PARADOXICAL CONTRACTION. 



If we start from 4 C. with the water-level 

 at the top of the tube, at one time heating the 

 flask up to boiling point, and at another time 

 cooling it till the water within is all frozen, 

 and in each case roughly measuring the amount 

 of expansion by collecting the water which over- 

 flows from the top of the tube through a rubber 

 connection, we shall find that by simply cool- 

 ing it four degrees from 4 to zero C., and 

 then freezing it at the latter temperature with- 

 out cooling it any further, we shall cause it to 

 expand about twice as much as by heating it 

 ninety-six degrees from 4 to 100 C. 



The paradoxical manner in which water both 

 contracts and expands with cold, is a matter 

 of the greatest importance to human and 

 animal life in temperate and arctic climates. 

 It causes the cooling in winter of the water in 

 lakes and ponds down to the temperature of 

 4 C., saves them from getting colder, and 

 limits the freezing to a comparatively thin 

 layer at the top. The influence through which 

 these results are obtained is the action of con- 

 vection-currents similar to those described in 

 the last section, but operating this time in an 

 exactly converse manner. There we had them 

 heating all the water above the level at which 

 heat is applied that of the flame ; here we 

 have them cooling the water below the level 

 at which cold is applied namely the surface 

 of the pond. 



