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COMPOSITION OF SEA WATER. 



Specimens of sea water from the open parts of 

 the Atlantic are very uniform in their composition, 

 whether taken in the latitude of Gibraltar or of 

 the Hebrides ; but such is not the case with the 

 waters of internal seas, nor again between the 

 waters of the coast and those of the offing. The 

 subiittoral sea-zone is that of the maximum of 

 marine life, and it is along the coast-line that all 

 those changes are to be observed, from super-saline, 

 normal, brackish to fresh, which are severally de- 

 pendent on the amount of surface evaporation, the 

 influx of rivers, and on the equalizing action of 

 winds and tides. 



The density of water taken at the surface is less 

 than of that taken at depths • the degree of salt- 

 ness, also, increases in the same direction. The 

 water from the surface contains less air than does 

 that from depths, and the difference may equal 

 one hundredth of the volume of water. Again, 

 analyses of the waters of the Black Sea, the Sea of 

 Azof, and the Caspian have shown that, though the 

 salts which they contain are the same, the propor- 

 tions are different. These varying conditions have 

 a marked influence in local assemblages of marine 

 animals. 



From a series of observations taken within depths 

 of eight fathoms, Admiral Smythe puts the tempe- 

 rature of the Mediterranean surface waters at rather 

 more than three degrees higher than those of the 

 Atlantic for the same latitudes. This condition 



