THE OCEAN 329 



is so small that it cannot be tasted; yet the amount of salt in the 

 sea-water is more than 200 times that of calcium carbonate. The 

 reason is that calcium carbonate is being taken out of the water all 

 the time by animals which live there, to make shells, coral, etc., 

 while most of the salt carried to the sea stays in the water, and this 

 seems to have been true for millions and millions of years. 



Gases in sea-water. The sea-water contains dissolved gases 

 also. The most abundant are those of the air, namely nitrogen, 

 oxygen, and carbon dioxide. The amount of oxygen dissolved 

 in the ocean is rather more than -^^ of that in the air; the amount 

 of nitrogen about y^ that of the air, while the amount of carbonic 

 acid gas in the sea is 18 times that in the air. 



Much of the gas in the ocean was dissolved from the atmos- 

 phere. After being taken into solution at the surface, the gases 

 are distributed through the whole ocean, partly by the movements 

 of the water, and partly in other ways. 



The oxygen of the water is being used all the time by the ani- 

 mals which live in the sea, and its supply is being renewed all the 

 time by solution from the air. Animals and plants do not use the 

 nitrogen dissolved in the water, and the same nitrogen probably 

 stays there from year to year and from age to age. The carbon 

 dioxide is being used all the time by the plants of the sea, and 

 some if it is constantly escaping into the air. 



Salinity, density, and movement. Some parts of the sea are 

 more salt than others. There are several reasons for this: (1) The 

 salt is left behind when ocean water evaporates. Since evapora- 

 tion is more rapid in some places than in others, the water becomes 

 more salty where evaporation is great, as generally where the 

 climate is hot. (2) Where the amount of rainfall is great, the water 

 is freshened. (3) Where rivers enter the sea, they bring in fresh 

 water. In all the above ways the saltness of the sea-water at the 

 top of the ocean is being changed all the time. 



The more salt there is in water, the heavier it is. Every change 

 in its saltness changes its density, and unequal density causes 

 movement. When the surface water becomes more dense than 

 that beneath, it sinks, and the lighter water comes in over it from 

 all sides. When the surface water of one place becomes less dense 



