The Sea-water and its Physical and Chemical Properties 



47 



temperature reaches +1-86°. The whole layer then has the maximum density a^ = 

 8-18. If cooling proceeds further the temperature falls only at the surface until this 

 reaches the freezing point tc — —0-53°, where ct,^ = 8-00, while the remainder of the 

 water mass remains at 4-1-86° and a^^ = 8-18. On further loss of heat ice is formed and 

 the density is raised by the liberation of salt until it reaches 8-18 when convection starts 

 again and continues as long as ice continues to form. 



If, on the other hand, the surface layer has a salinity greater than 24-695% then the 

 vertical convection continues until the whole layer reaches the temperature of the 

 freezing point and proceeds further without interruption as long as fresh ice continues 

 to form. The difference between the two densities cr^g, and ua^ is, however, not large. 

 As shown in Fig. 24 these differences are largest at salinities of 6-7%o and very small 

 between 20%o and 35%o. 



C-25 



0-20 



^ 015 



's' 010 

 005 







10 15 20 25 30 35 40 

 5, %o 



Fig. 24. Density at the freezing temperature and maximum density of sea water as a function 



of salinity. 



Two adjacent water masses of different salinity will not, as far as their salinity is 

 concerned, be in equilibrium. In solutions of different concentrations in contact in 

 this way the material dissolved in the water will move from the region of higher con- 

 centration to that of lower concentration, that is, in the direction of the concentration 

 gradient. Known as molecular diffusion, it follows the same laws as thermal conduct- 

 ivity. If the salinity gradient is —{dSjdx) x 10~^, where S is given in %o, there will, by 

 diffusion, pass in unit time (sec) through unit area at right angles to the direction 

 of the gradient (1 cm-) an amount of salt Mg given by Mg = —K(dSjdx) x 10"^ 

 where k is the molecular diffusion coefficient with the dimensions (g cm"^ sec~^). 

 The change with time in a given distribution of salinity follows from the differential 

 equation dSjct =^ k{c'^SIcx^) where /c is a constant independent of the time and the 

 distance (Fickian-diffusion equation). The diffusion coefficient for sea-water is very 

 small (0-0189 g cm"^ sec"^ at 35%o), molecular diffusion thus proceeds extremely 

 slowly, and long periods are needed to eliminate larger differences in salinity by pure 

 molecular diffusion. In this respect diffusion is quite analogous to thermal con- 

 ductivity. 



Osmotic pressure is a phenomenon that is closely related to the properties of dilute 

 solutions described above. It is of very considerable importance for the biology of 

 living organisms in the sea. If a tank II (see Fig, 25) filled with sea-water of salinity 



