22 THE OCEAN. 



CHxVPTER III. 



COMPOSITION OF SEA-WATEB.— SPECIFIC WEIGHT. — SALT MABSHES, NATURAL AND 

 ARTIFICIAL.— VARIOUS SUBSTANCES. — DIFFERENCES OF SALTNESS. — MARINE 

 SALT. 



Besides the ooze, the remains of animalculoc, and innumerable frag- 

 ments held in suspension, the sea- water is also charged with chemical 

 substances in solution, which give it a specific gravity considerably- 

 superior to that of fresh water. This varies in all seas, according to 

 the quantity of the substances dissolved, the amount of evaporation, 

 the contributions of rain and rivers, the direction of the currents and 

 counter-currents. In the polar seas the specific gravity of the waters 

 is also modified by the formation, or melting, of the ice. Every 

 variation of temperature, every local movement of the sea, causes a 

 more or less perceptible modification in the proportion of the salts 

 dissolved, and in the specific gravity of the water. Thus we can 

 only obtain an average for the various conditions of the fluid mass in 

 the different seas. 



The mean specific gravity of oceans with deep basins is nearly 

 1028 ; that is to say, sea- water weighs 2-8 per cent, more than the 

 same bulk of distilled water. In the Mediterranean, w^here the heat 

 of the sun evaporates more water than the rivers bring down to it, 

 the average specific gravity exceeds 1029 ; in the Black Sea, on the 

 other hand, where very considerable rivers of fresh water discharge 

 themselves, the specific gravity is reduced to 1016. And all the 

 intermediate degrees between these extreme specific gravities are 

 found, according to the varied physical conditions which exist, in 

 other seas. Furthermore, it seems to be established that the waters 

 of the ocean in the southern hemisphere are, on an average, lighter 

 than those of the northern hemisphere.* 



The average quantity of all the salts contained in the sea, or the 

 snltness of sea- water, was estimated by Bibra and Bischof at 35*27 

 parts in 1000 ; but much more complete observations made since by 



* Horner; J. Davy. — Bischof, Lehrhuch der chemischen Geologic. 



