This section is from the book "An Experimental History Of The Materia Medica", by William Lewis.
Common or Culinary Salt; called, from its most obvious source, sea salt; though found also, in immense quantities, in the bowels of the earth. It is a perfectly neutral salt, com-posed of a peculiar acid denominated from it the marine acid, and of the mineral alkali natron. It disolves in less than thrice its weight of boiling water, and does not, like the other neutral salts, concrete again in the cold, so long as the evaporation of the fluid is prevented; cold water dissolving nearly as much of this salt as boiling water. By gentle continued evaporation it shoots into cubical crystals, seve-ral of which unite together into the form of hollow truncated pyramids. The crystals, ex-posed to the fire, burst and crackle,† soon after melt, and appear thin and limpid as water: if the salt be melted along with other fusible salts or with vitreous matters, it does not perfectly unite with them, but flows in part distinct upon the surface. After suffering a considerable heat, it liquefies in the air.
1. Sal Gemmae. Sal gem, rock salt, fossil common salt. This is met with in several parts of the world, but in greatest plenty in certain deep mines, of prodigious extent, near Cracow in Poland: some is likewise found in England, particularly in Cheshire. It is for the most part very hard; sometimes pure, transparent and colourless; more commonly mixed with earthy or stony matters, of an opake whiteness, or of a red, green, blue, or other colours. These last sorts are purified, for the common uses of salt, by solution and crystallization.
† Sal decre-pitatum.
2. Sal muriaticus Pharm. Lond. Sal ma-rinus hifpanus Pharm. Edinb. The salt extracted from sea water and saline springs. Sea waters yield from one fiftieth to about one thirtieth their weight of pure salt: from . several springs much larger quantities are obtained: those in our own country at Nantwich, Northwich, and Droitwich, afford from one sixth to one third their weight. Sea water contains, besides the common salt, a portion of purging bitter salt, and of another saline substance which remains dissolved after the crystallization of the latter, of a very pungent taste, scarce reducible into a crystalline form, composed of marine acid and calcareous earth: from both these salts the spring waters are usually free. There are two general methods of extracting the common salt from these natural solutions of it: the one, a hasty evaporation, continued till the salt concretes and falls in grains to the bottom of the pan, from whence it is afterwards raked out, and set to drain from the bittern: the other, a flow and gradual evaporation, effected in the warmer climates by the fun's heat, by which the salt is formed, not into small grains, but into large crystals, called bay-salt. The salts obtained by these two processes differ in some respects from one another: that got by hasty evaporation, especially if a boiling heat, or one approaching to it, be continued during the time of the salts concreting, is apt to liquefy in a moist air; an inconvenience which the crys-tallized sort is not subject to: the crystals are found likewise to be stronger than the other, and to answer better for preserving provisions. Both sorts prove impure and brown-coloured if the solutions are evaporated directly, but of perfect whiteness if previously clarified by boil-ins with a little ox blood, or other like sub-stances, which concreting by the heat, invifcatc the unctuous matter, and carry it to the surface in form of scum. Both sorts generally retain a portion of the bitter salt; whose basis being an earth, solutions of them deposite this earth on the addition of any alkali.
Common salt differs from other saline sub-stances in occasioning drought, and tending, not to cool, but rather to heat the body. It prevents putrefaction less than most others, and in small quantities, such as are taken with food, promotes it: by this quality it probably promotes also the resolution of the aliment in the stomach, at the same time that it proves a mild stimulus to that viscus itfelf. Salted animal foods are generally, perhaps justly, accounted one of the principal causes of the scurvy at sea; not that the salt is of itself prejudicial, but on account of its being incapable of preserving the animal subjects, for a length of time, in a perfectly uncorrupted state. Pure sea salt, and sea water, are rather falubrious than hurtful, both in the true scurvy, and in impurities of the blood and humours in general. In considerable doses, they act as purgatives: Hoffman ob-serves, that an ounce of the salt, dissolved in a proper quantity of water, occasions commonly six stools or more, without uneasinsfs; that this salt checks the operation of emetics, and carries them off by stool; that in glysters it is more effectual, though used only in the quantity of a dram, than any of the purgatives; and that where other glysters fail of opening the belly, a solution of common salit takes place.
* A remarkable instance of the efficacy of common salt given in very large doses in a worm case, is related in the Medical Transactions of the London College, vol. i. A person reduced to the utmost extremity with pain in the stomach, obstinate constipation, and contracted limbs, was advised, after many remedies had been used in vain, to drink salt and water. He drank within an hour two pounds of salt mixed in two quarts of water. It occasioned violent vomiting, which brought up a quantity of small worms, and its operation ended with purging and a profuse sweat. Great rawness and fore-ness of the gullet and stomach, with unquenchable third, and dysury, remained. These symptoms went off by free dilution; and he ventured the third day after to repeat the dose of salt, which had effects similar to the former. A perfect cure was the consequence of this singular practice.
The common sorts of sea salt, contrary to other neutral ones, part with a little of their acid in the boiling down of solutions of them to dryness (a). To this cause are attributed the weakness of the salt prepared by that process, and its disposition to deliquiate in the air; both which imperfections are said to be corrected by a small addition of fresh acid when the salt begins to concrete. Hence alio distilled sea water is manifestly impregnated with acid, so as to be unfit for drinking or for the common purposes of life; unless a little chalk, vegetable ashes, or other like substances, be added in the distillation, to absorb and keep down the acid extricated by the heat * (a): by this means the distilled fluid proves perfectly sweet.