.: Potassium, Sodium, Calcium, Magnesium, Iron, 

 Manganese, Aluminium, Zinc, and Copper, are basic in 

 their character, i. e., they unite with the acid bodies 

 that have just been described, to produce salts. Each 

 one is, in this sense, the base of a series of saline com- 

 pounds. 



ALKALI-METALS. The elements Potassium and Sodium 

 are termed alkali-metals. Their oxides dissolve in and 

 chemically unite to water, forming hydroxides that are 

 called alkalies. The metals themselves do not occur in 

 nature, and can only be prepared by tedious chemical 

 processes. They are silvery-white bodies, and are lighter 

 than water. Exposed to the air, they quickly tarnish 

 from the absorption of oxygen and moisture, and are 

 rapidly converted into the corresponding alkalies. 

 Thrown upon water, they mostly inflame and burn with 

 great violence, decomposing the liquid. Exp. 11. 



Of the alkali-metals, Potassium is invariably found in 

 all plants. Sodium is especially abundant in marine and 

 strand vegetation ; it is generally found in agricultural 

 plants, but is sometimes present in them in but small 

 quantity. 



POTASSIUM AND ITS COMPOUNDS. 



Potassium, Sym. K ; * at. wt. 39. When heated in 

 the air, this metal burns with a beautiful violet light, 

 and forms potassium oxide. 



Potassium Oxide, or Potash, K 2 0, 94, is the so- 

 called "actual potash " that figures in the analyses of 

 plants and valuation of fertilizers. It is, however, scarcely 

 known as a substance, because it energetically unites 

 with water and forms hydroxide. 



* From the Latin name Kalium. 



