35 
Pure rock-salt consists of simple chloride of sodium 
= Na Cl= 39*66 sodium, and 60-34 chlorine ; the blue 
variety contains usually traces of copper, the red traces of 
oxide of iron, and the grey some bituminous clay or 
gypsum. The compact rock-salt, as it is given off in 
the mother-liquid which remains after the boiling of 
most brines, contains, moreover, either bromide or iodide 
of magnesium, or both, but only in small amount. The 
crepitating salt of Wieliczka, which, when thrown in water, 
gives out bubbles of gas with a crepitating or crackling 
noise, contains carburetted hydrogen with some carbonic 
oxide. Many kinds of red-coloured rock-salt contain nu¬ 
merous shells of small infusoria (polygastriccc loricato), which 
tell of their marine origin. It is almost equally soluble in 
cold water and hot, and requires about three times its bulk 
to dissolve it; the solution has a purely salt taste, some¬ 
what astringent, and gives, with nitrate of silver, a cheesy 
deposit of chloride of silver, which is insoluble in nitric acid. 
Before the blow-pipe it melts easily, and without noise, to 
a colourless pearl, which gradually evaporates, and colours 
the flame bright yellow; when melted with protoxide of 
copper and bi-sulphate of potash, or salt of phosphorus, a 
fine blue flame of chloride of copper appears on the char¬ 
coal. When sprinkled with sulphuric acid it gives off 
hydrochloric acid, which disappears with frothing even 
when cold, and still more so when sufficiently heated. 
Rock-salt is the most widely diffused salt of sodium, and 
frequently constitutes very remarkable beds, or it fills up 
cavities, sometimes also blocks in layers, especially in the 
middle and later stratified rocks, where it generally accom¬ 
panies anhydrite and gypsum, and is free from petrifactions. 
Sometimes, however, its presence can only be recognised 
in more or less rich bitterns or mineral springs. The beds 
of rock-salt of Wieliczka in Galicia are widely celebrated 
on account of their extent and purity. At Cardona in 
Spain a whole mountain of salt stands exposed to the air, 
in the western Kirgisen-steppe of Southern Russia, it lies 
only a few feet beneath the surface of the earth. The 
salt-beds of Swabia are found 20—40 yards deep in 
the lower shell-limestone in three different groups; 
those .of Yic in Lothringen 58 metres deep in the lower 
lias, those of Chester 212 feet deep in variegated sand¬ 
stone ; the salt of Salzburg, Hall in the Tyrol, Reichen- 
hall, Ischl, Hallein, etc., belongs to the so-called Hasselt 
mountains, a bituminous clay mixed with gypsum, which 
is put into Alpine limestone found there, and lixiviated 
underground. The saturated brine is. then taken out 
and boiled down in the salt-pans. The salt-springs of 
Kreuznach, which are rich in iodine, rise in the carboni¬ 
ferous rocks, and are gradually concentrated before they 
are brought to the salt-pans. 
There are extremely rich salt-springs at Kissingen and 
at Nauheim in the Wetterau ; the latter are hot, and both 
are chiefly used as baths. The springs of Canstatt and 
Mergentheim in Swabia, also contain rock-salt. 
The salt lakes of inland countries, as, for example, the 
Dead Sea, and also sea-water, contain a large quantity of salt, 
and the latter is frequently used for obtaining what is called 
sea-salt, as, for instance, on the coasts of Istria and Dalmatia. 
When the brines of salt-springs are evaporated they 
form small, hollow, funnel-shaped, ladder-like crystals 
(Fig. 21), which sink into the soil, and are brought into the 
market as kitchen-salt. They always contain some water, 
and can only be freed from it by grinding and drying, or 
by melting it. In this way the greater part of kitchen- 
salt is prepared from the more impure kinds of rock-salt 
by solution and evaporation. A more simple method 
would perhaps be, to melt the rock-salt and allow it to 
trickle down, by which process it could be obtained in the 
desired shapes, and reduced to powder. Rock-salt is used 
principally for seasoning food and preserving meat, fish, 
etc., and has been so used from the earliest times. It also 
appears to be an indispensable element in the food of all 
animals, since all the fluids of the animal body contain 
more or less chloride of sodium. It is also much used as a 
manure. It is employed in the manufacture of soda, soap, 
and glass, and in the preparation of chlorine and hydro¬ 
chloric acid in many manufactures. The red salt figured 
is also used for baths, in imitation of the natural springs, 
as a remedy in scrofula, glandular swellings, and the like. 
/ 
Fig. 17.— Soda-Saltpetee, Niteate of Soda. 
The primary form is an obtuse rhombohedron of 
106° 35', similar to that of calcite, and as represented in 
Fig. 17 ; the cleavage is in the direction of the primary 
form, the crystals are colourless, of vitreous lustre, of 
1-5—2-0 hardness and 2-09 specific gravity, transparent to 
translucent. It more frequently occurs in granular masses, 
similar to those of saltpetre. The chemical constituents 
are simple nitrate of soda — Na N, consisting of 36*75 
parts of soda and 63*25 of nitric acid. The natural nitrate 
of soda of Chili contains, in addition to 94*291 of nitrate of 
soda, small quantities of chloride of sodium, sulphuret and 
nitrate of potash, nitrate of magnesia, iodide and bromide 
of magnesium and of water. It behaves like nitrate of 
potash before the blow-pipe and on charcoal, but it colours 
the flame yellow. The pure salt is soluble in three times 
its volume of water, and has a sharp, saline, and somewhat 
cooling taste. When it is sprinkled with sulphuric acid 
and heated, nitric acid vapours are given off. 
Nitrate of soda is found in considerable masses in the 
district of Atakama in Chili, and is known in commerce as 
Indian or Chilian saltpetre. From a solution of this a 
tolerably pure soda-saltpetre can be obtained by crystal¬ 
lization ; the crystals take the form of rhombohedral aggre¬ 
gations, with ladder-like hollowed planes, and, like potash- 
saltpetre, is much used in the preparation of nitric acid, 
but not for gunpowder. 
Fig. 23.—Sulphate of Soda, Glaubee Salts. 
The primary form is an oblique rhombic prism of 
99° 36 ; and 80° 24 y , which can only be split in the direc¬ 
tion of the acute lateral edges. It is usually found only 
in crystalline grains or needles, or in mealy crusts. By 
solution and crystallisation, however, very large crystals, 
with truncation of the basal edges, angles, and lateral 
edges (Fig. 23), may be obtained, also twins and crystal¬ 
line masses. Colourless, lustre vitreous, translucent, of 
uneven fracture, 1*5—2*0 hardness and 1*48 specific gra¬ 
vity. The elements of the pure salt are simple sulphate 
of soda, with ten equivalents of water; Na S + 10 H, 
consisting of 19*2 soda, 24*8 sulphuric acid, and 56*0 of 
water. When native it is almost always rendered impure 
