656 OYSTERS, AND ALL ABOUT THEM. 



and four hours later, when the temperature of the water had 

 risen to 50 C. above zero, their movements were once more 

 fully established. Other gill and mantle lobes which had 

 been three hours in water of a temperatue of i C. to 2 C. 

 below zero moved quite lively on the following day. This 

 recalls to me a very weighty difference between fresh and 

 salt water, which is often overlooked. 



It is generally known that fresh water is densest and 

 heaviest at a temperature of 4 C. above zero. When any 

 portion has arrived at this temperature during freezing 

 weather, its sinks to the bottom of the body of fresh water, 

 where it remains until the entire mass above it is of the 

 same density. That portion which first becomes lower in 

 temperature than 4C. then expands, rises to the surface, 

 and stiffens into ice as it reaches the temperature of o. 



The fact is less known that with sea- water the lower 

 the temperature the greater the density and weight of the 

 water. Therefore, it also sinks to the bottom until it 

 has reached the temperature at which it forms ice, which, 

 when it holds 3 per cent, of salt in solution, is 2-28 C. 

 below zero. It is evident, then, that water may be found 

 at the bottom over the sea flats of a temperature of 2 C. 

 below zero, while, during the most severe cold, water at the 

 bottom of the lakes and deeper rivers of North Germany is 

 found to be constantly several degrees warmer than this. 

 When, finally, the sea-water, from the surface to the 

 bottom, has reached its freezing point, it does not become 

 solid ice for the whole thickness, but thin layers of ice, at 

 greater or less distances apart, are formed in it. These 

 layers, which are crystallized from the salt water, are free 

 from salt, are hence lighter than the surrounding water, 

 and accordingly ascend to the surface ; consequently, those 



