170 HEAT OF THE WATERS 



heat of the water is always set free, as is requisite 

 to keep the temperature of the newly-formed ice 

 and of the still liquid water up to 0°. The water 

 running under the ice retains therefore, even in 

 the coldest days of winter, the constant tempera- 

 ture of the freezing point. The surface of ice 

 acquires, as the cold increases, the temperature of 

 the air, and gradually transmits it to the interior 

 of its mass ; and for this reason, the formation of 

 ice is always going on, more and more slowly how- 

 ever as the thickness of the layer increases. 



Since ice is lighter than water (its specific 

 gravity being 0*916), you would expect that it 

 could only appear at the surface. Nevertheless 

 in rivers we very often find the bottom at the 

 beginning of a frost, coated with ice. The for- 

 mation of this groimd-ice is explained by the fol- 

 lowing circumstances. The growth of ice, like 

 that of any other crystalline substance, is very 

 much promoted by the presence of a solid nucleus, 

 as of a crystal already formed, or of any other 

 solid particle. Thus you will never see the ice 

 first formed in the middle of a bowl of water, but 

 it always starts from the edges, and goes on grow- 

 ing from thence to the middle. However, if a 

 piece of ice is hung in the midst of some freezing 

 water, it will forthwith grow in size by the forma- 

 tion of new ice. If there are no such solid parti- 

 cles present, the temperature of water may be 



