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■will precipitate tlie iron, and leave the earthyfalts untouched : or elfe the mixed falts may bedried and calcined for an hour in a red heat,by which the. acid will be expelled from thefulphat of iron, and the fulphat of lime alfowill be rendered fcarcely foluble, whilft theaffufion of cold water, and digeftion for fomehours, -will completely diffolve out the fulphatof magnefia. If only the latter fait and fulphatof iron were prefent, the only infoluble refiduewill be the brown colcothar, or oxyd of iron,belonging to the fulphat, and from which thequantity of the latter may be eftimated.

The Sulphats of lime and of magnefia may befeparated in two methods. If the magnefianfait is very fmall in proportion to the other, theaddition of perfectly cauflic ammonia will fepa-rate the magnefia (though not totally) and leavethe lime untouched, provided the mixture bekept for fome hours in a well corked bottle, toexclude the carbonic acid of the atmofphere.But as it is more commonly required to feparatefulphat of lime from a larger quantity of fulphatof magnefia, advantage is taken of the verydifficult folubility of the former and the readyfolubility of the latter. The mixed folutionstherefore are to be concentrated highly by eva-poration ; when, after fome hours repofe, mollof the fulphat of lime will feparate alone, andmay be removed. The folution may then beevaporated to drynefs and ftrongly calcined,and then pulverized and digefted with three orfour times its weight of cold or lukewarm water,by which the whole of the magnefian fulphatwill be diffolved, and what little fulphat oflime may remain after the firft procefs, willremain untouched. The folution may then beboiled with carbonat of potalh, to decompofethe carbonat of magnefia, and the latter, whenwafhed, dried, and ignited in a ftrong redheat till it is fully calcined, will be pure mag-nefia. We may add, that when calcined mag-nefia contains any lime, it may be eafily detectedby rubbing a quantity of it with fome diftilledwater, putting the whole in a phial, and lhakingthe mixture frequently during digeftion for afew hours. On fubfidence, the clear liquorwill be a lime-water , if any lime be contained inthe magnefia, as may be difcovered by its be-coming turbid on blowing through it with thebreath, or adding a few drops of a carbonatedalkali; but if the magnefia be pure, the waterwill have diffolved nothing.

Sulphat of magnefia is largely ufed in medi-cine, both directly and for the preparation of

Magnefia and its Carbonat , the methods of pro-curing which have been defcribed under thefearticles.

Ammoniaco-Magnesian Sulphat.

When pure ammonia is added to fulphat dfmagnefia, a great part of the earth is precipitated,but the reft remains in folution, and, by evapo-ration, a triple fait, compounded of fulphuricacid, magnefia, and ammonia, is formed. Thisfait is ail'o procured more fpeedily, »aiid withlefs lofs, by adding to a folution of fulphat ofmagnefia one of fulphat of ammonia, each fome-what concentrated. An abundant depofition ofre.ular, tranfparent, fhining. cryftals takesplace immediately, which are the triple fait inqueftion. The form of this fait is-generallyo&ohedral f : its tafte is fait and bitter. Whenheated, it firft melts in its own water of cryftal-lization, after which it is decompofed, ammo-nia is firft given out, then acidulous fulphatofammonia rifes, and fimplefulphat of magnefiaremains.

This triple fait is much lefs foluble in waterthan either of the fulphats of which it is formed,as is obvious by its precipitating on the additionof the two folutions. It is compofed, accordingto Fourcroy, of 66 per cent, of fulphat of mag-nefia, and 32 of fulphat of ammonia.

Sulphat of Alumine.

The fimple combination of fulphuric acid andalumine, was often produced in the refearchesof the various eminent chemifts who have turnedtheir attention to the analyfis and manufactureof alum ; but a more accurate knowledge of itis due to Vauquelin. Under the article Alum ,we have fully detailed the many interefting ex-periments that have been made on this valuablelalt, and given the p oofs of the necefiity-of pot-afh and ammonia to its compofition.

Sulphat of alumine, therefore, differs from-alum in not containing any alkali, and on thisaccount it is fcarcely cryftailizable.

Sulphat of alumine is made by diltblving purealumine in equally pure fulphuric acid, at aboiling heat, evaporating the lolution to com--pleat drynefs, and then re-diffolving the refiduein water. In this Hate, if the previous deficca-tion has been compleat, an aftringent lolutionis obtained, which is not fenfibly acidulous, andby halty evaporation only gives a falinejnagma 5but when evaporated ilowly, and with greatcare, it may be made to appear in the form offoft, flexible, fhining crylLUine plates. But if

i Fourcroy.