764 



HISTORY OF THE SEA. 



fresh. This conies from the various matters which it holds in 

 solution. This difference varies with different seas ; with the 

 quantity of matters held in solution ; with the amount of eva- 

 poration; the size and number of rivers flowing into the 

 various seas ; the ice melting into them ; the currents, and 

 various other causes. The average quantity of salts held in 

 e3lution in sea water is estimated at 34.40 parts in 1,000, and 

 this average is the same in all sea3. The quantityof common 

 salt held in solution is always a little more than three-quarters 

 (75.786) of the total mineral matter held in solution. Thj 

 salt of the sea averages, if the water is evaporated, about two 

 inches to every fathom ; so that, were the ocean dried up, a 

 layer of salt about two hundred and thirty feet thick would 

 remain on the bottom, or the whole salt of the sea would 

 measure more than a thousand millions of cubic miles. This 

 vast quantity of salt in the sea explains how the enormor $ 

 beds of rock salt were formed, when the lands now expose 1 

 were covered by the waters. 



. Beside the oxygen and hydrogen which constitute its water % 

 the sea contains chlorine, nitrogen, carbon, bromine, iodine, 

 fluorine, sulphur, phosphorus, silicon, sodium, potassium, 

 boron, aluminium, magnesium, calcium, strontium, barium 

 From the various sea-weeds most of these substances can be 

 obtained. Copper, lead, zinc, cobalt, nickel and manganese 

 l ave also been found in their ashes. Iron has also been ob- 

 tained from sea water, and a trace of silver also is often 

 deposited by the magnetic current established between the 

 sheeting of ships and the salt water. Though only a trace is 

 :hus found, yet it has been estimated that the whole waters of 

 ihe ocean contain in solution two million tons of silver. In 

 •.he boilers of ocean steamships, which use sea water, arsenic 

 has also been found 



Sea water also retains dissolved air better than fresh water, 

 md the bulk of this in ocean water is generally greater by a 



