CHAPTER II 



Physical Properties of Sea Water 



The physical properties of pure water depend upon two variables, 

 temperature and pressure (Dorsey, 1940; International Critical Tables), 

 whereas for sea water a third variable has to be considered — namely, the 

 salinity of the water, which will be defined and discussed below. Some 

 of the properties, such as compressibility, thermal expansion, and refrac- 

 tive index, are only slightly altered by the presence of dissolved salts, but 

 other properties that are constant in the case of pure water, such as 

 freezing point and temperature of maximum density, are dependent, in 

 the case of sea water, on salinity. Furthermore, the dissolved salts add 

 new properties, such as osmotic pressure and electrical conductivity. 



When dealing with sea water as it occurs in nature, it must also be 

 taken into account that important characteristics are greatly modified by 

 the presence of minute suspended particles or by the state of motion. 

 Thus, the absorption of radiation in the sea is different from the absorp- 

 tion of radiation in pure water or in ''pure" sea water because the waters 

 encountered in nature always contain suspended matter that causes 

 increased scattering of the radiation and, consequently, an increased 

 absorption in layers of similar thickness. The processes of heat conduc- 

 tion, chemical diffusion, and transfer of momentum from one layer to 

 another are completely altered in moving water, for which reason the 

 coefficients that have been determined under laboratory conditions must 

 be replaced by corresponding eddy coefficients. Some of the physical 

 properties of sea water, therefore, depend only upon the three variables, 

 temperature, salinity, and pressure, which can all be determined with 

 great accuracy, whereas others depend upon such variables as amount of 

 suspended matter or character of motion, which at present cannot be 

 accurately determined. 



Salinity, Temperature, and Pressure 

 Salinity. Salinity is commonly defined as the ratio between the 

 weight of the dissolved material and the weight of the sample of sea 

 water, the ratio being stated in parts per thousand or per mille. In 

 oceanography another definition is used because, owing to the complexity 

 of sea water, it is impossible by direct chemical analysis to determine the 



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