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mother water that remains after moll of the faitlias be en extracted, contains much muriat ofmagnefia, and this is advantageousy convertedinto the fulphat of magnefia, as will be mentionedunder that article.

There is {till another method of extruding thefait from fea-water, which is by collecting thefand that has been repeatedly moiftened by thefea-water and dried, and lixiviating it in refer-roirs, where it forms a very ftrong brine, whichis then boiled down as ufual. This way ismuch pradifed on the weftern coaft of France ,particularly in Lower Normandy , and at the iflesof Oleron and Rhe. s

The fpot being chofen (which fhould be on alevel fhore with a clean fand) the neceflarybuildings are ereded namely, evaporating pans,ltore-houfes, covered {beds, &c. and an areaof three or four acres is feleded a little belowthe level of the fpring tides and above the neap.The furface is carefully levelled by the plough,and rolled frnooth and hard. It is then filledto the heighth of feveral inches with fand takenfrom the edge of the fea at low water, and thefand is alfo drenched with fea-water as the tideflows in. It then lies expofed to the fun andwind, which foon diflipate the fuperfluouswater, and the furface of the fand appears co-vered with a white efflorefcence. It is thenturned over frequently with a kind of {hovel,changing the furface feveral times a day till thewhole is thoroughly dry. This faline fand isthen carried to the flieds, and the procefs re-peated with frefh fand till a large quantity iscollected, which generally employes the wholefummer. To make the l'alt, the dry fand istaken out of the flieds and thrown into fmallround pits about -2-f feet in diameter, and 12inches deep, the bottoms of which are linedwith hard rammed clay mixed with choppedltraw, to prevent the water from oozing through.The fand is then covered with fea-water, orwith the weaker ley of former operations, andafter {landing fome hours is drawn off intorefervoirs or barrels, whence the evaporatingpans are immediately fupplied. The fand islixiviated a fecond time, and this ley is refervedfor a frefh portion of fand. The boilers are oflead, about 3-* feet fquare, and 4 or 6 inchesdeep. They are heated with wood of any kind,or fometimes with reeds, and a boiler of thiskind is worked off in from two to three liours.The fait is raked out as it forms, and drained inhollow cones, as in other places. Three pans ofthis dimenfion yield together about 5olbs. of fait.

The fait procured in this way is white andfmall grained, but it is very apt to be damp,and is a weak-bodied fait little fit for prefervinganimal food for any length of time.

In fome northern countries fome advantageis made of the effect of cold in concentratingbrine, by freezing at firfl only the more waterypart, of courfe leaving the unfrozen part pro-portionably richer in fait. 'The winters of thiscountry are not cold enough in general for thispurpofe, but it is ufed occafionally on the Balticcoafts. The cold however mult not be toointenfe, otherwife the brine itfelf freezes. Fro-zen fait water is not in hard folid maffes likefrefh water, but it is foft and crumbly or rotten.The efficacy of this method of concentratingbrine is very confiderable.

Though much of the French fea-falt is veryindifferent, the Dutch refine it into a verv ex-cellent fait, which is ufed in pickling the"her-rings for which this nation is fo juftly famous,and which trade is under the ftrideft infpedionas to the goodnefs of the fait, and the care tobe taken in every ftep of the bufinefs. It is notprecifely known whether the Dutch do anything more than boil the fait again and evaporate{lowly, except that it is the conftant cuftomwhen the brine begins to fait, to add a fmallquantity of very four whey, which is found byexperience to be a very important addition.

* * * *

It has been fuppofed by fome eminent obfer-vers that when a confiderable depth of brinenot faturated, remains at reft for a time, thereis a gradual but invifible fubfidence of the fait,fo that the lower part of the column of brinewill be fenfibly falter than the upper. To thishas been attributed the increafe of faltnefs in-the fea, in proportion to the depth whence thewater is taken. With regard to land brine-fprings, it is indeed certain that the deepeft areufually much the falteft, but this may be readilyaccounted for from the conftant infiltration offrefh water which almoft always lies above thefait rock, and fometimes in vaft quantity.Hence it is neceflary in molt places (as in.Chefhire for example) to line the tides of thebrine well with very ftrong planking, and eventhis cannot entirely keep out the frefli water,,which always finds fome way in.

But an experiment of Haller {hews that thereis actually a fubfidence of fait, or at leaft fome-thing takes place which produces a fimilar

* Journ. de> Mines. No. 7, j>. 61. and Iincy. Meth. Arts & Metiers, Article Su/inn.