430

MANUFACTURES.

Manufac- thrice a week in the summer. The first saline crusttures. which is formed is small-grained, the latter large. Bay salthas generally a little tinge of colour, green or brown,according to the soil on which it is formed. It is onlymade in the summer months.

2 d, partial Another way of making salt from sea water, which

evapora- j s practised much on the French and other coasts

tlon " of temperate climates, is partly by the atmospherical

evaporation, and partly by boiling, for the summers arenot hot enough in such climates to make salt by mereexposure of brine to the air. The general mode of pro-ceeding is that already mentioned ; that is, the sea wateris exposed during the summer in shallow artificial pools,where it becomes highly concentrated, and this is after-wards boiled down in iron pans in the usual mode.This way is adopted in some parts of England, particu-larly at Lymington . The mother-water which remainsafter most of the salt has been extracted, contains muchmuriate of magnesia, and this is advantageously con-verted into the sulphate of magnesia.

3d method. There is still another method of extracting the salt fromsea water, which is by collecting the sand that has been re-peatedly moistened by the sea water and dried, and lixivia-ting it in the reservoirs, where it forms a very strong brine,which is then boiled down as usual. This method is muchpractised on the Western coast of France , particularly inLower Normandy , and at the Isles of Oleron and Rhe.

The spot being chosen, (which should be on a level shorewith a clean sand,) the necessary buildings are erected,namely, evaporating pans, store-houses, covered sheds,&c., and an area of three or four acres is selected a littlebelow the level of the spring tides and above the neap.The surface is carefully levelled by the plough, androlled smooth and hard ; it is then filled to the heightof several inches with sand taken from the edge of thesea at low water, and the sand is also drenched with sea• water as the tide flows in. It then lies exposed to the

sun and wind, which soon dissipate the superfluous water,and the surface of the sand appears covered with awhite efflorescence. This is turned over frequentlywith a kind of shovel, changing the surface several timesa day till the whole is thoroughly dry. This salinesand is then carried to the sheds, and the process is re-peated with fresh sand till a large quantity is collected,which generally employs the whole summer. To makethe salt, the dry sand is taken out of the sheds andthrown into small round pits about two and a half feetin diameter, and twelve inches deep, the bottoms ofwhich are lined with hard-rammed clay mixed withchopped straw, to prevent the water from oozing through.The sand is then covered with sea water, or with theweaker lie of former operations, and after standing somehours is drawn off into reservoirs or barrels, whence theevaporating pans are immediately supplied. The sandis lixiviated a second time, and this lie is reserved for afresh portion of sand. The boilers are of lead, aboutthree and a half feet square, and four or six inches deep.They are heated with wood of any kind, or sometimeswith reeds, and a boiler of this kind is worked off infrom two to three hours. The sali is raked out as it isformed, and drained in hollow cones, as in other places.Three pans of the above dimension yield together aboutfive pounds of salt.

dthmethod. The salt procured in this way is white and small-grained, but it is very apt to be’damp, and is a weak-bodied salt little fit for preserving animal food for anylength of time. In a few Northern Countries, some

advantages is made of the effect of cold to concentrate Mechanicalbrine, by freezing at first only the more watery part, of Processes,course leaving the unfrozen part proportionally richerin salt. The winters of this Country are not coldenough in general for this purpose, but it is occasion-ally used on the Baltic coasts. The cold, however, mustnot be too intense, otherwise the brine itself freezes.

Frozen salt water is not in hard solid masses like freshwater, but is soft and crumbly or rotten. The efficacy ofthis method of concentrating brine is very inconsiderable.

(613.) Description of the Cheshire salt district. —We Cheshire have stated the other repositories of salt to be salt rocks saIt dls "and brine springs, which in England are found only in trlct ‘Cheshire and its immediate neighbourhood. A veryinteresting account of this salt district has been given byMr. Holland, in a Memoir published in vol. i. of thefirst series of the Transactions of the Geological Society ,from which it appears that the rock salt occurs from28 to 48 yards beneath the surface of the earth. Thefirst stratum or mine is from 15 to 21 yards in thick-ness. It very much resembles in appearance brownsugar-candy, is perfectly solid, and so hard as not tobe broken but with great difficulty by iron pikes orwedges. Of late, the workmen have been accustomedto blast it with gunpowder, by which means they loosenand remove many tons of it together. Beneath thisstratum there is a bed of hard stone, consisting oflarge veins of flag, intermixed with some rock salt, thewhole from 25 to 35 yards in thickness. Under this jj x tent ofbed is a second stratum or mine of salt, from five to the Che-six yards thick, many parts of it perfectly white and shire s aclear as crystal, others browner, but all purer than the dlstr11 ' ‘upper stratum, yet reckoned not quite so strong. Onlythese two distinct beds of the fossil salt have been metwith at Northwich ; but it has been ascertained thatthe same limitations do not exist throughout the wholeof the salt district, three distinct beds, at least, beingfound in some situations, separated in like manner fromeach other by intervening strata. The great body ofthe rock salt, both in the upper and the lower beds,is composed of crystals of muriate of soda, intimatelymixed with certain proportions of clay and oxide of iron,containing likewise small proportions of certain earthysalts. At the same time, particularly in the lower strataof the rock, there are found separate crystalline concre-tions of muriate of soda in a purer state, variouslydisposed, sometimes occurring distinctly in the cubicalform, in other places in masses of larger size and irre-gularly shaped. Above the whole mass of salt lies abed of whitish clay, which has been used in the Liverpoolearthenware; and in the same situation there is foundalso a quantity of gypsum. , 0 f

(614.) Rock salt-pits are sunk at great expense, andare very uncertain in their duration, being frequently de- *° ne>stroyed by the brine springs bursting into them, anddissolving the pillars that support the roof; throughwhich the whole work then falls in, leaving vast chasmsin the surface of the earth. In forming a pit, a shaftor eye is sunk, similar to that of a coal-pit, but moreextensive. When the workmen have penetrated to tl.-esalt rock, and made a proper cavity, they leave a suffi-cient substance of the rock (generally about seven yardsin thickness) to form a solid roof; and as they procee ,they hew pillars out of the rock, to sustain the 100 .Gunpowder is then employed to separate what is meanto be raised, which is conveyed to the surface in hugecraggy lumps, and is drawn up above ground in capa