1 66 The Realm of Nature CHAP. 



one-fifth, of the whole atmospheric pressure ; thus sea-water 

 in contact with air absorbs twice as much nitrogen as oxygen. 

 Still the proportion of oxygen in the air which is breathed in 

 the water by sea creatures is twice as great as that in the 

 atmosphere. At the average pressure and 32 F., 100 parts 

 of water by volume absorb from air 1-56 parts of nitrogen 

 and 0-82 of oxygen ; at 70 F. the quantities absorbed are 

 i-oo part of nitrogen and 0-52 of oxygen, and so on in 

 inverse proportion to the temperature. The amount of ab- 

 sorbed nitrogen in sea-water does not change after it has 

 sunk below the surface ; thus by finding how much nitrogen 

 is dissolved in any part of the ocean one can calculate the 

 temperature the water originally had at the surface, and also 

 the amount of oxygen which must have been absorbed at 

 the same time. The creatures living in the sea, and dead 

 animals and plants decaying, diminish the amount of oxygen, 

 so that the full quantity which was absorbed by the sea- 

 water is hardly ever found in samples taken from a con- 

 siderable depth. If any part of the ocean were quite 

 stagnant, and never renewed from the surface, the dissolved 

 oxygen would in time become exhausted. The chemists of 

 the Challenger and of other deep-sea expeditions have 

 never found a sample of sea-water free from oxygen, and 

 this is a sure indication that all parts of the ocean are 

 moving, however slowly. Very little carbonic acid is ab- 

 sorbed from the air, on account of the small proportion of 

 that gas in the atmosphere ; but the oxygen, when used 

 up as described above, is changed in great part into car- 

 bonic acid, which remains in the sea- water chemically 

 combined with the carbonates. 



226. Pressure and Sea -water. Professor Tait has 

 found by experiment that sea -water is very slightly com- 

 pressed by its own weight. Under the surface the pressure 

 increases about I ton per square inch for every mile of 

 depth. At the bottom of the deepest part of the ocean the 

 vast pile of water exerts a pressure more than 500 times 

 that of the atmosphere on the surface, or about 4 tons to 

 the square inch. At this depth 11,000 cubic feet of sea- 

 level air would be squeezed into 22 cubic feet ; but 1 1,000 



