PHYSICAL PROPERTIES OF SEA WATER 15 



observations. The freezing point of sea water as a function of salinity 

 and chlorinity is shown in fig. 2. 



The freezing point of sea water is the "initial'^ freezing point — 

 namely, the temperature at which an infinitely small amount of ice is in 

 equilibrium with the solution. If ice forms in a closed system, the 

 concentration of the dissolved solids increases, and hence the formation 

 of additional ice can take place only at lower temperatures. 



The vapor pressure of sea water of any salinity S (in per mille) 

 referred to distilled water at the same temperature can be computed 

 from the following equation: 



ey, = ea{l - 0.00053 S), (II, 8) 



where ey, is the vapor pressure of the sample and ea is the vapor pressure 

 of distilled water at the same temperature. The vapor pressure of sea 

 water mthin the normal range of concentration is about 98 per cent of 

 that of pure water at the same temperature, and in most cases it is not 

 necessary to consider the effect of differences in salinity, since variation 

 in the temperature of the surface waters has a much greater effect upon 

 the vapor pressure. 



Other Properties of Sea Water 



Maximum Density. Pure water has its maximum density at a 

 temperature of very nearly 4°, but for sea water the temperature of 

 maximum density decreases with increasing salinity, and at salinities 

 greater than 24.70 °/oo it is below the freezing point. At a salinity of 

 24.70 °/oo, the temperature of maximum density coincides with the 

 freezing point: d^j = — 1.332°. Consequently, the density of sea water of 

 salinity greater than 24.70 °/oo increases continuously when such water 

 is cooled to its freezing point. The temperature of maximum density is 

 shown in fig. 1 as a function of salinity and chlorinity. 



Compressibility. Ekman has derived an empirical equation for the 

 mean compressibility of sea water between pressures and p bars, as 

 defined by a^.t?,? = q;8.,?.o(1 — hp). The numerical value decreases with 

 increasing temperature and increasing pressure and varies approximately 

 between the limits 4.6 X 10-^ and 4.0 X 10-^ 



The true compressibility of sea water is described by means of a 

 coefficient that represents the proportional change in specific volume if 

 the hydrostatic pressure is increased by one unit of pressure: K = ( — 1/a) 

 (da/ dp). It can be computed if the mean compressibility is known. 



Viscosity. When the velocity of moving water varies in space, fric- 

 tional stresses are present. The frictional stress, r, which is exerted on 

 a surface of area 1 cm^ is proportional to the change of velocity per 

 centimeter along a line normal to that surface (r = /x dv/dn), the coeffi- 

 cient of proportionality (fi) being called the dynamic viscosity. This 



