Geology 



though much smaller quantities of carbonic acid and 

 water vapour. 



The oxygen is an invisible gas without any notice- 

 able colour, is an extremely active chemical element, 

 and serves for the breathing of animals and the com- 

 bustion of many substances such as wood and coal. 

 It is found not only in the atmosphere, but in combina- 

 tion with another gas, hydrogen, forms the substance 

 water which plays such an important part in the 

 activities of the Earth and its inhabitants. 



Oxygen is also found, in combination with other 

 elements, in many minerals and rocks, and thus forms 

 part of the solid Earth also. 



The nitrogen in the air is an inert body ; it does 

 not enter readily into chemical combination with the 

 other constituents of the atmosphere, though it can be 

 made to do so under certain conditions, and occasionally 

 combines in small quantities with the oxygen when a 

 lightning flash occurs. 



By the agency of minute bacteria which are to be found 

 upon the roots of peas, beans, and certain other plants, the 

 nitrogen is caused to combine with oxygen and other 

 elements to form the compounds called nitrates, which 

 perform an important part in the economy of plants. 



The carbonic acid, though normally present in 

 quantities not greater than 35 parts in 100,000 

 parts of air, nevertheless, is absolutely essential to 

 the life of plants and animals, and is responsible 

 for setting in motion very many of the chemical 

 changes which take place during the rotting away of 

 rocks under the influence of the atmosphere a process 

 usually spoken of as weathering. 



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