148 APPLIED SCIENCE 



162. Sublimation. Most solid substances melt when 

 a certain amount of heat is applied to them. Upon being 

 heated further they vaporize. There are a few substances, 

 like ammonium chloride, which vaporize without melting. 

 To purify such substances, they must first be heated and 

 their vapors collected. This process of purification is called 

 sublimation. 



163. Distillation. Distillation is the process by which 

 a liquid is boiled and its vapor condensed. It is used, like 

 the processes of crystallization and sublimation, for purposes 

 of purification. If impure water, for instance, is placed in 

 a boiler to which a condensing apparatus (an apparatus for 

 cooling the steam) is attached, the vapor or steam given off 

 when the water is boiled, is condensed. It then becomes 

 pure or distilled water, all the non-volatile impurities having 

 been left in the boiler. The water has a lower boiling point 

 than the impurities, hence it boils first, and is thus enabled 

 to leave the impurities behind. 



164. Chemical Properties of Coal. The principal ma- 

 terials used for fuel are petroleum and coal. Ordinary hard 

 coal is called anthracite coal, and the soft, lumpy kind that 

 crumbles very easily is called bituminous coal. All fuels 

 are composed of carbon, or compounds of carbon and hydro- 

 gen, called hydrocarbons, combined with such impurities 

 as ash, sulphur, nitrogen, etc. 



When fuel burns the chemical change which takes place 

 is that the oxygen of the air combines with the hydrogen 

 and carbon. The manner in which coal burns depends upon 

 its composition, the nature of the fire, and the air supply. 



If the draught of air is insufficient, the gases are only partly 



