of British and Foreign Salt . 1 
Now these proportions were, as nearly as could be expected, 
obtained in the analysis of North wich salt; from whence we 
may conclude, that the only sulphate which it contains, is 
gypsum, or the sulphate of lime. 
It must be remembered, however, that the calcareous sul- 
phate, contained in any variety of common salt, cannot be in 
a state of complete desiccation, but would lose 22 parts out of 
ico, by exposure to a red heat.* It becomes necessary, there- 
fore, either to increase, in the proportion of 5 to 4, our esti- 
mate of the sulphate of lime obtained by the foregoing rule, 
or, more simply, to assume that 100 grains of sulphate of 
barytes, indicate 73 grains of sulphate of lime, dried at 16b 0 
Fahrenheit, = 57 ignited. 
(F. ) When sulphate of lime and sulphate of magnesia, 
were both ascertained, and other sulphates also might possi- 
bly be present, as in the varieties of salt from sea water, the 
calculation became a little more complicated. In this case, 
after determining the quantity of both sulphates, (by the pro- 
cesses D. &c. ) I estimated how much sulphate of barytes they 
ought respectively to afford ; and then compared the estimated 
quantity, with that which was actually obtained. The earthy 
carbonates, for example, precipitated from 1000 grains of 
Lymington salt, which had previously been digested with al- 
cohol, were converted into 31 grains of calcined sulphates, 
consisting of 19 grains of dry sulphate of magnesia, and 12 
grains of dry sulphate of lime. Now from the magnesian 
sulphate 38 grains of sulphate of barytes should result, and 
* This I find to be the loss sustained by ioo grains of artificial selenite, dried at 
i6o°, and then ignited. The same quantity of crystallized native selenite, I learn from 
Dr. Marcet, loses 20.7 grains, by being calcined in a strong red heat. 
O 2 
