but, having already given an outline of this distinction (vol. i. pp. 10, 11), we must refer the reader to the principal acid salts, under their alphabetical heads.
II. Alkaline Salts. See Alkalies, Barilla, Pearl-ash, Pot-ash, and Soda.
III. Neutral Salts, are such as are obtained from a combination of acids and alkalies.
The principal and most useful of this class is, the Common or Sea-salt. It differs from all other neutral substances of this nature, in its taste being purely saline, and occasioning thirst, after it has been swallowed. The primitive figure of its crystals is that of a cube; but, on evaporating a solution of salt, the smaller cubic particles assume the secondary form of hollow squares. When the crystals are perfectly pure, they are not affected by most air; and, in this state, 100 parts contain, according to Bergman, 52 of muriatic acid, or spirit of salt; 42 of soda, or mineral alkali; and 6 of water of crystallization: though we believe, with Gren, that the proportion of the fluid part is greater, and that of the two first ingredients somewhat smaller. Nature furnishes us with this neutral salt, either in a solid state, in mines; or dissolved in the sea; or in saline springs.
If it be obtained from the bowels of the earth, it is called Rock-salt; and is generally very hard, and transparent, though it is sometimes opaque, white, and sometimes of red, green, blue, or other shades. The purest of this kind is colourless; the other species are purified by solution in water, and by re-crystallization, before they can be employed for culinary purposes. The principal mines of Rock-salt are in the vicinity of Cracow, in Poland, and at Northwich, in the county of Chester.
The salt, however, which is thus easily procured, and in very large masses, by no means affords a sufficient supply: hence numerous persons are employed in extracting it from sea-water, or from saline springs. The former yields only from one-50th to one-30th part of its weight, but the latter produce the greatest quantities; and we are informed by Dr. Browning (Art of Making Common Salt, &c. 8vo.), that the celebrated saline springs of Northwich, Nantwich, and Droitwich, in Cheshire (whence Great Britain is principally supplied), contain more than one-sixth part of good salt.
Common salt is obtained from these natural solutions by two different methods: First, the saline fluid is speedily evaporated, till the salt begin to concrete, and settle in the form of grains at the bottom of the pans; after which it is put into proper vessels for draining the brine; and, when the process is completed, it is called bay-salt. By the second method, the evaporation is slow and gradual; so that it is continued only till a saline crust is formed on the surface of the liquor; which soon shoots into crystalline cubes.
In the first volume of the Transactions of the Royal Society of Edinburgh, we meet with a memoir, by the Earl of Dundonald, containing an account of a new expedient of purifying sea-salt. He observes that the common salt is mixed with various substances, which in a great measure render it unfit for the important purpose of preserv-
