WATER 109 



buoyancy of ice. The coldest water would 

 continually sink to the bottom and there 

 freeze. The ice, once formed, could not be 

 melted, because the warmer water would stay 

 at the surface. Year after year the ice would 

 increase in winter and persist through the 

 summer, until eventually all or much of the 

 body of water, according to the locality, 

 would be turned to ice. As it is, the tempera- 

 ture of the bottom of a body of fresh water 

 cannot be below the point of maximum den- 

 sity ; on cooling further the water rises ; 

 and ice forms only on the surface. In this 

 way the liquid water below is effectually pro- 

 tected from further cooling, and the body of 

 water persists. In the spring the first warm 

 weather melts the ice, and at the earliest 

 possible moment all ice vanishes. 



Such are the important thermal properties 

 of water, and in briefest outline their unique 

 fitness for the living mechanism. No other 

 known substance could be substituted for 

 water as the material out of which oceans, 

 lakes, and rivers are formed, and as the sub- 

 stance which passes through the meteorolog- 

 ical cycle, without radical sacrifice of some 

 of the most vital features of existing condi- 

 tions. Ammonia in these respects is the only 

 substance now known which approaches the 



