SODIUM CHLORIDE BERTHOLLET'S LAWS 409 



tion, because the water has evaporated from them while the salt has 

 remained in solution. The salt of sea water serves as the source not 

 only for its direct extraction, but also for the formation of other masses 

 of workable salt, such as rock salt and of saline springs and lakes. 



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

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

 advantage is taken of high tides. These basins are sometimes separated 

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

 in April 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 bed 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 table salt begin to 

 separate out when the concentration attains 28 p.c. of salt (which cor- 

 responds to 28 of Baume's hydrometer). It is 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 table 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. 

 In the water of oceans (Chapter I.) there is held so large an amount of 

 sodium chloride that on evaporation this salt, notwithstanding its great 

 solubility in water, soon reaches saturation and separates out. From 

 100 parts of sea water there separates out, by natural and artificial 

 evaporation, about one part of tolerably pure table salt at the very 

 commencement of the operation ; the total amount held in solution 



" 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. If I mistake 

 not, this industry is now no longer carried on, because the deposits of Stassfurt provide 

 a much cheaper salt, owing to the evaporation and separation of the salt being there 

 carried on by natural means and only requiring a treatment and refining, which is also 

 necessary in addition for the ' summer salt ' obtained from sea water. 



