Another noticeable and not easily accounted for feature in the geology of rock-salt is its frequent association with bitumen and petroleum, which are found with salt in tbe oil formations of Pennsylvania. Bastennes, where bitunien was long worked; is close to the salt deposits of Dax, the foot of the Pyrenees; 9,nd petroleum floats in small quantity on the surface of a aping near Orthez, and has been found in a boring in the neighbourhood of Sallee in the same district. Petro leum and bitumen also occur not far from Volterra in Tuscany, where the largest rock-salt works of Italy exist, and near to which are Count Larderel's celebrated boracic acid springs ; and they are worked in some quantities in Wallachia, where also much rock-salt is found. Petroleum has lately been discovered in Hanover, not far from the German salt deposits already mentioned. Bitumen colours the lowest beds of the rook-salt mines of Nancy. It is found in and around the Dead Sea in numerous places, while both bitumen and petroleum occur abundantly at Baku, on the Caspian, near some large salt deposits both old and recent. A good deal of organic matter, both vegetable and animal, exists in the sea, and as its waters became concentrated, such organic matter would concentrate with them. Large quantities of shells filled with petroleum are spoken of as being found in Pennsylvania, and myriads of shells saturated with bitumen occur in the old workings of Bastennes ; but whether or no there is any tendency of orgailic matter in presence of strong brine, through the avidity of brine for water, to develop itself into these hydrocarbons, remains for the present a mystery unsolved. Such a union of facts as here given testifies strongly to the theory that rock-salt is a true sedimentary rock, end that it probably owes its origin to the slow evapo ration, in the course of enormous lapses of time, of salt lakes or inland seas fed from the waters of the ocean. The sea as it now exists may owe some of its saltness to the solution of rock-salt formed during previous geological periods, and subsequently depressed beneath the present ocean. Probably such cases of solidification and re-solution have been frequently repeated, but that the present known formations of rock-salt owe their origin to an evaporation of salt water, such as is now going on in certain quarters of the globe, rather than to any eruptive sg-ency, there can be hardly any room to doubt.
The very general distribution of salt in almost every known region of the globe, the facility with which it can be quarried from the mountain sides, or obtained by evaporation from the waters of the sea, or of salt lakes, the fact of its being a prime essential in the economy of life, and a staple raw product of numerous important industries, have led to the introduction of the salt manu facture in one form or other into almost every country, and shed an interest over all facts connected with its production. In considering the various processes by which satt is manufactured, the methods employed in European countries will alone be studied, especially comparing the English manufacture with those in use abroad, where important differences exist. The subject will be divided into 3 heads :—(1) The production of salt from sea-water by spontaneous evaporation ; (2) the mining of rock-salt ; (3) the production of white salt from brine by evaporation with artificial heating.
Sea-Salt or Bay-Salt (FR., Sel marin ; GER., Meer-salz).—The production of salt from sea-water by spontaneous evaporation varies much with the general atmospheric conditions. It was at one time practised in England ; at Lymington in Hampshire, at Hayliug Island near Portsmouth, and at Saltcoats on the Ayrshire cciast, the evaporation of sea-water for the production of salt in " salterns " or " brine-pans " was formerly s, staple industry. Since the suppression of the duty
on salt, and the development of the production in Cheshire and Worcestershire, the sea-salt industry has been reduced to one or two establishments round the coast where coal is cheap, as at N. Shields, where salt is made by artificial evaporation from strong brine produced by dissolving lock-salt to saturation in sea-water. But the employment of solar heat is common in countries where the climate is more suitable ; hundreds of thousands of tons of salt are annually produced in this way along the W. shores of France and Portugal, in the Bay of Cadiz, along the E. of Spain and S.-E. of France, and along the coasts of Italy, Austria, Greece, Turkey, and Russia. The manu facture of salt from sea-water is in fact an industry of high importance, employing much labour, and affording large revenues.
Sea-water differs but little in its composition, whether taken at the surface or at the lowest depths, tides and currents apparently maintaining it in a perfect state of mixture. Some enclosed seas, such as the Red Sea and the Mediterranean, appear to be rather richer in saline matter than the waters of the ocean ; others, as the Black Sea and the Baltic, are soniewhat poorer. Under the tropics, and where dry winds prevail, there is some trifling augmentation of the saltness of the ocean, whilst at the poles, ancl near the mouths of some great rivers, the water is rather less salt, but these differences are completely local and relatively insignifics,nt. Table I. (opposite), gathered from various authorities, thongh it may be considered fanciful in respect of the combinations in which the various elements are supposed to exist, will convey an idea of the composition of sea-waters.
For the better understanding of the processes of manufacturing sea-salt, it will be convenient to consider what are the general results of the concentration of sea-water by evaporation. Usiglio, in some observations very carefully made in the neighbourhood of Cette upon the water of the Mediterranean during evaporation, describes the reaction° and the order in which they take place. The sp. gr. of tho water there is 1-023. When the clear water ia submitted to concentration by evaporation, uo deposit takes place until the water attains a sp. gr. of 1-05, when a little fcrrio oxide and calcium carbonate begin to go down. This continuea till the sp. gr. is 1.12. at which point, selenite (hydrated calcium sulphate) also begina to separate, and continues till the sp. gr. is 1.25. Meanwhile, as soon as 1.21 is reached, i. e. when the original volume of the water is reduced from 1000 parte to 95, magnesium sulphate cryatallizes out with the selenite, accom panied by aome sodium and tnagnesium chlorides. Sodium bromide likewise begina to &posit so soon as 1.231 is attained. The precipitation of these 3 salts continues steadily to progress until close upon sp. gr. 1.3, and the volume of the solution is reduced to 16 parts, or about wts of what it was. Its percentage composition will then be :—Magnesium sulphate, 11.45 per cent. ; magnesium chloride, 19.53 ; sodium chloride, 15.98 ; sodium bromide, 2.04 ; potassium chloride, 3.30; water, 47.7. So that when the water had only reached a sp. gr. of 1.21, the only substances widish had separated were (in percentagea of the original water) :—Ferric oxide, 0.0003; calcium carbonate, 0.0117; selenite, 0.1466 ; but between 1.21 and 1.231, the composition of the deposit became :—Calcium sulphate, 0.0283; magnesium sulphate, 0.0621 ; sodium bromide, 0.0222 ; magnesium chloride, 0.0153 ; sodium chloride, 2.7107. Thus between these two last-named densities, nearly 2.84 per cent. of saline matter crystallized out of the °elution, 95i per cent. of this being sodium chloride.