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effential difference between the common andglacial acid, is that the latter from the mode inwhich it is prepared contains a fmaller portionof water than the former, and that to this isowing both its volatility and property of conge-lation; it is incidentally mixed with fulphureousacid, but the prefence or abfence of this, doesnot appear to be of any material importance.

SULPHATS.

Under this article we {hall defcribe the faltsproduced by the union of fulphuric acid withthe alkalies and earths. The metallic fulphatsare defcribed under the feveral metals.

All the fulphats form cryftafizable faltswith thefe bafes, except with alumine, whichrequires the prefence of potafli or ammonia tobe able to cryftailize. The fulphats are fcarcelydecompofable by heat alone, but when fufed incontadl with charcoal or any carbonaceousmatter, they aie converted more or lefs com-pleatly into fulphurets , as has been defcribedunder that article. They are all infoluble inpure alcohol. The folutions of thefe falts are de-compofed by the other falts of barytes entirely,and by the falts of lime nearly fo, the acid form-ing a precipitate with thefe earths. A fimilardecompofition takes place when any fulphat isadded to the foluble falts of lead, filver, andother metals whole fulphats are little foluble.None of the fulphats are entirely decompofedat a moderate temperature by any other acid,the fulphuric Handing the higheft in the orderof affinity, with a very few exceptions. Butfome of the acids partially decompofe the ful-phats, and by taking part of their bafe they re-duce the remainder to the ftate of acid ful-phats. The nitric and muriatic acids are ofthis kind, and the tartareous, with regard to thefulphat of potafli. But the acids that are fixedin the fire, fuch as the boracic, phofphoric,and arfenic, decompofe the fulphats totally in ared heat long contained.

Sui.phat or Potash, or Vitriolated Tartar.

_ This fait may be formed by the direct com-bination of fulphuric acid and potafh, or car-bonat of potafh, but being of little value it isnever prepared in this way, as it is producedin the diltillation of nitric acid, being the refi-due of the decompofition of nitre by fulphuricacid. Phis refidue, which is a hard, white,acid, faline mafs, contains fulphat of potafli,generally with a little nitrous acid or with an

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excefs of fulphuric acid. It fhould be diffolvedto faturation in boiling water, nearly laturatedwith carbonat of potafli, in cafe the acid is inconfiderable excefs, and on cooling graduallythe fulphat of potafli feparates in final! cryflals.The whole cryftalline mafs taken out after theliquor is perfectly cold, and has flood at reftfor a day, is nothing but fulphat of potafli,which by re-foiution in warm water, and cool-ing more gradually, may be obtained in largercryftals. The form of this fait is generally thatof a fix-fided prifm terminated by as many py-ramids, like that of a quartz cryftal.

Sulpliat of potafli is the leaft foluble of allthe fulphats, requiring about fifteen times itsweight of cold water, but only five parts ofboiling water, fo that much of it readily fepa-rates from the hot folution when laturated.The fait is very hard and readily reducible topowder, and its tafte is faline and bitter. It isperfectly permanent in the air, neither deliquef-cing nor effervefcing.

On account of the fparing folubility of thisfait it is often feparated in the form of a precipi-tate from watery folutions as foon as it is formed.Thus if moderately concentrated fulphuric acidbe added to a ftrong folution of potafh orcarbonat of potafh, the mixture becomes almoftimmediately thickened with a large quantity ofhard white powder which is the fulphat of pot-afh formed and precipitated at the moment ofmixture.

This fait is one of the mod eafily diftin-guifhable of the faline fubftances, by its liard-nefs, little folubility, and bitter tafte, and ge-neral pyramidal form of its cryftals.

When a cryftal of fulphat of potafli is fud-denly heated it decrepitates, and fplits afunderwith fome violence. It then parts with itswater of cryftaliization which is not more than1.4 in ico, according to Dr. Thomfon. At ared heat it melts, but does not undergo furtheralteration.

According to Mr. Kirwan, 100 parts of thedry decrepitated fait, confift of 54.8 of potafli,and 45.2 of acid, which agrees very nearlywith the experiments of Dr. Black; and 100grs. of the fait diffolved in water, and precipi-tated by muriated barytes give 135.25 grs. ofignited fulphat of barytes. But on the otherhand, Dr. Thomfon, 1 found that 100 parts ofthe cryftallized fait loft 1.4 by fufion, which istherefore the water of cryftaliization, and thatthe refidue diffolved in water and precipitatedby muriated barytes gave only 128 parts ofChemiftry.

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SOL. II.