4 AGRONOMY 



hydrogen are joined to one of oxygen. When symbols in 

 parenthesis are followed by a number written below the line, 

 as Ca(NO g ) 2 , it signifies that the elements and quantities in 

 parenthesis are to be multiplied by the number so written. 

 Calcium phosphate (Ca g (PO 4 ) 2 ) would also be correctly 

 written CaP O . 



m m m 



Chemical compounds. When salt and sand, or charcoal and 

 sulphur, are stirred up together, the result is a mere mechan- 

 ical mixture. In a chemical compound an entirely new sub- 

 stance is formed, which may possess characteristics quite unlike 

 those of the combining elements. Under certain conditions 

 charcoal and sulphur may be made to form a chemical union 

 in which two atoms of sulphur join with one of carbon to pro- 

 duce carbon disulphide (CS 2 ). Sulphur, as everybody knows, 

 is a yellow solid and carbon a black solid, but the carbon 

 disulphide formed by their union is neither black, yellow, nor 

 a solid, but is a colorless liquid resembling water. Moreover, 

 while either pure carbon or sulphur may be eaten without 

 harm, when they are chemically combined they form a deadly 

 poisonous liquid, which, exposed to the air, soon turns to a 

 heavy, suffocating, and highly inflammable gas. Again, when 

 carbon is burned in the air, as in ordinary wood and coal fires, 

 one atom of carbon unites with two of oxygen and forms a 

 colorless, suffocating gas known as carbon dioxide (CO 2 ). In 

 a similar way sulphur may be burned to form sulphur dioxide 

 (SO 2 ). Calcium carbide, from which acetylene gas is produced, 

 is another union of carbon and is represented by the formula 

 CaC 2 . It is interesting to note that all the chemical elements 

 unite with others in definite and unvarying proportions. No 

 matter how much oxygen may be present when carbon is 

 burned, the molecule of carbon dioxide formed always consists 

 of two atoms of oxygen and one of carbon. Carbon, however, 

 may be made to form other combinations with oxygen. When 

 there is a lack of oxygen, carbon monoxide (CO) may be 



