156 CHEMISTRY. 



viz., oxygen, nitrogen, carbon, and hydrogen are the chief 

 elements concerned in the formation of the earth. Especial- 

 ly is this true of orgianic substances, both vegetable and ani- 

 mal. In some of these, it is true, there are lime, phosphorus, 

 sulphur, iron, etc. ; but these are generally in small quan- 

 tities, while the great bulk of them is made up of combina- 

 tions of the four grand elements which we have mentioned. 

 Then of substances not living, the earth's envelope of air, 

 fifty miles thick, is a mixture mostly of two of these ele- 

 ments, oxygen and nitrogen, and all the water is composed 

 of oxygen and hydrogen. And to come to the solid crust 

 of the earth, carbon is seen in the enormous quantities of 

 coal treasured up in the bowels of the earth for the use of 

 man; carbon and oxygen united with a metal form the 

 limestone rocks and ranges of mountains ; oxygen is a large 

 constituent of the granite and other hard rocks ; and of the 

 compound mixture under our feet which we call earth the 

 four grand elements form a very large proportion. 



207. Chemical Changes in Air and "Water. These elements 

 are continually the subjects of chemical changes. You have 

 already seen how that mixture of gases, the air, is constantly 

 changing by means of the chemical operations going on in 

 the lungs of animals, in the leaves of vegetables, in combus- 

 tion, in the various arts of man, and in the decay of animal 

 and vegetable substances. Though, therefore, the atmos- 

 phere which envelops the earth is to-day composed of oxy- 

 gen, nitrogen, and carbonic anhydride, in precisely the same 

 proportions as that which enveloped it when our first parents 

 were in the Garden of Eden, yet it is not the same air, but 

 its elements have from that time to this been going through 

 many changes, entering into the composition now of liquids, 

 now of solids, and now of gaseous substances. The ele- 

 ments of water are also continually changing, though per- 

 haps not to such an extent as those of air. In 9, Part L, 



