The salt thus obtained, partakes or the colour of the bottom on which it is formed; and is hence white, red, or gray.

Sea water contains, in 1000 parts, 25 of chloride of sodium, 5·3 sulphate of magnesia, 3·5 chloride of magnesium, 0·2 carbonate of lime and magnesia, 0·1 sulphate of lime, besides 1⁄2000 of sulphate and muriate of potash. It also contains iodide of sodium, and bromide of magnesium. Its average spec. grav. is from 1·029 to 1·030.

[Figs. 962-964 enlarged] (155 kB)

Sea-water and weak brines may be concentrated either by the addition of rock salt, by spontaneous evaporation in brine-pits (see suprà), or by graduation. Houses for the last purpose are extensively employed in France and Germany. The weak brine is pumped into an immense cistern on the top of a tower, and is thence allowed to flow down the surface of bundles of thorns built up in regular walls, between parallel wooden frames. At Salza, near Schönebeck, the graduation-house is 5817 feet long, the thorn walls are from 33 to 52 feet high, in different parts, and present a total surface of 25,000 square feet. Under the thorns, a great brine cistern, made of strong wooden planks, is placed, to receive the perpetual shower of water. Upon the ridge of the graduation-house there is a long spout, perforated on each side with numerous holes, and furnished with spigots or stopcocks for distributing the brine, either over the surface of the thorns, or down through their mass; the latter method affording larger evaporation. The graduation-house should be built lengthwise in the direction of the prevailing wind, with its ends open. An experience of many years at Salza and Dürrenberg has shown, that in the former place graduation can go on 258, and in the latter 207 days, on an average, in the year; the best season being from May till August. At Dürrenberg, 3,596,561 cubic feet of water are evaporated annually. According to the weakness of the brine, it must be the more frequently pumped up, and made to flow down over the thorns in different compartments of the building, called the 1st, 2d, and 3d graduation. A deposit of gypsum incrusts the twigs, which requires them to be renewed at the end of a certain time. [Figs. 962.] and [963.] represent the graduation-house of the salt-works at Dürrenberg. a, a, a, are low stone pillars for supporting the brine cistern b, called the soole-schiff. c, c are the inner, d, d the outer, walls of thorns; the first have perpendicular sides, the last sloping. The spars e, e, which support the thorns, are longer than the interval between two thorn walls from f to g, [fig. 963], whereby they are readily fastened by their tenons and mortises. The spars are laid at a slope of 2 inches in the foot, as shown by the line h, i. The bundles of thorns are each 11⁄2 foot thick, from 5 to 7 feet long, and are piled up in the following way:——Guide-bars are first placed in the line k, l, to define the outer surface of the thorn wall, the undermost spars m, n, are fastened upon them; and the thorns are evenly spread, after the willow-withs of the bundles have been cut. Over the top of the thorn walls are laid, through the whole length of the graduation-house, the brine spouts o, o, which are secured to the upper beams; and at both sides of these spouts are the drop-spouts p, p, for discharging the brine by the spigots s, s, as shown upon a larger scale in [fig. 964.] The drop-spouts are 6 feet long, have on each side small notches, 5 inches apart, and are each supplied by a spigot. The space above the ridge of the graduation-house is covered with boards, supported at their ends by binding-beams q. r, r show the tenons of the thorn-spars. Over the soole schiff b, inclined planes of boards are laid for conducting downwards the innumerable showers. The brine, which contains at first 7·692 per cent. of salt, indicates, after the first shower, 11·473; after the second, 16·108; and after the third, 22. The brine, thus concentrated to such a degree as to be fit for boiling, is kept in great reservoirs, of which the eight at Salza, near Schönebeck, have a capacity of 2,421,720 cubic feet, and are furnished with pipes leading to the sheet-iron salt-pans. The capacity of these is very different at different works. At Schönebeck there are 22, the smallest having a square surface of 400 feet, the largest of 1250, and are enclosed within walls, to prevent their being affected by the cold external air. They are covered with a funnel-formed or pyramidal trunk of deals, ending in a square chimney, to carry off the steam.

[Figs. 965-967 enlarged] (227 kB)

[Figs. 965], [966], [967.] represent the construction of a salt-pan, its furnace, and the salt store-room of the works at Dürrenberg; [fig. 967.] being the ground plan, [fig. 966.] the longitudinal section, and [fig. 965.] the transverse section, a is the fire-grate, which slopes upwards to the back part, and is 311⁄2 inches distant from the bottom of the pan. The ratio of the surface of the grate to that of the bottom of the pan, is as 1 to 59·5; that of the air-hole into the ash-pit, as 1 to 306. The bed under the pan is laid with bricks, smoothly plastered over, from b to c, in [fig. 966.] Upon this bed the pillars d, d, &c., are built in a radiated direction, being 6 inches broad at the bottom, and tapering to 11⁄2 inch at top. The pan is so laid that its bottom has a fall towards the middle of 21⁄2 inches: see e, f, [fig. 966.] The fire diffuses itself in all directions under the pan, proceeds thence through several holes g, g, g, into flues h, h, h, which run round three sides of the pan; the burnt air then passes through i, [fig. 967.], under other pans, from which it is collected in the chimneys k, k, to be conducted into the drying-room. At l, l, there is a transverse flue, through which, by means of dampers, the fire-draught may be conducted into an extra chimney m. From the flues k, k, four square iron pipes n, n, issue and conduct the burnt air into the main chimneys in the opposite wall.