28 THE OCEAN. 



tlie substances which they contain in solution, we may conclude that 

 the proportions of these substances have continually varied during 

 the geological eras. The saltness would be modified from age to age, 

 according to the various quantities of soluble substances which the 

 rivers brought down to the ocean ; and which it returned again to the 

 land, either directly, by depositing them on the shore, or indirectly, by 

 fixing them in the tissues of its plants, corals, and other organisms 

 which people its expanses. By ingenious comparisons between the 

 conditions of the present day and those Avhich seem to have existed in 

 former times in the sedimentary beds, several geologists have attempt- 

 ed to determine if the substances in solution in sea-water have aug- 

 mented or diminished. But the conclusions at which they have 

 arrived rest, at present, on data too hypothetical for us to regard 

 them as a new conquest of science. It is only certain that in our 

 day the proportions of the substances dissolved have not ceased to 

 vary in every sea. We can judge of this by the. enormous difference 

 that there is between the saltness of the waters of the Caspian and 

 those of the Black Sea, two separate basins which formed a part of 

 the same ocean at a geological epoch still comparatively recent. 



Sea- water contains also a great quantity of the atmospheric gases, 

 the proportions of which constantly change with heat, light, the 

 motion of the waves, and barometric pressure. Salt water retains 

 dissolved air better than fresh water, and the bulk which it ab- 

 sorbs is generally greater by a third than that found in rivers. It 

 varies from a fifth to a thirtieth, and gradually increases from the 

 surface to a depth of about 325 to 380 fathoms.* Carbonic acid gas 

 is also contained in a relatively very considerable proportion in 

 sea-water, as might have been expected from the swarming myriads 

 of marine animals. Under the influence of light, plants decom- 

 pose this gas, which diminishes during the day and is increased 

 again during the night. As to the quantity of dissolved oxygen, it 

 varies inversely; during the day it increases by degrees, to be again 

 reduced in the hours of darkness. As by a sort of respiration the great 

 sea, that immensity alive with organisms, absorbs and disengages 

 alternately the gases necessary to the maintenance of life, and mea- 

 sures each breath by the daily course of the sun. 



• Bischof, Lehrbuch der chemischen und physikalischen Geologic. 



