50 MINERALOGY. 



in hot and dry weather succeeding a rain. It is also pro- 

 duced in the same way in Egypt, and appears in light 

 tufts, which are gathered up by a sort of broom, and 

 the nitre is separated from its impurities by lixiviation, 

 and is crystallized by evaporation. It is found diffused 

 in the earth on the floor of some of our Western caverns, 

 and is apt to appear in crusts or needle-like crystals on 

 their walls. This salt is that one of the ingredients of 

 gunpowder that furnishes the oxygen required for the 

 combustion. 



97. Nitrate of Soda. This is very much like nitre; like 

 that, it deflagrates when thrown upon the fire, and it has 

 the same supply of oxygen in it that nitre has ; and it 

 would answer the purpose equally well in gunpowder if 

 it did not readily deliquesce that is, gather moisture 

 from the atmosphere. Both this salt and the nitre are 

 used for obtaining nitric acid. 



98. Common Salt. This is composed of chlorine and 

 sodium, two elements which never appear in nature un- 

 combined. Its crystals are the cube and its secondaries. 

 A peculiarity in the arrangement of its crystals when 

 they are formed in a solution was shown in 22. This 

 mineral is very thoroughly diffused in the earth, mostly, 

 however, in solution, nearly one thirtieth of all the water 

 in the sea being common salt. Lakes that have no outlet 

 to the sea are very salt. This is the case with the Great 

 Salt Lake of this country, the Dead Sea, and the Caspian 

 Sea. Over one fifth of the water of the Great Salt Lake 

 is salt, and the proportion is even greater in the case of 

 the Dead Sea. There are famous salt mines where the 

 solid mineral is obtained in Poland, Hungary, Spain, Sici- 

 ly, and Switzerland. In the extensive mines near Cracow 

 chapels, halls, etc., are excavated far below the surface, 

 their roofs being supported by immense pillars of salt, 

 which, on being lighted up, present a magnificent ap- 

 pearance. In Northern Africa there are hills of salt. In 

 this country vast quantities of salt are obtained by evap- 



