258 



DEPTHS OF THE OCEAN 



the oxygen returns, producing thus an annual change in the 

 gaseous conditions of the deeper parts of the oyster-" polls.'* 

 In autumn the state of things may become critical for the oysters, 

 which are suspended in baskets at a depth of i-J-2 metres ; it 

 happens occasionally that the animals all die at this time by 

 suffocation through want of oxygen or by sulphur poisoning. 



The water may, on the other hand, become over-saturated 

 with oxygen, as occurs sometimes in the Kattegat, or in spring 

 in some parts of the oyster-" polls," where plant life is particularly 

 luxuriant. 



Carbonic acid. Carboiiic add occurs combined as carbonates and bicar- 



bonates, and only in very small quantities as a free gas. The 

 quantity varies considerably, among other things because of the 

 activity of plants and animals, as above mentioned. Usually 

 there is about 50 c.c. of carbonic acid in i litre of sea-water, 

 but of this only a few tenths of a cubic centimetre is free gas in 

 solution. 



Carbonic acid has probably been present from the formation 

 of the primitive ocean, together with the salts of the sea, but 

 the quantity varies from place to place and from time to time, 

 depending on the number of plants and animals, on the com- 

 position of the bottom, and more especially on atmospheric 

 conditions. At times considerable quantities of carbonic acid 

 gain access to the water through submarine volcanic activity, 

 but this has probably less influence on the variations than the 

 atmospheric conditions. August Krogh has made some very 

 valuable investigations on this point, and has arrived at the 

 conclusion that the sea is a sort of regulator for the amount of 

 carbonic acid in the atmosphere. When there is much carbonic 

 acid in the air, much will be absorbed by the sea ; this is the 

 case near land, and especially where there is a dense population 

 and extensive industrial activity, or near active volcanoes. The 

 tension of carbonic acid is everywhere small, but it is on the 

 average greater over the land than over the sea. Now, if the 

 tension in the air over a certain portion of the sea is smaller 

 than it is in the sea, the latter will give off carbonic acid to the 

 air. The sea thus has a regulating influence on the variations 

 in the carbonic acid of the atmosphere. Many important 

 questions arise with regard to these relations, but we cannot 

 enter into further detail here ; investigations on the subject 

 are few. 



Nitrogen. NUrogeu is SO inert a gas that it is of little importance in 



oceanography. It is absorbed from the atmosphere in con- 



Krogh's 

 investigations. 



