Maximum Density 31 



swollen, their scales standing out from the body, their 

 eyes btdging. 



Maximum density — ^Water contracts on cooling, as do 

 other substances, but not to the freezing point — only 

 to 4° centigrade (39.2° Fahrenheit). On this pecu- 

 liarity hang many important biological consequences. 

 Below 4° C. it begins to expand again, becoming lighter, 

 as shown in the accompanying table: 



Hence, on the approach of freezing, the colder lighter 

 "water accumulates at the surface, and the water at the 

 point of maximum density settles to the bottom, and 

 the congealing process, so fatal to living tissues generally 

 is resticted to a thin top layer. Here at 0° C. (32° F.) 

 the water freezes, expanding about one-twelfth in bulk 

 in the resulting ice and reducing its weight per cubic 

 foot to 57.5 pounds. 



Stratification of the water — Water is a poor conductor 

 •of heat. We recognize this when we apply heat to the 

 bottom of a vessel, and set up currents for its distribution 

 through the vessel. We depend on convection and not 

 ■on conduction. But natural bodies of water are heated 

 and cooled from the top, when they are in contact with 

 the atmosphere and where the sun's rays strike. 

 Hence, it is only those changes of temperature which 

 increase the density of the surface waters that can pro- 

 duce convection currents, causing them to descend, and 

 deeper waters to rise in their place. Minor changes 

 •of this character, very noticeable in shallow water, occur 



