20 HISTORY of the SOCIETY*. 



To purify common fait, by diifolving it in water, decom- 

 pounding the bitter falts, and precipitating their earthy bans, 

 by adding a fixed alkali, whether foffil or vegetable, is a tedi- 

 ous procefs, and by far too expenfive to be employed for ceco- 

 nomical or mercantile pnrpofes. It is even imperfect ; as it is 

 almoft impoffible, after that procefs, to feparate from the fea- 

 falt the Glauber fait, or vitriolated tartar, or fait of Sylvius, 

 which are produced according as trie foflil or vegetable alkali 

 is ufed. 



Lord DUNDONALD obferved, That hot water faturated with 

 fea-falt, will flill diflblve a great part of the bitter earthy falts. 

 His method, therefore, of purifying the common fait from thefe 

 bitter falts is, To take a conical vefTel, having a hole in the fmall 

 end of it, which is to be undermoft ; to place it, filled with 

 common fait, in a moderate* heat ; to take one twentieth part of 

 the fait contained in it, and putting it in an iron pan, to dif- 

 folve it in its proper proportion of water, fo that the water 

 {hall be completely faturated with the fait ; and then to pour 

 this folution boiling hot on the fait in the conical veflel, which 

 is to be purified. The boiling water being already faturated 

 with fea-falt, will diffolve no more of it, but will diflblve much 

 of the bitter earthy falts ; and this folution will gradually drop 

 out at the hole in the bottom of the cone. When it ceafes to 

 drop,'the fame procefs is to be repeated by means of frefh por- 

 tions of the fame parcel of fait, already partly purified, till it 

 be brought to the required degree of purity. Lord DUNDO- 

 NALD reckons, that three fuch wafhings make the common fait 

 of this country purer than any foreign fait ; that each wafhing 

 makes it 4.4 times purer than before ; fo that (difregarding 

 fractions) after the fecond wafhing it will be 20 times, after the 

 third 91 times, after the fourth 410 times, and after the fifth 

 1845 times purer than at firft. 



THE fuperiority of fait thus purified to common fait, is 

 equally obvious to the tafte, and by its effect in preferving fifh, 



flefh 



