278 SALTS. 



lime. This rule is not without exceptions, but it 

 enables the chemist to determine beforehand innume- 

 rable cases of precipitation. 



695. SOLUTION AND CHEMICAL COMBINA- 

 tion differ TioN. Solution differs from chemical com- 

 bination in tne varying proportions in 

 which it occurs according to tempera- 

 ture, and in the absence of any change of chemical 

 properties. Nitre, for example, dissolves in water, at 

 100, in nearly double the quantity which will dis- 

 solve at 70. At the same time, it forms a solution to 

 which it has imparted its own chemical properties 

 unchanged. 



596. Another important distinction is 



State another , 



important found in the following fact. While chem- 

 twn ' ical combination is most active between 

 bodies whose properties are most opposed, such as fat 

 and resins, solution occurs most readily in the case 

 of similar substances. The metals dissolve in mer- 

 cury. Salts dissolve in water. Fats and resins dissolve 

 in alcohol and ether, which, like themselves, contain 

 much hydrogen. 



697. CRYSTALLIZATION. In passing from 



What is said , r 



o/ crystalline the liquid to the solid condition, the par- 



arrangement? ^^ Qf mogt bodieg assume a crystalline 



arrangement. Their mutual attraction is more than 

 a mere force which draws and binds them together. 

 It groups them in regular forms. The crystals thus 

 produced are often too small to be separately seen. But 

 even where this is the case, the crystalline structure is 

 readily observed. Surfaces of zinc, or cast iron, ex- 



