QOO ANALYSIS OF BRITISH AND FOREIGN SALT. 



Sulphate of From the table it may be seen, that the proportion of sul- 



phate of lime is greater in foreign bay salt, than in any va- 

 riety of British salt, even than in those which are prepared 

 from seayvaier with a boiling heat. The only explanation 

 of this fact that occurs to nte is, that, during the rapid 

 evaporation of seawater, u considerable part of the calcare- 

 ous sulphate is precipitated at an early stage of the process, 

 and is partly removed in clearing the boiler ; a process, which 

 can scarcely be performed during the formation of bay salt 

 in pits, the sides of which are composed of moist clay. The 

 remainder of the selenite, thus precipitated by the rapid 

 evaporation of seawater, enters into the composition of the 

 pan-scale. 

 Same kinds of I" tne course of this inquiry, I was induced to repeat the 

 salt differ. same experiments several times, on various specimens of salt 

 bearing the same designation ; and was surprised to find, 

 that the results by no means corresponded. In one instance, 

 for example, fishery salt was found in 1000 parts to contain 

 no less than l6 parts of sulphate of lime; while another 

 specimen, nominally the same, contained only llf parts of 

 selenite in the same quantity; and a third only 5|. At 

 length it occurred to me, that these differences were proba- 

 bly owing to the circumstance of the salt having been taken 

 from the boiler at different periods of the evaporation. I 

 requested, thereibre, to be furnished with specimens of salt, 

 drawn at different stages of the process, from a given por- 

 tion of brine, evaporated in the same boiler. These were 

 pubmitted to analysis; and the results arc shown in the fol- 

 lowing table. 



Common salt drawn from the boiler, -2 



hours alter the first application of 



heat 



Salt drawn 4 hours after do. 



Salt drawn 6 hours after do. ....... 



Tirst nird? salt Hence it appears, that there was a gradually increasing 

 contains most purity in the salt from sulphate of lime, as the process of 

 lime ° evaporation advanced ; the greatest part of this earthy com- 

 pound being deposited at an early stage of the process. 

 Different specimens of the same kind of salt may, there- 

 fore, differ in chemical purity as much from each other, as 



from 



