48 



77?^ Sea-water and its Physical and Chemical Properties 



S%o is separated from a tank I containing distilled water by a semi-permeable mem- 

 brane M which is permeable only for water and not for the substances in solution, 

 water will pass from tank I through the membrane M into tank II which contains the 

 the salt solution, and as a result the pressure in the tank II will rise. The sea-water could 

 be said to draw the pure water through the membrane. This process will continue until 

 the excess pressure in TI exceeds that in I by a fixed value P. This excess pressure at 

 which the system is in equilibrium is termed the osmotic pressure. According to physi- 

 cal chemistry it has been shown (see Nernst, Theoretische Chemie, 4th ed. 1903, 



Fig. 25. For explanation of the osmotic pressure. 



p. 157) that there is a relationship between the osmotic pressure and the depression 

 of freezing point which for sea- water at 0° takes the form P = —M-AAta- Stenius 

 (1904; see also Thompson, 1932) found the proportionality value 12-08 atm 

 for the constant in this equation. For other temperatures Pq must be multiplied by 

 (1 + 0-003670- Table 12 gives values for the osmotic pressure at 0° according to 

 Stenius. 



The size of the osmotic pressure gives an idea of its biological importance. Or- 

 ganisms that live in the water are usually covered by a skin that is partly permeable 

 to water. They live in osmotic equilibrium with their environment. If one of these 

 organisms is placed in water of lesser salinity, water will pass in through its skin into 

 its body ; if the salinity is higher, water will be removed. Both processes, if they occur to 

 any extent, are unfavourable to the life of the organism since thecapacity of adaptation 

 is fixed within narrow limits. 



6. Other Physical Properties of Sea-water 



Other properties of sea-water that are also of importance in oceanography and 

 should be briefly mentioned are the heat capacity and the thermal conductivity, the 

 surface tension and the internal viscosity. 



{a) The heat capacity of the specific heat of a body is the number of calories required 

 to heat 1 g of the material through 1 °C. The specific heat of pure water is dependent 

 on the temperature and shows a minimum of 0-947 at 34°C. It rises more rapidly to- 

 wards lower than towards higher temperatures and at 18°C it is 0-999. 



A series of experimental determinations of the effect of the salinity was made by 

 Thoulet and Chevallier (1899) and their results have been utilized by Kriimmel to 

 prepare the figures shown in Table 14. The experimental value for the specific heat 

 of sea-water c^ is less than would be expected from the amount of salt in solution. 



