PLATINA. 175 



of slaves allowed to be employed by those to whom leases 

 of these mines had been granted. The ruby family of mi- 

 nerals is composed of seven species. They are" all extreme- 

 ly hard, and several of them highly valued on account of 

 their beauty. 



The saline minerals comprehend all the combinations of 

 alkalies with acids which exist in the mineral kingdom : 

 such are salt-petre or nitrate of potash ; common rock salt, 

 or muriate of soda; and sal-ammoniac, or the muriate-of 

 ammonia. Common salt is found in immense masses under 

 the earth's surface in many countries, particularly in Poland, 

 Hungary, and England. The salt-springs in some parts of 

 the United States owe their origin to beds of fossil salt. 

 The rain-water, which penetrates to their surface, effects the 

 solution of a certain portion of them with which it comes in 

 contact, and thus becomes, in some cases, it is said, ten times 

 Salter than the water of the sea. The injiammable minerals 

 comprehend all combustible bodies, except metals and the 

 diamond ; and include sulphur, resins, bitumens, and gra- 

 phite. Among the bitumens are found the several varieties 

 of mineral coal that are used for fuel, gas-lights, and other 

 purposes. At Pittsburgh in Pennsylvania there are inex- 

 haustible quantities of coal of a superior quality ; it is found 

 also in other parts of the state, in some parts of New- York, 

 and in Rhode-Island. It not only enhances the value of the 

 lands in which it is found, and through which it must pass, 

 but is a source of national wealth. In England there are 

 vast beds of coal which often lie at the depth of a hundred 

 feet beneath the surface of the earth. Near White-haven 

 there are some coal mines that extend half a mile under the 

 sea. 



The metallic minerals comprehend all the mineral bodies, 

 that are composed either entirely of metals, or of which 

 metals constitute the most considerable and important part. 

 It is from the minerals belonging to this class that all metals 

 are extracted ; and for this reason they have been called 

 ores. They are found in a native state, either simple, con- 

 sisting only of one substance, or compound, when composed 

 of two or more substances. We shall briefly describe a few- 

 of the most useful metals. The first is platina. This is the 

 heaviest of metals, and is found among the gold ores of South 

 America in the form of small grains or scales. Its colour is 



