SALINITY, GASES 63 



absorbed by the sea. The tension of car- 

 bonic acid is usually greater over the land 

 than over the sea, and the tension may be 

 less in the air over the sea than in the 

 sea itself, in which case carbonic acid would 

 pass from the sea to the air. Thus according 

 to circumstances there is an interchange of 

 carbonic acid between the air and the sea, 

 so that the sea has a regulating influence 

 upon the amount of carbonic acid in the 

 atmosphere. Beneath the surface the amount 

 of free carbonic acid in the water has been 

 shown in some places to increase with increase 

 of depth. 



Gases in the Black Sea, — The chemical 

 conditions in the Black Sea, as compared with 

 those in the open ocean, are peculiar, and 

 may be briefly referred to. It has been cal- 

 culated that the quantity of water intro- 

 duced annually into the Black Sea from the 

 Mediterranean by the undercurrent through 

 the Bosphorus is only 1 -2500th part of the 

 volume of the Black Sea, so that, while the 

 surface waters are being constantly renewed, 

 the water of the deeper layers can be renewed 

 only once in 2500 years or longer. Conse- 

 quently the upper layers down to about 125 

 fathoms are normal in character, and contain 

 a quantity of dissolved air sufficient to sustain 

 life, but the oxygen of the deeper layers can 



