10 ELEMENTARY AGKICULTURAL CHEMISTRY 



ing in the production of evil-smelling gases or vapours. When 

 oxidation by the air occurs, heat is evolved. 



Valency. — Elements differ in their power of combining 

 with other elements. Thus chlorine can only combine with 

 hydrogen atom for atom, or one atom of chlorine is chemically 

 equivalent to one atom of hydrogen, as is seen in the com- 

 pound HCl ; oxygen has usually twice the combining power 

 of hydrogen, or one atom of oxygen is chemically equivalent 

 to two atoms of hydrogen, as seen in the compound H^O ; 

 nitrogen is possessed of even greater value in combination : 

 one atom of it can combine with three atoms of hydrogen, as 

 NH3 ; lastly, carbon forms a compound CH^, showing that 

 one atom of carbon is equivalent to four of hydrogen. The 

 number of atoms of hydrogen which one atom of a given 

 element can combine with or replace is called the valency of 

 the element. The valency of chlorine is 1, that of oxygen 2, 

 that of nitrogen 3, and that of carbon 4. Or chlorine ia 

 said to be monovalent, oxygen divalent, nitiogen trivalent, and 

 carbon tetravalent. The valency of an element varies in 

 different compounds, and generally those compounds of an 

 element in which it has one particular valency possess many 

 properties in common, but quite distinct from the properties 

 which it has in another class of compounds in which its 

 valency is higher or lower. Thus divalent iron, which occurs 

 in all ferrous compounds, gives quite a distinct set of re- 

 actions from trivalent iron, which occurs in all ferric com- 

 pounds. 



Volatile: — capable of being converted from a liquid or solid 

 into a vapour or gas by heat. The term is somewhat loosely 

 used in two senses : 



(1) (the proper use) When a substance on heating is con- 

 verted into a vapour or gas without undergoing any chemical 

 change. In this case, on cooling, the gas or vapour is trans- 

 formed again into the original solid or liquid — e.g.^ camphor, 

 water. 



(2) When a substance, on being heated, is converted into gas 



