which causes a constant flow of heat across the water-ice surface of separation. In this way, ice 

 particles whose temperature is 0° are, so to speak, surrounded by pockets of warm water during 

 ice formation which protect them from excessive cooling. Conversely, during ice melting, when 

 heat is introduced from without, the ice particles are surrounded by a pocket of cold water which 

 prevents the ice temperature from rising above 0°. 



Figure 29 shows the temperature variation during ice formation in agitated fresh water (ac- 

 cording to Altberg). If water is cooled considerably and at the same time vigorously agitated, the 

 temperature sometimes drops to -0. 2° before ice formation begins. Then, as the amount of ice 

 increases, the temperature of the water rises to 0°. 



(^20 



o:w 



q:oo 



-0.10 



-0:20 



10 20 30 40 50 60 



Figure 29. Changes in water temperature during ice formation. 



Thus, the basic condition necessary for ice formation is a certain supercooling of water. 



Furthermore, for the conversion of water from its liquid into its solid phase, it is necessary 

 to remove a comparatively large amount of heat from it — the heat of crystallization. Consequently, 

 the second necessary condition for ice formation is the assurance that water will, by some means 

 or another, lose a great deal of heat. 



LITERATURE: 4, 5, 40, 62, 141. 



Section 37. Ice Formation in Fresh Waters 



Let us assume that we have a certain volume of fresh water which is being cooled from the 

 surface, and which is at rest. Since the freezing point of fresh water is lower than its temperature 

 of maximum density, by the time the temperature of the surface layer reaches the freezing point, 

 all convective currents have ceased. A certain supercooling of a very thin surface layer causes the 

 formation of nuclei of crystallization in it. The distribution of these nuclei along the surface of the 

 water will naturally be chaotically uniform, but further developments cannot of course be uniform 

 in all directions. 



90 



