Page:Proceedings of the Royal Society of London Vol 1.djvu/377

Rh of 100 or 110 degrees. This process is so slow as to last seven or eight days, instead of eight or ten hours, which is the shortest time employed in the ﬁrst mode of evaporating the same quantity of brine, and accordingly the salt forms in large cubical crystals, prOper to the muriate of soda.

Of these varieties, the stoved salt is that which is usually em- ployed for domestic purposes. The common salt is consumed prin- cipally in the salting of provisions that are not intended for sea- voyages. But for this last purpose, the large-grained or ﬁshery salt is peculiarly ﬁtted.

With respect to the preparation of salt from sea-water, the author takes notice of the process employed on the coasts of Scotland, both east and west, where, from the cheapness of fuel, artiﬁcial heat alone is used; and of the salt-works at Lyming'ton in Hampshire, where advantage is taken of a milder climate for removing ﬁve sixths of the water by spontaneous evaporation, previous to its admission into the boilers.

Since the evaporation in each of these Works is conducted rapidly during the formation of the salt, it generally resembles the stoved salt of Cheshire; but in consequence of the heat being slackened during Sundays, a larger kind is then formed, and it is termed Sunday salt. At Lymingtun, there are also formed, by drippings from the salt during its drainage, large stalactical masses, termed salt-cats, weighing sixty or eighty pounds each; but these do not ex- ceed 11ﬁrth part of the salt prepared at Lymington.

Of the several salts above described, the large-grained ﬁshery salt is that which most resembles the foreign bay-salt in appearance; and in fact (mys the author,) a large proportion of what is sold in London as bay-salt, and esteemed as of foreign manufacture, is this Cheshire salt.

Dr. Henry next gives a table of the results of his chemical expe- riments on eleven varieties of salt; and in this it appears, that the Lymz'ngton cat is that which contains the smallest quantity of impu- rity, and the Lymington or Scotch common salt the largest quantity; the ﬁrst amounting to only 12 parts in 1000, and that of the last to 64.

But it appears that all the kinds of Cheshire salt are nearly equal to the Lymz'ngton cat in purity, and perhaps superior in respect to the quality of the extraneous matter, which varies in different kinds from 13% to 17:;- iu the thousand. The foreign bay-salts, on the contrary, have as much as from 35 to 40 parts of impurity. Of these about 10 parts are insoluble, and consist chieﬂy of argillaceous earth, coloured by oxide of iron. The native rock salt of Cheshire also contains as much or more of insoluble impurity, which is chieﬂy a marly earth, with some sulphate of lime. The earthy muriates of lime and magnesia abound most in salt which is prepared by rapid evaporation of sea water.

Since common salt contains extremely little water of crystalliza-