THE NON-LIVING WORLD 147 



and quartz, or " rock crystal," is one of the commonest 

 of them. Rarer ones are rubies, emeralds, sapphires, 

 topazes, amethysts, opal and the substance hydrophane, 

 before referred to.* The most brilliant of crystals, dia- 

 monds, are, as before said,t formed of carbon, and, of 

 course, are neither silicious nor calcareous. Great crystal- 

 line masses of rock-salt are found in many places. 



Grranite consists of an accumulation of crystals of three 

 kinds intermixed namely, quartz, and two silicates of 

 alumina : mica and feldspar. Gneiss is a highly crystal- 

 line rock, composed of a feldspar with mica and quartz. 



Porphyry "and basalt are allied to granite. It has 

 been found by experiment that such rocks can be 

 rendered liquid at very high temperatures, and liquid 

 rocks of such kinds exist naturally in the, form of the 

 lavaj which is emitted from burning mountains or 

 volcanoes. 



Our atmosphere, the aeriform envelope of the earth, 

 or air, is not like water, a substance resolvable into 

 gases, chemically combined, but is a mixture of gases 

 and vapours. About one-fifth part of it consists of 

 oxygen and almost all the rest of nitrogen. Carbonic 

 acid is always present in small but varying quantities, 

 as a general rule about five volumes of it to 10,000 

 volumes of air. 



There is also some ammonia and a certain quantity of 

 the vapour of water. The amount of this aqueous vapour, 

 however, varies greatly, as it must do from what we have 

 already seen respecting the conversion, by heat, of liquid 

 water into vapour or steam, and its condensation, at a 

 lower temperature, into its liquid condition once more. 



* Se ante, p. 103. t See ante, p. 140. 



\ See |*>, p. 163. See ante, pp. 86 and 87. 



