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to cfcape the evaporation from the heated body) which will fill the Space leSt by fuch particles ; and thus the body will by flow degrees be in part at leaft decomposed, being continually di- verted of fomc of its principles, driven from it by fire. Vid. Mem. Acad. Scicnc. Par. 1736.
This fcems all that can be done in the diflillation by the re- tort ; and it is eafy to fee how flow and imperfect this muft needs be, compared with an evaporation in the open air, where there 'Is fpace eanly given to every particle, as it arifes or iflues from the Alum laid on the coal, and influenced by its heat. This however is not all ; for the immediate con- tact of the coal may eafdy be conceived to be of great fer- vice in the procefs, fincc the coal continually furniflies a fupply of Sulphureous or oily matter, which, Mr. Lemery has proved, is ever of great Service in the diSengaging and vo- latilizing of acids.
An accident which was difcovered Some time afterwards) in regard to the four Alums tried on the burning coal, the two with, and two without effect, which Served greatly to con- firm this reasoning in regard to the different effects of fire in the open air, and in clofe veffels. It was difcovered, that the veffels employed in the diftillations from thofe Alums had fucceeded differently in the fire, two of them having re- mained perfectly whole, and the other two having cracked in Several places. On enquiring to which of thefe the Several Alums, which had been the fubject of the Succeeding obfer- vaiions had belonged, it was found, that the two which an- swered to the experiment had been the refidue of the diflil- lation in the veffels which remained whole, while the Alums taken out of the two cracked retorts, had no change made Ul them, on the charcoal, but remained wholly unmoved and unaltered on it ; thefe having Suffered their ultimate decom- poSurc, in regard to thefe procefles, before, by means of their having had a communication with the open air, by theSe cracks ; while the other Alums, which had been urged in whole veffels, were yet fubject to all the natural changes from heat and an open air.
.Mr. Lemery has proved, that the common natural white vitriol is amixtbody, compounded of green vitriol and Alum. It is very certain that we are not able, by any known procefs, to make white vitriol from thefe ingredients ; but it is alfo certain, that tho' thefe two falls, being diffolved in the fame water, and afterwards cryftallized in the common method, form their cryftals quite feparate ; yet nature does many things, which art in vain attempts to imitate, and may cafily have effected this mixture in the bowels of the earth, which we in vain attempt by chymical veffels and by furnaces ; Since all trials prove the certainty of Mr. Lemery's aflertion, in regard to this body.
The Separation of the cryftals of vitriol and Alum, when we make the Solution of both together, is eafily accountable for, on this principle, that they do not begin to formthemfelves in the fluid at the fame time ; Alum being more difficultly Soluble in water than vitriol, muff be alfo more ready to form itfelf again into cryftals, and (hooting before the vitriol finds oc- cafion to Shoot, it cannot but be Separated from it, and fhoot alone. But we find that nature has found a nicer combination of thefe two Salts than we know how to make ; for tho' it is eaflly proved to demonftration, that white vitriol, as we See it, does contain both vitriol and Alum ; yet they arc So clofely combined, that, when the whole is diflblvcd, they cryftallize to- gether, and continue mixed. Mem. Acad. Scienc. Par. 1736. Mr. Geoffroy has entered more nicely into the origin of this Salt, than any other author. We know, that befide the ores which contain together vitriol, fulphur, and Alum, there are Some peculiar to Alum alone ; and moft writers on thefe Subjects have agreed that the baSis of Alum is white unvitrifiable earth, of the nature of chalk, which fixes the common vitriolick acid into the form of this Salt ; and Mr. Geoffroy has proved from experiment, that this earth, which is the baSis of this fait, is contained in no Small quantities in Several of the common foflil fubffances, as the boles, clays, and the like ; and many of thefe, even after burning, have furnifhed tkis curious enquirer with Alum, on the mixing them with oil of vitriol, or of Sulphur. When this is known, it will appear lefs ftrange, that glaSs fhould Sometimes afford Alum ; Since it may naturally be SuppoSed often to contain the fubftance of this earth, to which there needs only the joining this com- mon acid, to produce the fait j and to this was owing the re- markable quality of a certain coarfe glaSs, made Some years ago in France, the bottles made of which, fpoiled all the wine that was put into them, and that often in a very little time, by impregnating it with real Alum. Mem. Acad. Par. 1728. The method by which Mr. Geoffroy fucceeded beft in mak- ing his artificial Alum, was this. He took pieces of common earthen veffels, of the coarSer and more porous kind, and cauSed them to imbibe a large quantity of oil of vitriol, of which they would receive a larger quantity than the fame earths would when unburned, as their pores were now more open : the acid caufed Some effervefcence with them, and in fine became mucilaginous ; and this mucilage, expoSed to the air, afforded cryftals of pure Alum y of a regular figure, and of all the properties of that fait. Broken tobacco-pipes, wetted with Spirit of fulphur, in the Same manner afforded cryftals
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of Ahum \ and when the pipes had rcmanied dry a considerable" time, there (hot out from them a bcautiSuI and regular Alumen plumofum ; the Small remainder of the acid having had time to work in the pores of the fubftance, So as to afford this eS- fioreScence.
