744 SALT 



obtained until a further boring of 348 feet was made. At Droitwich the borings are 

 only to a depth of 175 feet, and so abundant is the supply of brine, that if the pumps 

 cease working, it speedily rises to within nine feet of the surface, and if left unremoved 

 soon overflows. The freedom of the brine from dilution by freshwater springs is from 

 time to time tested by the hydrometer. From the pumps the brine is directly conveyed 

 by means of pipes to reservoirs, from which, as the evaporation proceeds, it is admitted 

 into the pans. As the water is vaporised, the salt is deposited and falls to the bottom 

 of the pan ; it is then drawn to the sides by the workmen, until a heap is accumulated, 

 and from this portions are ladled out into rectangular wooden boxes with perforated 

 bottoms, allowed to drain and solidify, removed from the boxes, and placed in the 

 drying room ; the salt of coarser grain is simply drained roughly in baskets and 

 dried. The grain of the bait, i.e. its occurrence in larger or smaller crystals, is entirely 

 the effect of temperature ; the fine-grained or table salt is produced by rapid heating, 

 and is formed at that end of the pan next the fireplace ; the coarse or bay-salt is 

 formed by the slow evaporation which goes on at the other end ; whilst an intermediate 

 variety, common salt, is produced in the middle. A pan may sometimes be slowly 

 evaporated for the express purpose of obtaining bay-salt. 



In the preparation of salt various substances have been added to the brine, with a 

 view of improving the quality of the product : these have been chiefly bodies containing 

 albuminous matters, which, coagulating upon the application of heat, entangle all solid 

 impurities and carry them to the surface ; blood, white-of-egg, glue, and calves' feet 

 have thus been extensively used. There is also another class of substances employed 

 for a different purpose. When a concentrated solution of any saline matter is evaporated, 

 much annoyance is caused by a layer of the solid salt forming on the surface of the 

 liquid and impeding evaporation: this is called a 'pellicle'; to obviate this, and 

 to avoid the loss of labour entailed by constant stirring, oils, butter, or resin, have been 

 added to the brine. The effect of the latter is said to be perfectly magical, the intro- 

 duction of a very few grains being amply sufficient to clear the largest pan, and to 

 prevent any recurrence of the ' setting over.' 



When it is required to prepare salt from the weak brines which are of common 

 occurrence in France and Germany, the second method is resorted' to, and the 

 brine is concentrated by natural evaporation previous to the application of artificial 

 heat : this concentration was formerly effected by distributing the brine over flat 

 inclined wooden surfaces, but it is now brought about by allowing the brine to trickle 

 in a continuous stream through walls of thorns exposed to the sun and wind. This, 

 which is called the method of graduation, is employed, among other places, at 

 Moutiers in France, and at Nauheim, Durrenberg, Kodcnberg, and Schonebeck, in 

 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 Schonebeck, the 

 graduation-house is 5,817 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 Sal/a and Durrenberg 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 Durrenberg, 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, 2nd, and 3rd graduation. 

 A deposit of gypsum incrusts the twigs, which requires them to be renewed at the 

 end of a certain time. Figs. 1747 and 1748 represent the graduation-house of the 

 salt-works at Durrenberg. 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 / to <?, Jig. 1748, 

 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 1 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, I, 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- 





