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manufacture of fal ammoniac from fulphat ofammonia and common fait, which laft in thiscountry furnifhes the greater part of the Glau-ber’s fait in ufe.

This fait is alfo obtained by the decompofi-tion of muriat of foda by pyrites, and by gypfum,(as deferibed under that article) and is the in-termediate ftep in the manufacture of foda fromcommon fait.

Sulphat of foda in its common ftate formslong tranfparent cryftals, which are ufually fix-fided prifms with dihedral fummits. Some-times they are obtained of an enormous fize,nearly a foot in length, and an inch in breadth,but commonly their length is from one toabout -three inches. The tafte of fulphat offoda is fait, bitter, and unpleafant. Thi cryf-tals are foluble in lefs than three times theirweight of cold water, but at a boiling heat nowater is required, as they readily liquefy intheir water of cryftallization. Hence this faitis one of thofe that cryftallizes with the great-eft eafe from a hot concentrated folution.Confiderable cold is produced during the folu-tion of the cryftals in water. When the cryf-tals are heated they liquefy in their water ofcryftallization, and if the heat is continued tillthe water is entirely evaporated, a hard whitefaline mafs remains, which is the fait unalteredexcept by the lofs of water, fo that on resolu-tion it is obtained in cryftals as before. If thedry faline mafs be boiled with water till thelatter is fully faturated, and the folution pouredinto a thin phial whilft yet fealding hot, andimmediately corked perfectly tight, the liquordoes not folidify on cooling if kept quite atreft, but remains fluid and tranfparent. But onopening the cork, and, if neceffary, giving thebottle a gentle fhake, an inftant folidificationbegins at the furface, rendering it opake, andproceeds rapidly downwards to the bottom tillthe whole is compleatly congealed, and maybe inverted without fpilling a particle. A fen-fible warmth is felt on the outfide of the bottleat the time of congelation. The caufe of thisis the extrication of the caloric of liquefaction,as explained under the article Caloric, p. 207.

When this fait in its cryftallized ftate is ex-pofed to a dry air it foon becomes coveredwith a mealy-looking powder, and after a timewhole falls into the fame white powder,which confifts of the fait merely deprived of moftof its water of cryftallization by the aCtion of theair, and reduced to about half its former weight.

* Eflays, vol i. p. 282.

Hence the fait fltould be kept in well-cloiodbottles to preferve its cryftalline form.

The compofition of this fait may be inferredfrom the following fads. According to Klap-roth , 15 1000 grs. of the fait recently cryftallizedand dried on blotting paper were reduced bythorough drying on a fand-bath to.420 grains,whence the quantity of water of cryftallizationis 58 per cent. Alfo 100 grs. of the dried faitdiffolved in water, and decompofed by acetitedbarytes gave 168 grs. of dry fulphat of barytes,whence 42 grains of dry Glauber’s fait (thequantity contained in 100 of the cryftallizedfait) would give 70.56 grains of fulphat ofbarytes.

Laftly, from the refpeClive quantities of ful-phuric acid and foda required for mutual fatu-ration, and from the quantity of dry fulphat offoda thereby produced, Mr. Kirwan eftimatesthe proportions of ingredients in 100 parts ofthe cryftallized fait to be 58 of water, 23^52fulphuric acid, and 18.48 foda.

Some of the decompofitions of fulphat offoda by other falts are interefting. Of thefethe moft curious is that of the formation ofthis fait in a mixture of fulphat of magnefiaand muriat of foda, when expofed to a freezingtemperature, which was firft remarked byGren. c Thefe falts do not decompofe eachother in a temperature above freezing, but doit readily and almoft compleatly below thispoint. What is remarkable, this decompositionwhen once effected is permanent at any tempe-rature, for on railing the heat, Epfom fait andcommon fait are not regenerated as might havebeen expended.

Sulphat of foda is decompofed by muriat oflime, and as the fulphat of lime thence refult-ing is nearly infoluble, moft of it precipitates,and the folution contains little elfe than theregenerated muriat of foda, provided the quan-tities of each fait have been duly proportionedto each other. When muriated magnefia andfulphat of foda are in folution together, the ad-dition of lime caufes the following fucceflivedecompofitions, viz. the lime firft decompofesthe muriated magnefia, muriat of lime isformed and pure magnefia is precipitated ; andthe muriat of lime thus formed then decom-pofes the fulphat of foda, and muriat of fodaremains in folution, whilft fulphat of lime pre-cipitates and mixes with the magnefia, whichhas before fubfided. It is upon thefe two cir-cumftances, namely, the decompoiition of mu-

« Ac. Chim. tom. 24.

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