62 



CHEMISTRY FOR AGRICULTURAL STUDENTS 



and to regard each compound as containing one, two, or more 

 combining weights of the element. 



If hydrogen, having the lowest combining weight, be taken 

 as unity, that of oxygen is taken as i6, that of carbon as 12, 

 and that of mercury as 200. Water would then contain two 

 combining weights of hydrogen to one of oxygen, and may be 

 represented as HgO, taking H and O to represent respec- 

 tively single combining weights of the elements. Carbonic 

 acid gas would contain one combining weight of carbon to 

 two of oxygen, and may be represented as CO2, where C 

 represents a combining weight of carbon. Red oxide of 

 mercury would contain one combining weight of each element, 

 and may be represented as HgO where Hg represents a com- 

 bining weight of mercury {hydrargyrum). 



In the earth's crust about seventy elements are known to 

 exist, but of these it is only necessary to give the combining 

 weights of twenty-two. These are : — 