The Lapis Calaminaris, of Some kinds, alfo contains tins white earth, which is the bafisoS Alum; and the Same Mr. Geoffroy discovered this by accident, when having made a mixture oS this ftone with the vitriolick acid, in hopes of obtaining from it an artificial white vitriol, he procured inftead of what he hoped for, two different Salts, a green vitriol, and a pure and perfect Alum.
The procefs of making Alum at Whitby in Yorkfliire is thus defcribed by Mr. Ray, at the end oS his collection of En- glish words not generally ufed, p. 1 39. They take the mine or ore picked from the defle or rock, and laying it in great heaps, burn it with whins or wood, till it be white. When it is Sufficiently burnt, they barrow it into a pit, ten foot long, Six broad, and Seven fourths of a yard deep, where it is fteeped in water the fpace of eight or ten hours ; then they draw out the liquor, which is only a lixivium impregnated with the Alum mine, into troughs, by which it is conveyed to the Alum houfe, into a deep ciflern, about twenty yards in cir- cumference, and three yards and an half deep. After this firft water is drawn off the mine in the pits, they do not preSently caft away the minej but pour frefh water on it a Second time ; and after the Second water is drawn off, which is much weaker than the firft, they caft out the mine, and put in new, and pour on frefh water as before. Out of the ciftern they con- vey the lixivium by troughs into the pans, where it is boiled the fpace of twenty-four hours ordinarily. Then they take, the liquor out of the pans, and examine it by weight, to know how much lee made of kelp, it will require, which for the moSt part is fix inches of the pan's depth. This being put in, as Soon as the liquor boils or flows up, by the putting in oS an iron coal-rake, or other iron instrument, they draw it cfF into a Settler, and let it Stand about an hour, that the Sulphur and other dregs may Settle to the bottom ; which be- ing done, it is drawn off into coolers, where it continues about Sour days and nights. The cooler being drawn about half full, they pour into it a quantity of urine, viz. about eight gallons into a cooler that contains about two half tuns. Hav- ing thus flood about four days and nights, it is quite cool, and the Alum cryftalized to the fides of the cooler. Then they fcoope out the liquor, which they call the mother, into a ciftern, and put it into the pans again with new lixivium, to be evaporated by boiling, csV. The Alum that is Shotten and cryftalized on the Sides of the cooler they Scrape off, and wafh with fair Spring water, then throw it into a brag, where the water drains from it. Thence it is caft into a pan, called the roching pan, and there melted ; after which it is Scooped out, and conveyed by troughs into tuns, in which it flands about ten days, till it be perfectly cool and condenSed. 7 hen they unhoop and Stave the tuns, and taking out the Alum, chip it and carry it into the ftore-houSe. The exact proportion of kelp docs not appear in this procefs; for tho' the workmen told Mr. Ray, that the lee was Six inches of the pans depth, yet they did not tell him how deep the pans were. Phil. Tranf. N°. 142. p. 1054.
When a work is firft begun, they take Alum of the liquor only that comes from the pits of the mine, without any other ingredients; and So might continue, but that it would Spend So much liquor as not to quit coff.
As to the procefs oS making Alum at Civita Vecchia, See Hift. Acad. Scienc. 1702. p. 26. See alfo Phil. Tranf. N". 265. about the Alum at Solfatura.
Alum is alio prepared about Leipfic from a foffil earth, im- pregnated with aluminous principles.
This Some call common Alum, Alumen vulgar e, by way oS distinction from the Roche Alum.
The manner of preparing it is defcribed at large by Hoff- man, ObScrv. PhyS. Chym. 1. 3. ObS. 8. p. 302. See Burggr. Lex. Med. in voc.
Naturalifts Speak oS divers kinds of Alum, which may be re- duced to two, native and artificial. Native Alums are thoSc prepared and perfected under ground, by the Spontaneous operations oS nature, mixing the proper in- gredients, water, earth, and Salt, or Sulphur. Thefe arc either liquid, or Solid, and concrete. Liquid Alum appears to be the primitive kind, as being that out of which the reft are formed.
Liquid Alum is defcribed by Pliny, as formed oS water ex- uding out of the earth in winter, and maturated by the fun's heat in Summer. Plin. Hift. Nat. T. 2. 1. 35. c. 15. p. 716. See Liquid Alum. Solid, or concrete Alum is divided into Sciffilc Alum, called by the Greeks ^x^ r, i as being Soft and eafily Separable. This is Said to bear a near reSemblancc to our plumofe Alum, only differing in this, that the former inftead of whitifh, is. of a grecnifh hue. Some pretend to have done wonders with it againft haemorrhages-, looSneSs of the teeth, the itch,, ptery- gium, pernios, c^c. In Some of which its aluminous or aStringcnt quality fhould do more harm than good. Jumk. ConSp. Therap. Tab. 16. p. 453. Seq.
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