Page:Cyclopaedia, Chambers - Supplement, Volume 1.djvu/123

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metallic nature, tho' that metal be often contained in the Alum ftone. That the fulphur is the chief efficient and ma- terial caufc of this production, feems to appear hence, that many Alum flones diftilled per defcenfum, yield good brim- ltone, and all Alum (tones during calcination, emit a fulphu- reous fteam. An inquifitive naturalift gathered from the very fame rock, and that within a few inches of each other, both vitriol, Alum, and fulphur, all of them excellent in their kind. Indeed thofe three minerals are fo nearly allied, that an ingenious chemift allures us, he can by fome artifices con- vert Alum into vitriol, or vitriol into Alum, the fame to all intents and purpofes, with the natural, Phil. Tranf. N°. 104. p. 71. feq.

Alum is ranked by Homberg, and others, as an urinous neu- tral fair* on account of the urinous fmell it exhales by bomingj and its ufe in the volatilization of fixed falts. Yet fome deny Alum to belong to the clafsoF falts; and rank it rather among ftony fubftances ; by reafon that after difiblving Alum, and precipitating the folution with oil of tartar per dcUquinm, the coagulation at the bottom, refembles a ftony calx, and being expofed to the fire will neither melt nor fublime. Janet Chym. Kxper. Tit. Alum. Week. Mem. Ingen. N°. r 5. p. go. The ore of Alum, if mature, yields its fait immediately, and without trouble ; but if lefs mature, it requires a previous calcination; as is the cafe in many of our Engiifh Alum ftones ; and if very crude or immature, it null not only be burnt, but a long time expofed to the air before it will yield its felt- From this it appears, that the Alum is riot a genuine and native fait, but is compofed by the acid of fulphur, cor- roding fome peculiar earth or ftone, as vitriol is by its cor- roding fome metal ; and that, in both thefc operations, this fcorroding acid can fomctimes perform its bufmefs, while it lies m -the bowels of the earth ; and fometimes while it lies m the open air, tho' it failed of it while buried. Phllof. I ranf. N°. 104.

Alum diftilled into an acid fpirit, with copper, or iron; be- comes good vitriol ; and vitriol freed from its metallick parts becomes aluminous ; and, being diftilled, yields a fpirit not to be ihftinguifhed by the tafte from that of Alum, and even fcarce by the moft accurate fcrutiny. Reflified oil of vitriol, or fpi- rit of fulphur, of the fame degree of ftrength, will fometimes concrete into a folid and tranfparent fubftance, refembling cryftallized Alum ; and this fubftance is no way different whe° ther prepared by one or the other of thefe ways, and in both refembles the pureft Alum, fo as not to be diftinguifliable from it unlefs by farting it.

Alum Ores generally contain vitriol as weli as Alum, and ate capable of great fermentation, when expofed fo the air, tho' they would never have been fubjea to it while buried in the earth. They will become fo hot in the heap, that it is fcarce poffible to endure the hand upon them ; and fometimes will break out into abfolute flame. The acid and the fulphur they contain arc the occafion of this, as, according to Symp- lon and fome others, they are of all fubterranean fermenta- tions and heats ; and the whole is not badly explained, by the familiar inftance of adding water to reaified oil of vi- triol. The acid and the fulphur of that fluid are, as in thefe ores, fo combined as not to exert their forces naturally againft one another; but all is quiet as in thefe ftones while under- ground, but the water does to the one, what the air does to the other ; fends in a third, which not agreeing with either of the other, difturbs and fets their particles in motion, and the veffc) containing the liquid becomes as hot to the touch in the one cafe, as the folids themfelves are in the other. Syatp- fin, Zymolog. Chym.

The Alum found in fome parts of the world pure and free from all admixture, might be eafily miftaken for a native fait ; but this has only been wattled out by water from its ore, and afterwards left, on the evaporation of that water, in a dry form. This, tho' unknown among us, is a very com- mon appearance of this fait in many parts of the world. All the Mum we ufe is feparatcd by art from various fubftances : in Italy from a foft reddiih ftone ; and about Puteoli, from various earths. In the iflands of the Archipelago, white earths and ftones abound with it ; and in England; we have it In a bluifh, or whitilh ftone ; and in great plenty in the fubftance called Irifh (late. Hill's Hift. of Fof. p. 389, to 393; The feveral ores of Alum are, all fubjedt, at times, to be co- vered with an efflorefcence of the fait, in a fhapelefs form. But the iflands of the Archipelago are the places where Alum is found in the greateft purity; and greateft variety of forms ; and are the places whence the anticnts had, and where We are therefore to expert to find, the forts they ufed, arid have given us accounts of.

