MINKRALOOY. 5 



Stances on the earth, is, in one of its forms, the most beau- 

 tiful and valuable of all the minerals. When combined with 

 phosphoric acid and water, alumina forms the mineral tur- 

 quoise, formerly found in Persia only, but now produced more 

 abundantly from the mines in New Mexico. 



Nearly all of the alumina on the earth is combined with 

 silica, and the resulting compound is called the silicate of 

 alumina. The chief portion of the earth's crust consists of 

 the silicate of alumina, combined with other substances, thus 

 forming a great variety of mineral species. 



The white clay used as one of the components of porcelain 

 ware is a silicate of alumina. It is named kaolin. Common 

 clay is the same, though colored red by a little of the oxide of 

 iron. The rock slates of the earth were originally deposited 

 under water as clay beds. 



Oxygen, one of the three most abundant of the elements, 

 is an invisible gas. It is in evidence everywhere. The waters 

 of the oceans, and also the atmosphere, are chiefly composed 

 of it. With the exceptions of chlorine and fiourine, it is prac- 

 tically found to be combined with all the other elements. The 

 ores of many of the metals, including the second most abund- 

 ant, iron, are merely combinations of such metals with oxygen. 

 Though invisible alone, it usually causes great changes in the 

 character of the other elements when combined with it, notably 

 with the lightest of the gases, hydrogen, in forming water. 

 When combined with iron it forms a red rust ; with copper it 

 makes green verdigris ; with mercury, the red oxide. Hema- 

 tite and magnetite consist (jf iron and oxygen. The great 

 changes made by its combination with the metal aluminum 

 haye already been referred to, and also with silicon. 



Calcium is the element which, when combined with oxygen, 

 forms lime. The latter, an alkaline earth, is never found in 

 an uncombined state. Its most abundant combination is 

 with carbonic acid, resulting in the well known mineral, 

 limestone. When magnesia is included, the mineral dolomite 

 is produced, which in nianv cases forms mountains. A large 



