Native chloride of sodium, whether obtained from the waters of the ocean, from saline lakes, from salt springs, or mineral masses, is never perfectly pure. The foreign matters present in it vary with its different origins and qualities. These are, the sulphates of lime, magnesia, soda, muriates of magnesia and potash, bitumen, oxide of iron, clay in a state of diffusion, &c.
Muriate of potash has been detected, in the waters of the ocean, in the sal-gem of Berchtesgaden in Bavaria, of Hallein in the territory of Salzbourg, and in the salt springs of Rosenheim.
The more heterogeneous the salt, the more soluble is it, by the reciprocal affinity of its different saline constituents; and thus a delicate hydrometer, plunged in saturated brine, may serve to show approximately the quality of the salt. I find that the specific gravity of a saturated solution of large-grained cubical salt, is 1·1962 at 60° F. 100 parts of this brine contain 251⁄2 of salt, (100 w. + 34·2 s.) From mutual penetration, 100 volumes of the aqueous and saline constituents form rather less than 96 of the solution.
Among the varieties in the form of this salt, the octahedral, the cubo-octahedral, and the dodecahedral, have been mentioned; but there is another, called the funnel or hopper-shaped, which is very common. It is a hollow rectangular pyramid, which forms at the surface of the saline solution in the course of its evaporation, commencing with a small floating cube, upon which lines of other little cubes attach themselves to the edges of the upper face; whereby they form and enlarge the sides of a hollow pyramid, whose apex, the single cubic crystal, is downward. This sinks by degrees as the aggregation goes on above, till a pyramidal boat of considerable size is constructed.
A Table of the results of the Analyses of several varieties of Culinary Salt.
| Origin of the Salt. | Chloride of Sodium. | Muriate of Mag- nesia. | Muriate of Lime. | Sulphate of Soda. | Sulphate of Mag- nesia. | Sulphate of Lime. | Clay and other insoluble bodies. | Oxide of Iron. | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Sal-gem of Vic | - | white | 99 | ·30 | — | — | — | — | 0 | ·005 | 0 | ·020 | |||||||
| red | 99 | ·80 | — | — | — | — | — | 0 | ·002 | ||||||||||
| —— Cheshire, crushed | 98 | ·33 | 0 | ·02 | — | — | — | 0 | ·65 | — | 0 | ·002 | |||||||
| Salt from Salt Springs: | |||||||||||||||||||
| Schönbeck, Westphalia | 93 | ·90 | 0 | ·30 | — | 1 | ·00 | — | 0 | ·80 | |||||||||
| Moutiers | - | des cordes | 97 | ·17 | 0 | ·25 | — | 2 | ·00 | 0 | ·58 | ||||||||
| boilers | 93 | ·59 | 0 | ·61 | — | 5 | ·55 | 0 | ·25 | ||||||||||
| Château Salins | 97 | ·82 | 2 | ·12 | |||||||||||||||
| White of Sulz | 96 | ·88 | 3 | ·12 | |||||||||||||||
| Ludwigshall, middle grained | 99 | ·45 | — | — | 0 | ·05 | — | 0 | ·28 | ||||||||||
| Kœnigsborn, Westphalia | 95 | ·90 | — | 0 | ·27 | — | — | 1 | ·10 | ||||||||||
| Sea salt, half white | 97 | ·20 | 0 | ·004 | — | — | 0 | ·050 | 0 | ·120 | 0 | ·070 | |||||||
| ——, of Saint Malo | 96 | · | 0 | ·30 | — | — | 0 | ·45 | 2 | ·35 | |||||||||
| Common Scottish salt | 93 | ·55 | 2 | ·80 | — | — | 1 | ·75 | 1 | ·50 | |||||||||
| Lymington, common | 93 | ·7 | 1 | ·1 | — | — | 3 | ·50 | 1 | ·50 | 2 | ·00 | |||||||
| ——, cat | 98 | ·8 | 0 | ·5 | — | — | 0 | ·5 | 0 | ·1 | |||||||||
| Cheshire, stoved | 98 | ·25 | 0 | ·075 | 0 | ·025 | — | — | 1 | ·55 | |||||||||
The geological position of rock salt is between the coal formation and the lias. The great rock-salt formation of England occurs within the red marl, or new red sandstone, the bunter-sandstein of the Germans, so called, because its colours vary from red to salmon and chocolate. This mineral stratum frequently presents streaks of light blue, verdigris, buff, or cream colour; and is chiefly remarkable for containing considerable masses or beds of gypsum. At Northwich, in the vale of the Weaver, the rock salt consists of two beds, together not less than 60 feet thick, which are supposed to constitute large insulated masses, about a mile and a half long, and nearly 1300 yards broad. There are other deposits of rock salt in the same valley, but of inferior importance. The uppermost bed occurs at 75 feet beneath the surface, and is covered with many layers of indurated red, blue, and brown clay, interstratified more or less with sulphate of lime, and interspersed with argillaceous marl. The second bed of rock salt lies 311⁄2 feet below the first, being separated from it by layers of indurated clay, with veins of rock salt running through them. The lowest bed of salt was excavated to a depth of 110 feet, several years ago.
The beds or masses of rock salt are occasionally so thick, that they have not been yet bored through, though mined for many centuries. This is the case with the immense mass of Wieliczka, and the lower bed at Northwich. But in ordinary cases, this thickness varies from an inch or two to 12 or 15 yards. When the strata are thin, they are usually numerous; but the beds, layers, or masses never exhibit throughout a great extent any more than an illusory appearance of parallelism; for when they are explored at several points, enlargements are observed, and such diminutions as cause the salt to disappear sometimes altogether. This mineral is not deposited, therefore, in a geological stratum, but rather in lenticular masses, of very variable extent and thickness, placed alongside of each other at unequal distances, and interposed between the courses of the other formations.
Sometimes the rock salt is disseminated in small masses or little veins among the calcareous and argillaceous marls which accompany or overlie the greater deposits. Bitumen, in small particles, hardly visible, but distinguishable by the smell, occurs in all the minerals of the saliferous system.