In the ifland of Melo, and fome other places, there is found a vaft quantity of that kind they called Alumen plmnofum, or feathered Alum. This is a finely ftriated fait, and is an efflo- refcence from the Alum rocks in -thofe places. Liquid Alum is alfo found in great plenty in the many caverns in thofe iflands, and is no other than a folution of the Alumen plu- mofum and other efflorefcene'es of Alum, which cruft over the fides of thofe fubterranean caves, made in the water, with which their roofs and fides are continually dropping. And thefc waters, collefted in the fmall cavities in the floors

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of the fame grottoes, and about their entrances, when evaporated, leave behind the Alum which had been diflblved n them, which is then found in fmall lumps in thefe little holes, and is what the antients called round Alum, and what might eafily be miftaken for a native fait. The liquid, plumofe, and [round Alum, mentioned by the antients, might therefore be had at tins time, ifwedefired it: but they are not known among us, only that we fometimes hear of the name of Alu- men plumofuvu In the German fllops, in the place of this; however, they keep a fort of amianthus, which has no one character or property of Alum ; and it is very happy for the world,^ that no phyfician there ever thought of prefcribin^ it, as it might be of very dangerous coufequence,to take that amian- thus inwardly.

To dikover Alum among vitriol, or vitriol among Alum, Mr. Wry ufed to mix them with oil of tartar per deUquium , this deftroy.ng the power of the acid, breaks the combination of parts which formed the concrete, and immediately pre- tip.tates that fubftance, which by mixing with the acid con- itituted that particular body : this in Alum is a white earth and in vitriol is that metal which the vitriol happened to par! take of, be that iron, copper, or whatever. There is indeed a much more expeditious and ready method than this, which is to throw a piece of the matter to be tried upon a burning charcoal, and after the fwelling and evaporation occafioned by the heat, there will remain a white ipot on the coal, which is the white earth of the fait, if there were any Alum there, and if there were not; this would not happen. Some curious perfons have doubted, whether the white falts extrafled from vitriol by Mr. Lemery, and by him faid to be Alum, Were really Alum or riot, fmce they did not give this charaaer on the trial. Mr. Lemery had four of thefe falts, or Alums, the one ob- tained from the caput mortuum of fix pounds of Alum, which had been diftilled ; the other three he had prepared from the Caput tnortuum of Engiifh, of German, and of the native white vitriol The two firft of thefe fhewed no action at all upon the charcoal, but remained wholly unmoved there • the other two fwelled, and had all the changes that were to be expeaed. It is plain, from thefe experiments that this tell by the charcoal is a very uncertain one, fince it fometimes tails ; and m this particular inftance it failed, in regard to that fait which was inconteftably Alum, while it fucceeded with other falts which did not appear to be fo evidently fuch ; and the oil of tartar, which aaed evidently on all, is there- fore to be greatly preferred as a teft. It is eafy to conceive, that the fwelhng and buffering up of Alum, when laid upon a burning charcoal, is owing to a vifcous aqueous humidity in it, and perhaps a fulphureous one alfo, which when heated and ranfied makes thefe efforts to difengage itfelf from the mineral fubftance which it was before united with ; and by means of thefe, the acid is by degrees freed from the earthy matter; and evaporates, leaving that only behind, which being a white calcarious earth, makes the white fpot upon the coal, which is the criterion of this trial. But it is to be ° b [ ervcd > *at this humidity, which caufes the fwelling and bubbling of the matter, is not eflential to Alum; it is only its acid and white earth which are fo ; aTid it is equally certain from experiment, that the degree of the combinations of thefe fubftances is by no means fixed, but the proportion of the one to the other is much greater in fome Alums than in others ; and feveral fubftances may be all equally Alum, and yet may all contain this foreign matter in different proportions ; whether that be owing naturally to their combinations in the bowels of the earth, or be brought on by the different de- grees of calcination ; nay, the fame calcination made with the fame fire, and in the fame furnace, may according to different accidents, have very different effeas on feveral par- cels of Alum. Therefore the trial by charcoal, which does not aa upon any of the effential parts of Alum, muft be a trial of a very inferior kind to that by oil of tartar, which a3s immediately on that which conftitutes the very effence of this fait;

The decompoiition of Alum, which is riiade on the charcoal is however very perfea, in comparifon of that made by the ordinary way of clofe diftillation in the retort, as well as more eafy and expeditious : the firft is the work of a few minutes; whereas in the other, after the phlegm has been driven ofF by a fand heat,' it requires to be kept in a violent wood fire feventy two hours, in order to have if only two thirds decompofed. The account Mr. Lemery gives of this, is, that the operations differ greatly, in that the firft is performed in the open air, the latter in clofe vefiels. When the fire acts upon any body, fo as to diflodge from it thofe particles which are ready for feparatiorl, it is neceflary for the completion of this, that they have fome whither to go, and a place to receive them, where fomething elfe will give way to them In clofe vefiels, the little air that they contain is by no means difpofed to give place to the evaporations of a heated body - and the greateft effea that can be expeaed here muft be eitier from thevelfels not being firmly enough luted, or from the eva- poration of fome particles, much finer and more fubtile than the air itfelf, thro' the pores of the glafs, (if air itfelf can Be rendered by heat capable of fuch a minute divifion of its particles, and

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