10 DEPARTMENT OF AGRICULTURE GEORGIA. [296] 



state (mixed, but not combined with nitrogen), it const? 

 tutes about one-fifth the bulk, and considerably more than 

 one-fifth of the weight, of the atmosphere. In combina 

 tion with hydrogen, it forms eight-ninths of all the water 

 on the globe. In combination with silicon, calcium and 

 other minerals, it enters largely into all the solid constitu- 

 ents of the earth's crust ; silica, in its various forms of 

 sand, common quartz, flint, etc chalk, limestone and mar- 

 ble, and all the varieties of clay, containing about half 

 their weight of oxygen. It is moreover found in all the 

 tissues and fluids of all forms of animal and vegetable lii'e, 

 none of which can support existence independently of this 

 element." 



When substances are burned, . the oxygen of the air 

 forms chemical combinations with the substances consumed, 

 one of the principal results of which is carbonic acid, which 

 is also a result of slow combustion or decomposition such 

 as takes place in the decay of vegetable matter, the fer- 

 mentation of the compost heap, and the digestion of food 

 by animals. Animals inhale oxygen from the air, which, 

 forming chemical union with the carbon of the food, and 

 the impurities of the blood, is exhaled as carbonic acid. 



When oxygen unites rapidly with other substances, the 

 union is termed combustion ; when the union takes place 

 slowly, it is termed oxidation. 



The rusting of iron is a familiar example of oxidation. 

 The application of oil or other substance to the iron, to 

 exclude the oxygen of the air, prevents this oxidation or 

 rusting of the iron. 



Hydrogen is a colorless, tasteless, inodorous, inflammable 

 gas, which does not exist in a free state, being always 

 combined with some other element. 



The chief form in which it is found is in combination 

 #ith oxygen in water. 



If the two gases are mixed in the proportion of two of 

 hydrogen to one of oxygen, they will not unite, but simply 



