8o A TEXTBOOK OF OCEANOGRAPHY 



to the presence of a minute phosphorescent medusa (Pelagia 

 noctiluca). 



Variations in the colour of the sea due to the presence of 

 minute inorganic particles in suspension have not been 

 observed with any frequency, at any rate apart from localities 

 in direct neighbourhood of the coast. The Gulf of California 

 during the rainy season is coloured by the waters of the Rio 

 Colorado. 



In the English Channel, after several days of stormy 

 weather, the water takes on a milky-green appearance, due to 

 minute chalky particles in suspension. 



Off the mouths of tropical rivers the sea is coloured for 

 miles. The Congo and Amazon furnish good examples, the 

 muddy yellowish water being in marked contrast to the deep 

 blue of the open ocean. 



Other characteristics of sea-water which must be briefly 

 mentioned are 



1 . Specific heat and conductivity. 



2. Surface tension. 



3. Viscosity. 



4. Compressibility. 



5. Electrical conductivity. 



6. Radioactivitv. 



Specific Heat and Conductivity. 



The specific heat of a substance is the amount of heat, 

 expressed in calories, necessary to raise I gram of the substance 

 through a degree of temperature (Centigrade). It is now- 

 customary to take the amount required to raise the temperature 

 from i4'5 to 15*5 C. The salts dissolved in sea-water have 

 a lower specific heat than that of fresh water ; consequently the 

 specific heat of sea- water is less than that of fresh water. The 

 specific heat of sea-water of different degrees of salinity has 

 been carefully estimated by the French oceanographer, 

 Thoulet. 



