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SUL ( 385 )

without decompofmg It unlefs the quantity ofacid be very large.

On evaporating any of the folutions of fele-nite in acids, and allowing the folution to coolwhen highly concentrated, this fait feparates inlong tranfparent crydals, which is a very com-mon form for it to occur in chemical analyfes.It is then didinguiflied by its want of tade, itsdifficult folubility when removed into purewater, and by its affording a precipitate withthe oxalic and barytic falts. When a liquorcontaining felenite is rapidly evaporated, thisfait feparates as a brittle fhining pellicle.

When the crydallized fulphat of lime, nativeor artificial, is calcined in a red heat, it fplits,and falls into a mealy powder by the lofs of itswater of crydallization. This forms the plafterof Paris fo much employed in modelling andcements, as it has the property when mixedwith water to the confidence of a thin pafte, offolidifying in a few minutes, and becoming ahard brittle mafs.

Selenite, like all the other fu(phats, is infolu-ble in alcohol, and hence this fluid is a veryufeful tell to detect the prefence of felenite inany watery folution, and to feparate it withoutdecompofition. Mr. Kirwan* found the powerof this ted to be fo ex ten five that felenite maybe compleatly precipitated from water thatcontains it in the proportion of T oVo> or evenlefs, by any fpirit of wine whofe fpecific gra-vity is below 0.850. Hence much advantagemay be made of alcohol in analyfes of com-pounds that contain this fait.

Sulphat of lime is decompofed by the fixedalkalies, by lime, and by barytes, efpecially in aboiling heat, and dill more readily by the alka-line carbonats. For this purpofe reduce thefulphat to fine powder and boil it for a quarteror half an hour, with twice its weight of drycarbonat of potafh or foda diffolved in water,and a compleat decompofition will be effe£fed,the liquor will contain fulphat of potafh orfoda, and at the bottom will be the lime ofthe felenite, fully carbonated, from which thepure lime may be procured by compleat cal-cination in a platina crucible, or the proportionof lime may be edimated from the knownconftituent parts of the artificial carbonat,which, when dried and ignited, contains 55 percent, of lime.

Sulphat of lime when expofed to a very in-terne heat melts into an opake white vitreousmafs. When heated with charcoal, it is par-* On Mineral Acids. F £

tially and imperfeftly decompofed, and fomefulphuret is formed.

The compofition of fulphat of lime is givenwith fome variety, by different chemids.

Mr. Kirwan found that 439 grs. of a dilutefulphuric acid (compofed of 225 grs. of drongacid of 1.5654 fp. gr. at 6o° mixed with 225grs. of water) faturated 152 grs. of marble,which contained 83.6 grs. of lime. The abovequantity of dilute acid contained 119*5 °* ren ^acid (for the explanation of which fee the arti-cle Sulphuric Acid ) and confequently the acidand lime are to each other as 119.54: 83.6as 58.84 of acid to 41.15 of lime. But it isfcarcely poifible to obtain thefe quantities fromtoo parts of felenite, unlefs after very long andviolent ignition, on account of the obdinacywith which water adheres to the fait. Thiscircumdance too throws fome difficulty in theanalyfis of this fait, for if roo parts, for ex-ample, be merely ignited for a fhort time, andthe lime feparated accurately, if the entireweight remaining to make up the 100 parts beconsidered as real acid, it would give too higha proportion of the latter, fince felenite thusdried generally contains at lead 5 or 6 percent, of water.

Thus Klaproth obtained 26.5 grs. of ignitedcarbonat of lime from 40 grs. of ignited fele-nite, which would be equivalent to 36.87 ofpure lime from too of the felenite; but if allthe 63.13 remaining were coiifideved as realacid, it would probably make it much too high.

The edimation given by Mr. Chenevix ofthe proportions of fulphuric acid and lime inthe fulphat very drongly ignited, and the modeof making the experiment, have been alreadymentioned in the preceding article of Sulphat ofBarytes. Dr. Thomfon, p on repealing Mr. C’sexperiment obtained widely differing refuits,being 56 of acid and 42 of lime, whereas,Mr. C. obtained 43 of acid and 57 of lime.As Dr. Thomfon’s experiment much moreclofely coincides with thofe of Kirwan, Klap-roth , Bergman, and Wenzel, we cannot helpconfidering it as more correct, or at lead as moreapproaching to the point of mutual faturationof the acid and bafe. From all that has beendone, however, it is clear that when this ful-phat occurs in chemical analyfis it is not fafe toedimate its compofition from any afiumed pro-portions, but it fnould be decompofed by boil-ing with an alkali in the way already menti-oned, by which the quantity of lime may be

Hem of Chcmiftry, Yol. ii. p. 355.

3 c

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