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cipitate gently dried becomes a perfectly whiteextremely light taftelefs powder, which is com-mon magnefia.

A double decompofition takes place in thisprocefs, the fulphuric acid quitting the fulphatof magnefia to unite with the potafh, and themagnefia uniting with the carbonic acid of thealkali employed. The fulphat of potafh beinga fait of very fparing folubility in water, rendersit neceffary to wafh the precipitate fo abundantly.The drying of the carbonat of magnefia istedious, for being exceffively light and fpungy,it retains a large portion of water, which can-not be feparaled by filtration. In the manu-facture in the large way, the palte of magnefiaand water is fpread on flabs of chalk, whichabforbs the water eagerly and much haftens thedrying.

Common carbonat of magnefia confifts ofcarbonic acid, water and magnefia in fomewhatvarying proportions. By being calcined forfome time in a red heat, both the water andcarbonic acid are expelled,, and the entire lofsby calcination is on an average about 55 percent. On the other hind, this carbonat whenadded to an acid, effervefces violently, hut onlyparts with its carbonic acid in the effervefcence,and this lofs is about 34 per cent, confequentlythe conftituent parts of 100 parts of the com-mon carbonat of magnefia will be, when driedin a gentle heat,

Magnefia - - - - 45Carbonic acid - - - 34W ater -----21

100

The quantity of alkali commonly ufed in ob-taining magnefia is more than is necelfary todecompofe the magnefian fulphat, and probablyone-fourth or one-fifth lefs would be fufficient.

Common magnefia is fcarcely at all foluble inpure water, but with an additional quantity ofcarbonic acid its. folubility increafes largely.Hence in making common magnefia it is advife-able to boil the ingredients on mixture, by whicha corifiderable quantity of loofely combined car-bonic acid is expelled, which otherwife wouldunite with part of the magnefia, render it folu-ble in Water, and it would be carried away inthe waffiings. Hence the more the alkali em-ployed is faturated with carbonic acid (beyondthe exadt dofe neceffary for the precipitatedmagnefia) the lefs will be the product lefton the filter, unlefs the boiling be continued alonger time.

This leads to the other diftinct fpecies offolid carbonat of magnefia, namely the cryflat-lized, or that which is faturated with carbonicacid. It may be prepared in feveral methods.If carbonated alkali be mixed with fulphat ofmagnefia, without heat, and the mixture left tofpontaneous evaporation, fin all long four-fidedprifmatic cryftals of carbonated magnefia willbe depofited. A neater and better way of pro-curing them is the following. Pafs a ftream ofcarbonic acid for fome time through water, inwhich common carbonated magnefia is fuf-pended. The agitation occafioned by the bub-bles of gas will prevent the magnefia fromentirely fubfiding, and it will abforb much ofthe carbonic acid and diffolve in the water.Decant the clear liquor, and fuffer it to evapo-rate in a heat not exceeding that of a warmroom. After a while the carbonated magnefiawill feparate in cryftals. This cryftallized faitis compofed of about

Magnefia - - - - 25Carbonic acid - - - 50Water ----- 25

100

It is foluble in 48 times its weight of cold water,and in much lefs when the water alfo has anexcefs of carbonic acid.

Carbonated magnefia in any ftate is decom-pofed readily and compleatly by potafh, foda,lime, barytes and ftrontian; the carbonic aciduniting with thefe fubftances and leaving themagnefia pure. It is alfo decompofed by allthe falts with the balls of barytes, ftrontian, andlime. Hence a common caution in preparingmagnefia, to wafh the precipitate with water aspure as poffible and free from calcareous falts jfor as molt fpring-waters contain fome fulphatof lime, this fait is decompofed by the carbonatof magnefia, and carbonat of lime is formed,which mixes with the magnefia. This is oflittle importance medicinally when the magnefiais ufed in its common ftate, but after calcinationthe earth contains a ftnall portion of quick-lime,the effect of which might be felt in large dofes.

Ammonia and magnefia appear to have verynearly equal affinities for carbonic acid, andhence it is that either of the two will only par-tially decompofe the faturated carbonat of theother. Thus if ammonia is added to cryftallizedcarbonat of magnefia in folution, the precipitateis not pure magnefia (which it would be withthe other alkalies) but the lub-carbonat, or com-mon magnefia alba, and vice verfa if pure mag-