108 



ELEMENTS OF GENERAL SCIENCE 



120. Distillation. Formerly it was customary for ships to 

 carry with them enough fresh water to last throughout the 

 voyage, but the larger steamships now prepare their daily 

 supply from the salt ocean water by distillation. When water 

 containing salt or other solid impurity is boiled and thus evapo- 

 rated, the solid does not evaporate. Since the steam does 

 not contain the impurity, it condenses into pure water. The 

 process is called distillation. This is the most certain method 

 of securing absolutely pure water (fig. 58). 



Large distilling plants are employed to produce commercial 

 supplies of pure water for drinking purposes, for ice manu- 

 facture, and for other uses. Also, in private establishments 

 smaller plants are sometimes maintained. 



121. Boiling and melting points. Every substance has its 

 boiling and melting point, but these may be at temperatures 

 very different from those of water. The melting point may 

 also be called the freezing point. The approximate melting 

 and boiling points of a few substances are given below : 



Excepting for differences in temperature, most substances 

 behave very much like water in changing from solid to liquid 



