114 THERMAL CAPACITY SPECIFIC HEAT. 



water and aqueous bodies generally, the law of expansion is 

 unknown. 



It is a remarkable property of water that, at a tempera- 

 ture of about 4 C. or 40 F., its volume is a minimum and 

 therefore its density is a maximum;* and whether its tem- 

 perature increases or decreases from this point, the water 

 expands in volume. When the temperature descends to the 

 freezing point, there is a still further expansion at the 

 moment of congelation; for this reason, ice floats in 

 water. 



We can now see what takes place in a pond of fresh 

 water during winter. The fall of temperature at the sur- 

 face of the pond does not extend to the bottom, where the 

 water seldom falls below 4 C., whatever may be the exter- 

 nal temperature. As the temperature at the surface de- 

 scends, the water at the surface cools, and being contracted, 

 it becomes heavier than the water beneath, and sinks to the 

 bottom. The water from beneath rises and becomes cooled 

 in its turn; and this process goes on till all the water has 

 attained its maximum density, i.e., till its temperature is 

 4 C. But when all the water has attained this tempera- 

 ture, it will remain stationary ; and any further cooling of 

 the water at the surface will expand it, until it finally con- 

 geals. It is clear that the deeper the water is, the longer 

 will be the time before the whole of the water has attained 

 its maximum density, and therefore that ice will form much 

 less rapidly on the surface of deep than on the surface of 

 shallow ponds. 



It is from the fact that water expands in freezing, taken 

 in connection with the low conducting power of liquids 

 generally, that the temperature at the bottom of deep ponds 

 remains moderate even during very severe cold, and that the 

 lives of aquatic animals are preserved. 



* The results of Playfair and Joule give 3".945 C. us the temperature at which the 

 density is a maximum. Phil. Transactions, 1856. 



