98 Dr. Henry’s Analysis of several Varieties 
generally speaking, than salt which is prepared by the rapid 
evaporation of sea water ; but that it is contaminated with about 
three times the amount of impurities, discoverable in an equal 
weight of the Cheshire large-grained salt, and with more than 
twice that of those, that are found in the stoved and common 
salt of the same district. 
II. The insoluble matter in the foreign salt, after the action 
of boiling water, appears to be chiefly argillaceous earth 
coloured by oxide of iron, and is probably derived in part from 
the pits in which the sea water is submitted to evaporation. 
We may, perhaps, assign the same origin to the very minute 
portion of muriate of lime, which is not found in the salt pre- 
pared by evaporating sea water in metallic vessels, nor even 
in the mother liquor, or uncrystallizable residue. In sea salt 
prepared by rapid evaporation, the insoluble portion is a mix- 
ture of carbonate of lime with carbonate of magnesia, and a 
fine silicious sand ; and in the salt prepared from Cheshire 
brine, it is almost entirely carbonate of lime. The insoluble 
part of the less pure pieces of rock salt is chiefly a marly 
earth, with some sulphate of lime. The quantity of this im- 
purity, as it is stated in the table, is considerably below the 
average, which in my experiments has varied from 10 to 45 
parts in 1000. Some estimate of its general proportion, when 
ascertained on a larger scale, may be formed from the fact, that 
government, in levying the duties, allows 65 lb. to the bushel, 
of rock salt, instead of 56I0. the usual weight of a bushel of 
salt. 
III. The earthy muriates, and especially that with base of 
magnesia, abound most in salt which is prepared by the rapid 
evaporation of sea water. Now since common salt, in all its 
