8o A TEXTBOOK OF OCEANOGRAPHY 



to the présence of a minute phosphorescent médusa {Pelagia 

 noctiluca). 



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

 minute inorganic particles in suspension hâve not been 

 observed with any frequency, at any rate apart from locahties 

 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 Enghsh 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 océan. 



Other characteristics of sea-water which must be briefly 

 mentioned are — 



1 . Spécifie heat and conductivity. 



2. Surface tension. 



3. Viscosity. 



4. Compressibility. 



5. Electrical conductivity. 



6. Radioactivitv. 



Spécifie Heat and Conductivity. 



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

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

 through a degree of température (Centigrade). It is nov^^ 

 customary to take the amount required to raise the température 

 from i4"5° to 155° C. The salts dissolved in sea-water hâve 

 a lower spécifie heat than that of f resh water ; consequently the 

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

 spécifie heat of sea-water of différent degrees of salinity has 

 been carefully estimated by the French oceanographer, 

 Thoulet. 



