MAGNESIUM 31 



and slightly soluble in water. It is a very active substance, com- 

 bining directly with nearly all the known elements at ordinary 

 temperature. A compound formed through the union of any ele- 

 ment with oxygen is called an oxide. Water is the oxide of hydro- 

 gen (H2O). Iron rust is the oxide of iron. At high temperature 

 oxygen combines vigorously with carbon with accompanying 

 evolution of heat and light, as in burning. 



Oxygen is necessary for all animal life. Good blood owes its 

 red color to a good supply of oxygen. It is of interest to know that 

 nearly one-half of an average soil is oxygen by weight. Water is 

 90 per cent oxygen. By weight, air is 23.2 per cent oxygen. 

 Rocks of the earth consist of 44 to 48 per cent of this element. 



Silicon. — Next to oxygen, silicon is the most abundant element. 

 More than one-fourth of the crust of the earth is silicon. Quartz 

 sand is the oxide of silicon. Some mountains are largely sili- 

 con oxide. In its free state silicon is a brittle solid, having a 

 metallic luster. 



Aluminum is widely distributed in nature, constituting 8 per 

 cent of the earth's crust. It has a great attraction for oxygen. 

 It is found in pure clay and in nearly all common rocks. In the 

 pure form alxuninum is a silver-white, lustrous metal about one- 

 third as heavy as iron. It is used largely in the manufacture of 

 cooking utensils and for many other useful things, such as paint, 

 alum, etc. 



Iron — ^Pure iron is a white and fairly soft metal. It rusts 

 easily in moist air and in salty solutions. Many farmers put grease 

 on plow mold-boards and cultivator teeth to keep off the oxygen 

 and thus prevent rusting. Iron is very abundant and widely 

 distributed. All soils and rocks contain iron in some combined form. 



Calcium is always found in combined forms because it is a very 

 active element. In its pure form calcium is a fairly hard, silver- 

 white metal. This is a widely distributed element. It is an impor- 

 tant constituent of bones, egg shells, limestone, coral, marble and 

 nauurai cnajLK!. 



Magnesium is widely distributed in natinre, and occurs in large 

 quantities. As an element it is a silver-white metal which burns 

 in the air with a brilhant white light. More than one-haH of 

 flash hght is magnesiimi powder. Many magnesium compounds 

 are found in soils. Certain common rocks (dolomitic limestone) 

 are composed of calcium and magnesium carbonates in nearly 

 equal proportions. 



