x The Hydrosphere 167 



cubic feet of sea -water would only be reduced to about 

 10,000 cubic feet, the density being only slightly increased. 

 If sea -water were absolutely incompressible the oceans 

 would be about 200 feet deeper than they actually are. 

 Sea-water is perfectly elastic. When pressure is removed 

 from a portion it returns at once to its original volume. 



227. Heat and Sea-water. When sea- water is warmed 

 it expands, steadily diminishing in density as the tempera- 

 ture rises. The specific heat is less than that of fresh 

 water, for while 100 units of heat (65) are needed to 

 raise 100 Ibs. of pure water from 32 to 33, 93-5 units can 

 raise the temperature of 100 Ibs. of sea -water (density 

 1.0260) through the same range. Sea-water conducts heat 

 better than fresh water, so that the heat of the surface 

 penetrates to a greater depth in the sea than in a deep lake 

 in the same time. When heat is removed from sea-water, 

 i.e. when it is cooled down, its density increases steadily, 

 for its maximum density occurs below the freezing point. 

 The chilled surface layer in contact with a very cold atmo- 

 sphere always sinks, unless it is much fresher than the 

 lower layers, which only happens in polar regions or near 

 shore. Sea-water freezes about 28 F., or at a temperature 

 4 lower than fresh water, and in the process of freezing 

 most of the salts separate out, so that the ice formed is 

 nearly fresh, while the water yielding it is left much salter. 

 All the salts are not excluded equally, the ice retaining a 

 larger proportion of sulphates than of chlorides. 1 Sea-water 

 ice has a soft and spongy texture, full of cavities containing 

 residues of unfrozen brine, and the water produced by 

 melting it is consequently bitter and unwholesome. 



228. Circulation of Deep Fresh Lakes. When the 

 Sun shines on a deep lake in summer the upper layer of 

 water is warmed, and expanding maintains its position, 

 heat being passed on to the lower layers by the slow pro- 

 cess of conduction. There is no tendency to transmit the 

 heat by descending hot currents as in the sea. When 

 winter sets in, the surface water cools rapidly by radiation, 

 and contracting, it becomes denser and sinks allowing 

 warmer water from beneath to take its place. This process 



