POTASSIUM AND SODIUM. 217 



torn of clay are made near the sea, all communicating with 

 each other. The water is let into the one adjoining the sea 

 at high tide, and when they are all filled it is shut off. The 

 sea-water affords a bushel of salt to every 300 or 350 gallons, 

 while the brine from the best springs gives a bushel to every 

 40 gallons. 



300. Amount of Salt in the Sea. About one thirty-sixth 

 part of sea-water is common salt. The proportion in the 

 best of our salt springs is one seventh ; in the water of the 

 Great Salt Lake it is over one fifth ; and in the Dead Sea it 

 is even more than that. The whole amount of salt in all 

 the seas and oceans of the earth is estimated to be at least 

 five times the mass of the Alps. It is enough to cover an 

 area of seven millions of square miles with a layer a mile in 

 thickness. 



301. Sodium Carbonate, Na 2 CO 3 . This salt is contained 

 in the ashes of sea-plants, as the carbonate of potassium is in 

 those of land-plants ; and originally it was obtained almost 

 wholly from that source by lixiviation that is, by making 

 a lye. But it is now obtained entirely, because more easi- 

 ly, from common salt by certain chemical reactions, which 

 are somewhat complicated ; briefly, however, the process is 

 as follows: (1) Sodium chloride is heated with sulphuric 

 acid, forming sodium sulphate, or, as it is technically called, 

 "salt cake;" (2) this is mixed with coal and limestone, 

 heated in a furnace of peculiar form, and thereby converted 

 into very crude sodium carbonate ("black ash"); (3) this 

 is then purified by solution in water and crystallization 

 (" soda ash "). During the first step, the manufacture of 

 salt cake, immense quantities of hydrochloric acid gas are 

 given off, which are condensed in water. During the sec- 

 ond step abundance of calcium sulphide is formed as a waste 

 product. 



The importance of this manufacture will be faintly ap- 



K 



