101 
of British and Foreign Salt. 
the results by no means corresponded. In one instance, for 
example, fishery salt was found in 1000 parts to contain no 
less then 1 6 parts of sulphate of lime ; while another specimen, 
nominally the same, contained only 1 1 ^ parts of selenite in 
the same quantity ; and a third only 52. At length it occurred 
to me, that these differences, were probably owing to the cir- 
cumstance of the salt having been taken from the boiler at 
different periods of the evaporation. I requested, therefore, to 
be furnished with specimens of salt, drawn at different stages 
of the process, from a given portion of brine, evaporated in 
the same boiler. These were submitted to analysis ; and the 
results are shewn in the following table. 
Hence it appears, that there was a gradually increasing 
purity in the salt from sulphate of lime, as the process of 
evaporation advanced ; the greatest part of this earthy com- 
pound being deposited at an early stage of the process. Dif- 
ferent specimens of the same kind of salt may, therefore 
differ in chemical purity as much from each other, as from 
other varieties. But when the impurities, contained in a solu- 
tion of muriate of soda, are of a different species, from those 
of Cheshire brine, and consist chiefly of the earthy muriates, 
the order will be reversed, and the purest salt, as I have already 
suggested, will be that which is first deposited; the contami- 
nation with the muriate of lime or of magnesia, continuing to 
increase, as the process advances to a conclusion,* 
• I cannot on any other principle explain the considerable differences, as to the 
Common salt drawn from the boiler, 2 hours 
-s Sulphate of 
c . lime. 
after the first application of heat. - 
Salt drawn 4 hours after do. - 
Salt drawn 6 hours after do. - - - - 
