92 Eafy Methods of rendering eemmen Salt very pure. 



M. Gren was convinced by experiment, that the fulphate of niagnefia in fait waters form* 

 at a fufficient degree of cold all the fulphate of foda which can then be afforded, and of 

 which they did not before contain the fmalleft particle. He endeavoured, but conftantly 

 without fuccefs, to reproduce the muriate of foda, and the fulphate of magnefia, by an ele- 

 vation of temperature. So that fait waters which contain fulphate of magnefia change 

 their nature ; and their compofition is no longer the fame when they have been fubje6ted 

 to feme degrees of cold below the freezing point 5 and a fubfequent increafe of heat will 

 not reftore their former ftate. 



The quantity of muriate of magnefia in fait waters which contain fulphate of magnefia, 

 is increafed by a temperature beneath zero. It is therefore produ£live of a real lofs to 

 fuffer the fait water to remain m the bafons during the winter, becaufe they will after- 

 wards afford a muriate of foda of a bad quality, and the fulphate of foda which is ob- 

 tained will not compenfate for this difadvantage. It appears likevvife that the hope of con- 

 centrating fea water by frofl is chimerical. 



It was an important queflion to be rcfolved in the art of making fait, what might be the 

 means of feparating the deliquefcent falts even before the evaporation. la this refpedl 

 M. Gren has fucceeded. His procefles are founded on the decompofition, well known to 

 chemifts, of muriate of lime by fulphate of foda, «nd muriate of magnefia by cauflic lime. 



If the water contain only muriate of lime with the muriate of foda, the fulphate of foda 

 is fufficient : if there remain muriate of magnefia and fulphate of foda, lime will fuffice. 

 In this cafe muriate of lime is formed, which afterwards decompofes the fulphate of fod* 

 by double affinity. 



If the water contain the muriate and fulphate of magnefia without fulphate of foda, It 

 will be neceffary to employ lime and fulphate of foda at the fame time. 



The fame fubftances muft alfo be employed, if the water contain the muriates of lime 

 and magnefia together; which then entirely exclude the fulphates of foda and of magnefia. 



Laftly, if the fulphate and the muriate of magnefia be prefent with the fulphate of foda, 

 lime only need be employed, unlefs the quantity of the fulphate of foda be found infuf- 

 ficient. In this cafe a portion of the fait muft be added. 



Thefe procelTes introduce no foreign fait into the waters, becaufe they are already fatu- 

 rated with fulphate of lime, which docs not render the muriate of foda more impure, be- 

 caufe it feparates in the ftate of fchlot. They have the advantage to clear it of the ful- 

 phates of foda and magnefia, which alter the purity of the muriate of foda, and render it 

 bitter. Befides which, in the practice, no mother water remains. The whole of the fluid 

 may be evaporated to the laft drop, and all the fait extra£led, which will conftantly be 

 pure marine fait. The diredtors of fait works will eafily perceive how much profit will 

 thus be obtained with regard to time and fuel confumed in the evaporation and drying of 

 the falts. But this is merely a fecondary profit, greatly inferior to the principal advantage 

 of obtaining a pure fait not deliquefcent, but capable of refifting the impreffion of the air, 

 and of being preferved and tranfported without lofs. 



The ftrongeft objedion againft the ufe of thefe procefTes may arife from the neceflity of 

 purchafing fulphate of foda ; and this obje(flion would be very well founded, if the fait wa- 

 ters were not for the moft part of fuch a nature as to afford it themfelves at a fmall ex- 

 pcncc; that is to fay, by their maceration at a temperature below zero. It would be fuffi- 

 cient 



