SUMMARY 167 



In early times before the advent of railways almost all 

 commerce was carried on over the sea. Even now this is 

 the cheaper way of transportation. Modern methods of 

 conveyance have enabled man to live with comfort at a 

 considerable distance from the ocean, but the dry interiors 

 of continents still remain sparsely inhabited. All com- 

 mercial nations must have an outlet to the sea and to ob- 

 tain it much blood and treasure have often been spent. 



SUMMARY 



The earth has been called a water engine since water has 

 played such an important part in its history. Pure water 

 is a colorless, odorless, tasteless liquid, composed of two 

 gases, hydrogen and oxygen. Water may evaporate at 

 any temperature, but evaporation goes on most rapidly at 

 the boiling point. As water above 4 C. increases in 

 temperature, it increases in volume. When water changes 

 from a liquid to a gas, its volume increases more than 1700 

 times. Water in cooling grows denser until it reaches about 

 4 C. It then begins to expand and continues to do so until 

 it freezes at C. When it freezes it exerts a pressure of 

 more than 100 tons to the square foot. The entire mass of 

 a body of water must reach a temperature of about 4 C. 

 before it begins to freeze at the surface. 



Water is the greatest of all solvents but it does not dissolve 

 every substance. The higher the temperature of water, the 

 less air but the more solid matter it will hold in solution. A 

 mixture of ice, salt and water is called a freezing mixture be- 

 cause the solution attains a temperature lower than that of 

 melting ice. All solutions freeze at a lower temperature than 

 that at which pure water freezes. Water may also hold sub- 

 stances in suspension. The greater the 'movement of water 



