420 PRINCIPLES OF CHEMISTRY 



also for the formation of other masses of workable salt, such as rock 

 salt, and of saline springs and lakes. 2 bis 



The extraction of salt from sea water is carried on in several ways. 

 In southern climes, especially on the shores of the Atlantic Ocean and the 

 Mediterranean and Black Seas, the summer heats are taken advantage 

 of. A convenient low-lying sea shore is chosen, and a whole series of 

 basins, communicating with each other, are constructed along it. The 

 Upper of these basins are filled with sea water by pumping, or else 

 ad vantage is taken of high tides. These basins are sometimes separated 

 from the sea by natural sand-banks (limans) or by artificial means, and 

 in spring the water already begins to evaporate considerably. As the 

 solution becomes more concentrated, it is run into the succeeding basins, 

 and the upper ones are supplied with a fresh quantity of sea water, or 

 else an arrangement is made enabling the salt water to flow by degrees 

 through the series of basins. It is evident that the beds of the basins 

 should be as far as possible impervious to water, and for this purpose 

 they are made of beaten clay. The crystals of salt begin to separate 

 out when the concentration attains 28 p.c. of salt (which corresponds 

 to 28 of Baume"'s hydrometer). They are raked off, and employed 

 for all those purposes to which table salt is applicable. In the majority 

 of cases only the first half of the sodium chloride which can be separated 

 from the sea water is extracted, because the second half has a bitter 

 taste from the presence of magnesium salts which separate out together 

 with the sodium salt. But in certain localities as, for instance, in the 

 estuary of the Rhone, on the island of Camarga 3 the evaporation is 

 carried on to the very end, in order to obtain those magnesium and 

 potassium salts which separate out at the end of the evaporation of sea 

 water. Various salts are separated from sea water in its evaporation. 

 From 100 parts of sea water there separates out, by natural and arti- 

 ficial evaporation, about one part of tolerably pure table salt at tho 

 very commencement of the operation ; the total amount held in solu- 

 tion being about 2J p.c. The remaining* portion separates out inter- 



2 bls Chloride of sodium haabeen found to occur in the atmosphere in the form of a 

 fine dust; in the lower strata it is present in larger quantities than in the upper, 

 so that the rain water falling on mountains contains less NaCl than that falling in 

 valleys. Miintz (1891) found that a litre of rain water collected on the summit of 

 the Pic du Midi (2,877 metres above the sea level) contained 0'34 milligram of chlorid 3 

 ,of sodium, while a litre of rain collected from the valley contained 2'5-7'6 milligrams. 



5 The extraction of the potassium salts (or so-called summer salts) was carried on at 

 'the Isle of Camarga about 1870, when I had occasion to visit that spot. At the present 

 time the deposits of Stassfurt provide a much cheaper salt, owing to the evaporation and 

 separation of the salt being carried on there by natural means and only requiring a treat- 

 ment and refining, which is also necessary in addition for the ' summer salt ' obtained 

 ifroni sea-water 



