IS2 



PHYSIOGRAPHY. 



[chap. 



is said to have its maximum density. This can easily be 

 observed by repeating an old experiment devised originally 

 by Dr. Hope. Insert two thermometers (Fig. 38), at 

 different levels into a cylinder of water, and chill the water 

 by applying ice around the middle of the vessel. As the 

 water becomes cooled it grows denser, and therefore sinks 

 to the bottom, so that the loiner thermometer falls until it 

 reaches 39° Fahr. Further cooling then expands the water, 

 instead of condensing it, and consequently the cold water 

 rises, so that now the upper thermometer, which has mean- 

 while been almost stationary, 

 begins to fall, and continues 

 falling until, like the upper one, 

 it reaches 39° Fahr. The whole 

 body of water is then at its maxi- 

 mum density, and any further 

 reduction of temperature causes 

 expansion, the cold water becom- 

 ing specifically lighter and rising 

 to the surface. Gradually, the 

 upper thermometer sinks to the 

 freezing-point, and then a layer 

 of ice begins to form upon the 

 surface. This experiment roughly 

 imitates what occurs in a natural piece of water, such as 

 a lake : the surface freezes, while the bottom-water remains 

 several degrees warmer. 



At the moment of freezing, when the particles of water 

 are trying to arrange themselves in those crystalline forms 

 which were noticed in Chapter IV., there is an increase 

 of bulk much greater than that just described. 



Ice, being thus relatively much lighter than water, floats 

 upon the surface. Yet there are certain conditions under 



Fig. 38. — Hope's experiment; on 

 the contraction of w.iter. 



