Salt

The concentration of salts in the Dead sea increases to a depth of about 25o feet, after which it remains practically constant. At this depth and below it is a concentrated solution, which, indeed, is supersaturated when pumped up, for a slight deposition of salt takes place owing to diminished pressure. Noteworthy features of Dead sea water are its relative freedom from sulphates and the high proportions of potash and bromide. These factors, coupled with the circumstance that atmospheric conditions in Palestine are favourable to solar evaporation for about 8 months of the year, indicate that the production of salt, potash, and even bromine is feasible in the Dead sea area, the process as regards salt and pot ash being similar to that described below under Manufacture. The brines at Kharaghoda resemble sea water in the character of their dissolved salts, but are much more concentrated, and in some cases practically saturated as shown by analysis No. 2.

The following table gives the composition of some concen trated natural brines used for the production of salt in various countries, It has been suggested that the foreign brines, characterized by the absence of sulphates and carbonates, have been produced by a natural process akin to the "Permutit" process for softening water.

Rock Salt,

mineralogically termed halite, crystallizes in the cubic system, the more usual form being a cube, but octahedral, dodecahedral, tetrahexahedral and hexoctahedral faces sometimes occur. The word halite is sometimes used as a group name to include a series of haloid minerals, of which rock salt is the type. Rock salt commonly occurs in cleavable masses, sometimes in laminar, granular or fibrous form, the last being known as "hair salt." The crystals, which occasionally have distorted or cavernous faces, possess a perfect cubic cleavage, exhibit conchoidal frac ture and are rather brittle; the specific gravity varies from 2.1 to 2.6 and the hardness is 2.5 on the scale of Mohs.

Rock salt is colourless and transparent when pure, but is fre quently red, yellow or brown, and more or less opaque owing to the presence of impurities such as lime and magnesium salts, with marl and iron oxide as insoluble impurities. The proportion of sodium chloride varies from 94.5% upwards, depending on the pro portion of the foregoing impurities.

Rock salt occasionally exhibits double refraction, due perhaps to natural pressure. In some crystals small cavities are present, and these may contain saline solutions or gases such as carbon dioxide, and volatile hydrocarbons. Some crystals (Knister

salz) decrepitate on being dissolved owing to the escape of con densed gases. Pure rock salt is essentially sodium chloride, but it generally contains magnesium salts, and these impurities cause it to be deliquescent. Rock salt is highly diathermanous, or capable of transmitting heat rays and the shorter infra-red rays. This property is utilized in investigations of infra-red radiation up to about 16/2, and large rock salt have been used for this purpose with faces up to 6". The refractive index of rock salt varies from at wave-length o•5893µ (yellow light) to 1.44102 at (infra-red).

Rock salt is occasionally found as a sublimate on lava, as at Vesuvius, where it is associated with potassium chloride, but it occurs mostly in bedded deposits, often lenticular in shape and sometimes of great thickness. These deposits are generally asso ciated with other minerals such as gypsum, anhydrite, sylvine, carnallite and kieserite. The associated minerals are compounds of calcium, magnesium, and potassium, and it is inferred there from that rock salt has been formed by the evaporation of inland seas such as the Dead Sea and the Great Salt Lake of Utah, or per haps in some cases by the extreme desiccation of an arm of the sea, such as the Kara Bughaz, which forms a natural salt-pan on the east side of the Caspian sea. Such beds of salt are found in strata of very varied geological age ; the Salt Range of the Punjab, for instance, is probably of Cambrian age, while the famous salt deposits of Wieliczka near Cracow have been referred to the Plio cene period. In many parts of the world, including the British area, the Triassic age offered conditions especially favourable for the formation of large salt-deposits. In England all the known deposits of rock salt occur in the New Red Sandstone, and with one doubtful exception are of Triassic age. The great salt deposits of Prussian Saxony are of Permian age (Zechstein), whilst those of Bavaria and of Tirol are Triassic. In the United States of America salt is found in strata of very different ages : the Silurian in New York, Michigan and north Ohio; the Carboniferous in Pennsylvania ; the Permian in Kansas, Texas and New Mexico ; the Jurassic in Utah ; the Tertiary in Louisiana, Idaho and Wyoming; and the Recent deposits of Oklahoma, Nevada and other western states.