The annexed table shows the composition of the rock-salt of a few of the largest and best known mines of Europe. The Stassfurt salt, however, is not generally so impure as stated ; in the Paris Exhibition of 1867, the writer obtained a sample from a lot of rock-salt from Stassfurt there exhibited in irumeuse slabs and blocks, which gave on analysis no less than 99-95 per cent. of pure sodium chloride, and 0.05 of calcium sulphate, and was consequently nearly pure.
White Salt.—The third branch of the salt manufacture, viz. the production of white salt by the artificial evaporation of Nine, is by far the most important of all the methods by which salt is prepared, so far as England is concerned, for not only is the salt thus obtained in a far purer condition than by any other method, but by this method we alone produce in tl3e Cheshire and Worcestershire salt-works, probably as much as a third of the quantity of salt which is consumed in the whole of Europe.
The brine used in Cheshire is that alluded to as overlying the upper layer of the salt-measures, and it is raised by pumping, and fed into the various salt-works. Some of the salt-works raise brine for themselves by their own pumps, only paying a royalty to the landlord. The cost of the brine varies in the different districts : at Northwich, it is 4-9d. per ton of salt made ; at Winsford, it is 6d., and this latter price may be taken as a fair average. It is rare, however, to find saturated brine thus overlying the beds of salt. In other countries, this is seldom or never the case, and a description of the manufacture of white salt may be commenced by explaining the various methods used for obtaining the strong brine.
1. A shaft is sunk, and tho rock-salt, mined in the usual manner, JIB brought to the surface and dissolved in water. Son-water or the water of saline springs is sometimes used. In France, large wooden tubs called Gesqoirs aro generally employed. The rock is broken and placed on perforated shelves, or hung in baskets round the upper edges of the tubs. The solution is performed in a reser voir of ma-onry or of concrete lined with cement, and in this case a sort of wicker boat is sometimes surrounded with enipty casks to give it flotation, and laden internally with broken lock-salt, and floated OD the water of the reservoir. The salt dissolves, the stronger brine falling to the bottom and the weaker rising to the surface, where it continually meets with fresh salt. The process thus continues, till, if the supply of rock be properly maintained, the whole bulk of the liquid speedily becomes saturated brine.
2. Another method employed, more particularly in workiog beds of saliferous clay, is the creation of a solution-chamber (chambre (k dissolution) within the deposit itself (see Fig. 1205), into which water is introduced with c,er tain precautions, and this, dissolving the salt, and beeoming converted into brine, is pumped up for use.
The construction of such a chamber is carried out in the following man ner. A shaft E is sunk to the bed of salt or saliferous clay ; in this, a horizontal longitudinal gallery P D is first pierced, and from this, the transverse galleries 1m, and other longitudinal galleries r s; this cuts the beds into squares or rectangles and these aro further divided up by other smaller galleries, not shown, till the whole part to bo excavated is cut into galleries, the roof being supported by the pillars left between. The size of the chamber,
the distances separating the galleries, their height, &e., must depend on the nature of the ground, and in some degree on the fancy of the operator. By this means, is me.de a chamber of more or less rectangular form, the central gallery P D running throughout its entire length. Another transverse gallery A B is pierced in the same level, and at some yards distant from the end of the chamber, and the gallery P D is pr,donged to meet A B at C. A long wooden pipe is laid from C to D, pierced with holes in its sides and bottom, and its upper part covered with a pont roof, so as to throw off any materials which may fall on it. A good timber dam is made at G, and well pugged with clay, and a sluice is made at H, by which the brine can bo drawn off ; F is another small shaft, through which water can flow into the chamber. The gallery A B rnay serve for any number of such chambers on the same level. All being thus arranged, water is allowed to flow down through F till the bases of the pillars are submerged a few inches. This water is SOOD converted into saturated brine ; it is drawn off by opening the sluice II, and pumped to the surface. More fresh water is let in to a like depth, and flually the bases of the pillars get cut away by solution. Each time fresh water is let in, care is taken that it rises only just far enough to bathe the lower ends of the pillars, which are thus gradually dissolved upwards from their bases, the sides of the chamber becoming likewise excavated and enlarged by solution. This goes on until the pillars become completely dissolved, and the whole becomes one great chamber filled with salt water. Much care is to be observed in controlling the quantities of water fed into these chambers, in which the pressure should he maintained as moderate and unvarying as pOssible. The fresh water admitted, being lighter than the brine, has a tendency to rise to the sur face, and the solution is liable to go on almost entirely at the top of the chamber, the interior of which tends to assume the form nf an inverted cone from this cause. This involves loss, and where two or more chambers A B are being worked continuously, the erosion of the salt necessarily takes place in the form shown in Fig. 1206, the portion C remain ing untouched. A partial remedy for this inconvenience, constituting the best mode of working, and now usually adopted, is to introduce the fresh water in a small but constant and steady stream at F (Fig. 1205), while the brine is being drawn off in the same continuous manner at 11. This method of working saliferous clays is much useil in Germany ; at Dfirrenberg, in Saxony, are 33 of these solution-chambers, each possessing a rnean capacity of about 700,000 cub. ft.