THE GASES. 131 



THE G-ASES. "We have alluded to the part which gases 

 take in the formation of the crust of the earth, and to the 

 preponderance of oxygen in rocks. Hydrogen contributes 

 in like manner ; it is emitted from volcanoes, and exists in 

 coal. Nitrogen forms four-fifths of the atmosphere, and 

 enters into the composition of animal structure. Carbon 

 forms the principal part of coal ; it exists in the state of car- 

 bonic acid, in the atmosphere ; and forms an important part 

 in all carbonates. It is an essential element in vegetable 

 structure, and is produced wherever vegetable or animal 

 matters are undergoing decomposition. Chlorine occurs 

 chiefly in the sea, in rock-salt, and brine springs. Fluorine 

 exists in some rocks, in minute proportion. Phosphorus is 

 extensively diffused, but its proportions are extremely minute. 

 It is abundant in osseous and other organic remains. Sulphur 

 is found chiefly in the sulphurets and sulphates. 



Almost the whole of the simple substances have entered 

 into their present combinations as binary compounds. The 

 following constitute nearly all the binary compounds of the 

 accessible parts of the globe : 1, silica ; 2, alumina ; 3, lime; 

 4, magnesia ; 5, potassa ; 6, soda ; 7, oxide of iron ; 8, oxide 

 of manganese ; 9, water ; 10, carbonic acid. 



It is calculated that oxygen constitutes fifty per cent, of 

 the ponderable matter of the globe, and that the crust of 

 the earth contains forty-five per cent, of silica, and ten or 

 twelve of alumina. Potassa contributes only seven per cent, 

 of the unstratified rocks, but forms a considerable ingredient 

 in many of the stratified forms : soda nearly six per cent, of 

 some basalts, and other less extensive unstratified deposits, 

 and enters largely into the composition of the ocean. Lime 

 and magnesia are disseminated almost universally, in the 

 form of silicates and carbonates. Iron as an oxide, sul- 

 phuret, or carburet, constitutes at least three per cent, of 

 all known rocks ; and manganese forms about one per cent. 

 It has often been cited as a fact, which the present state 

 of our chemical knowledge does not enable us to explain, 

 that silica predominates in the unstratified, and carbonate 

 of lime in the stratified deposits. 



The mass of rocks is composed of not more than some 

 eight or nine simple minerals. These are 1, quartz ; 2, feld- 

 spar ; 3, mica ; 4 7 hornblende 5, carbonate of lime ; 6, talc, 



K 2 



