The Chemical Relations of Water 293 



4 C. is reached, after which the slow rise of the water- 

 level in the glass tube will indicate that the water 

 above this temperature is behaving in the normal 

 fashion. From this point, the expansion is continuous 

 until the water begins to boil. 



This temperature, 4 C., is known as the temperature 

 of the " maximum density of water ". The anomalous 

 behaviour of water at 0, when heated, is, of course, 

 paralleled by the reverse effect being produced when 

 water at 4 is cooled. This peculiarity is of great 

 importance as a natural agency in the preservation 

 of animal life in shallow waters in winter, and in 

 maintaining a temperate climate over portions of the 

 globe which would otherwise be regions with a very 

 low mean annual temperature. When water in a lake 

 is cooled by contact with the cold air above it, the 

 surface water, contracting, becomes denser, and con- 

 vection currents are set up whereby the warmer water 

 is constantly moving towards the surface and the 

 cooled water is descending. When the temperature 

 of the water at the surface has fallen to 4, further 

 cooling causes expansion and it no longer sinks, since 

 its density is now lessening. Continued exposure to 

 cold causes the surface water to lose heat until its 

 temperature has reached freezing-point, 'ice crystals 

 then begin to form slowly at the top of the water, 

 and float on account of their density being less than 

 that of water. A continuous sheet of ice forms as 

 a result of the agglomeration of the crystals. This 

 forms a protective coating for the water beneath, ice 

 being a bad conductor of heat. Thus the thicken- 

 ing of the ice-sheet proceeds slowly, and the water, 

 except that in immediate contact with the ice, is at 

 the uniform temperature of 4 C. This is warm enough 



