Transportation on Land 



313 



substances is of practical importance in many industries. One 

 familiar use is illustrated in distillation, a process which com- 

 prises boiling the liquid, and removing therefrom its vapor, 

 which is condensed to liquid by cooling. For example, pure 

 water for drinking and cooking is now provided on ocean liners 

 by distillation of salt water from the ocean. 



In distillation either the vapor or the residue or both may be 

 saved. Sugar sap, salt water, and the like may be vaporized for 

 the sake of the residue alone, that is, the sugar, salt, and so on. 

 In certain kinds of gold mining, mercury is employed to gather 

 the grains of gold ; then the amalgam of mercury and gold 

 is broken up by distillation in which the mercury is vaporized, 

 condensed, and saved, and the gold is secured as the residue. 

 Water which is thought to be polluted, but must be utilized for 

 drinking, may be vaporized, the vapor being condensed and 

 cooled as pure water, and the residue thrown away. 



TABLE OF SOME COMMON SUBSTANCES WITH THEIR BOILING TEMPER- 

 ATURES AT ATMOSPHERIC PRESSURE 



