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fold at a high price) of fubftituting for it Glau-ber’s fait, made by ftirring a warm faturated fo-lution of it till nearly cold, whereby it feparatedin fmall needled cryltals, very clofely refemblingthe form of the true Epfom fait. ' On the otherhand, when Glauber's fait was the moil in re-queft in this country, the Epfom fait makersufed to obtain the latter fa t in large cryilaisimitating the fuiphat of foda, by adding an ex-cefs of fulphuric acid, and probably by a flowerevaporation. Both thefe falts were occafionallycalled in medicine Sal Mirabile , and as themedicinal effe£t was the fame, this alfo furthercontributed to keep up the miilake. Thediitinftion between the two is now known tobe perfectly Ample, for the addition of anycarbonated fixed alkali will immediately rendera folution of fuiphat of magnefia turbid, butwill not alter the tranfparency of fuiphat offoda. The folution Ihould be fomewhat di-luted.

The method of preparing fuiphat of magnefia,from the bittern of fea-water, has been variouilyHated. It is ftrongly affirmed by fome, that noaddition is made to it, but that the whole pro-cefs is merely that of evaporating the liquor toa very high point of concentration, and oncooling, the fuiphat cryftallizes, which is puri-fied by a fecond folution and cryftallization.On the other hand it is ftated, (and, we mullacknowledge, with much more probability) thatfuiphat of iron, or the mother water of vitriol,or fulphuric acid, in fome form or other, isadded to the bittern. If Bergman’s analyfis offea-water is correcl, this addition is neceflary ;for, according to this eminent chemift, fea-water contains, in the Engliih wine pint, 186.5grains of muriat of foda, 51 grs. of muriatedmagnefia, and 6 grs of fuiphat of lime. There-fore, as during the evaporation only muriat offoda, with part of the fuiphat of lime, are fe-parated, the bittern, or refidue after the faithas been chiefly extracted, mud contain a largequantity of muriat of magnefia, fome commonfait, and only a very fmall portion of fuiphat oflime ; and, confequently, an addition of ful-phuric acid, in fome form or other, is requiredto decompofe themagnefian muriat.

Sulphat of magnefia has a fait and bittertafte, followed, when taken in quantity, with aflightly fweetifh flavour. The regular form ofthis fait is a four-fided prifm, terminated byfour-fided pyramids; but it is only obtainedregular by the flow and infenfible evapora-

tion of a faturated folution. In general it is inthe form of fmall white needled cryftals, pro-duced by the cooling of the hot faturated folu-tion. The common fuiphat is confiderably de-liquefcent, but this is probably owing to theadmixture of fome muriat of magnefia or lime ;for the large regular cryftals of this fuiphat arepermanent in the air, and fomewhat efflorefcewhen the atmofphere is dry and warm.

This fait is extremely foluble in water. Theprecife degree of folubiiity in fome meafuredepends on its purity : the pure fait diflblves innearly its own weight of cold water, and byincreafing the heat much more is taken up.The fait, when heated alone, melts in its ownwater of cryftallization, of which it contain*about 48 per cent,* after which it becomes abard opake mafs, which will bear a very ftrongheat without melting or parting with any ofits acid; fo that it is again equally refoluble inwater. In a very intenfe heat, or before theblow-pipe, it melts into an opake mafs.

This fait is not decompofed by any Angle acid. When the fixed alkalies are added to itsfolution, the magnefia is feparated pure; butwith the carbonated fixed alkalies, carbonat ofmagnefia is precipitated, as has been deferibedunder that article.

There does not appear to be any proper fuper-fulphat of magnefia, though the very largecryftals of this fait are faid to be made by flowiycryflallizing the folution with excefs of acid.If fulphuric acid is added to dry fuiphat of mag-nefia, the whole of the excefs of acid is volati-lized at a low red heat, without difficulty.

The a£lion of ammonia or its carbonat, onthis fait, will be mentioned in the next article.

Sulphat of magnefia, when well cryflallized,is eflimated by Lergman to contain 19 per cent,of magnefia, 33 of real fulphuric acid, and 48'of water. Mr, Kirwan eftimates that 100 partsof the fait, when perfectly dried at a ftrongheat, contain 36.68 of magnefia, and 63.32of acid, which very clofely agrees with theeftimation of Ber.man.

Sulphat of magnefia is frequently, in thecourie of chemical analyfis, mixed with fuiphatof lime, and often with fuiphat of iron, andthe feparation of thefe falts is of importance incoming at accurate refults. Several methodsmay be ufed. The following will feparate theiron : if the mixture of thefe falts be in folu-tion, and any cafual excefs of acid has beenfaturated, the addition of fuccinat of ammonia

Bergman on Magnefia.