come indirectly from the sea are in the more concentrated form of 

 guano, which is rich with phosphates and nitrates. 



While the quantities of many elements in the sea are dependent 

 on the plants and animals which alternately extract them and return 

 them to the sea on their death, the sea is a storehouse for many 

 other elements and compounds that are not influenced materially 

 by biological processes, and since nature has not already concen- 

 trated them for us, it is these that man has attempted to take directly 

 from the oceans. 



The value of the sea as a source of chemicals must depend on 

 the economics of the process of extraction. What is the point in 

 mining silver from the sea when we can mine it less expensively 

 from the land? Even though there is a vast abundance of elements 

 Suspended in the sea — about 50 of the 103 known elements — they 

 generally occur in low concentrations. As a result, large quantities 

 of water have to be processed for a relatively small return. So it is 

 not unusual that substances we might expect to obtain easily from 

 the sea are more economically obtained from the land. 



Common salt, so familiar and essential to our lives, and the basic 

 substance of our chemical industry, is an interesting exception. It 

 makes up by far the greatest part of the solids in sea water — it has 

 been estimated that one cubic mile of sea water contains about 

 166 million tons of salts. For centuries men have obtained common 

 salt by simple evaporation, relying on the heat of the Sun to drive 

 off the water until only salt remains. In many parts of the world — 



These three settling tanks at a magnesium 

 factory at l-iartlepool, England, show one 

 stage in the extraction of magnesium from 

 the sea. The magnesium salt is treated with 

 lime, producing magnesia which settles in 

 great tanks. The metal magnesium can 

 then be extracted from the magnesia by a 

 chemical and electrical process. 



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