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donald's patents, but we believe the profits have not yet an- 

 fwered the expeiicc. 



The n.-itiire of alum, and confcqucntly the true theory of 

 its manxifafturc, has only been known fince the publication 

 of Vauquclin's excellent memoir on the fiibjed in the An- 

 nales de Chimie ; it is not furprifmg, therefore, that all the 

 long-ellabliflied procelTes (hould be more or lefs dcfcrtive. 

 Perhaps the following method would be found to combine 

 more advantages, and be fubjefl to fewer inconveniences than 

 any which has been hitherto put into praftice. The ore 

 (hould be firll (lightly roafted with coal to drive off the bitu- 

 men, and forward the dccompofition of the pyrites, which 

 maybe further accelerated by moderate waterings, and expo- 

 fure of frefh furfaccs to the aftion of the air. When faline 

 efflorefcences appear at the top of the heaps of ore, and 

 their interior, upon being dug into, alfo feems penetrated 

 with white faline particles, let the ore be difpofed in alter- 

 nate ftrata with coal, and again roailed, fo as to decompofe 

 as much as poffible of the lulphatcd iron, and combine the 

 acid with the clay ; the flower and more gently this procefs 

 can be carried on, the more completely will its objeA be an- 

 fwered. The lixivium obtained from this roafted ore will 

 confift chiefly of fulphated alumine, nearly faturated with 

 earth, but, on account of the ableuce of potafh, perfeAIy 

 foluble. By the fubfequent boiling and agitation, part of 

 the fulphat of iron would be dcconipofed, and this oxydation 

 of the iron might perhaps be ftill further effected, by pouring 

 the liquor through heaps of faggots, expofed to the wind, as 

 is. done in the hotifes of graduation for brine in France and Ger- 

 many. The ferruginous and felenitic fediments being now 

 allowed to fettle, tiie clear liquor ought to be transferred 

 into another refervoir, and there mixed with a hot folution of 

 acidulous fulphat of potafh, fueh as remains after the diftil- 

 lation of aquafortis from nitre and fulphuric acid ; cryilals 

 will be immediately depofited of an alum much purer than 

 common ; and thefe, by a further clarification, may be made 

 equal to that of La Tolfa. 



§ J. McuiufaBurc of Alum ly Chaptal's procefs. 



An attempt had been made, but with little fuccefs, at the 

 manufaftory of JavcUe near Paris, to prepare alum by the di- 

 rect combination of its conftituent principles ; but it was not 

 till the admirable and deciflve experiments, in the large way, 

 by Chaptal, publilhed by him in the genuine fpirit of philo- 

 fophic liberality, that the praiticability of this method could 

 be faid to be eilabliflied. According to the modern way of 

 preparing sulphuric acid, the rcquifite proportions of 

 fulphur and nitre being mixed together, are brought to com- 

 builion in a clofed chamber lined with lead ; tlie fulphur is 

 thus acidified and converted into vapour, which by degrees 

 unites with the water that overfpreads the floor of the cham- 

 ber, and forms a liquid, diluted, fulphuric acid. A fimilar 

 procefs was inftituted by Chaptal, only fubftituting dried 

 clay for the water ; the rcfult of which was fo favourable, 

 that a large manufactory on the fame plan was ftt on foot ; 

 which, having continued in full activity for feveral years, and 

 producing alum only inferior to that of La Tolfa, merits 

 a particular defcription. 



The chamber in which the combuftion is performed is 91 

 feet long, 48 feet wide, and 3 l feet in height to the pitch of 

 the roof. The walls are of common mafoniy, hned with a 

 moderately thick coating of white plafter ; the floor is a 

 pavement of bricks, fet in a mortar, compofed of baked and 

 unbaked clay ; and this firft pavement is covered by a fecond, 

 in which the bricks are made to overlie the joints of the 

 lower ones, and are themfelves firmly connected to each 

 Other by a cement:, compofed of equal parts of pitch, tikr- 



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pentine, and wax, made boiling hoi, t\nd poured bctw<en 

 the joiuls inllcad of mortar. Tlic loof is of wood, and the 

 beams are fet at much lifa dillanccs than common ; they arc 

 alfo channelled with deep longitudinal grooves, fur ihc pur- 

 pofe of receiving the pLuikn that fdl up the fpaee between 

 the beams ; fo that the whole of this great area of tarpcolry 

 does not prcfcut a fingle nail. The chamber thut con- 

 llruCted was covered on the fides and top with a layer of the 

 cement jull mentioned, applied as hot as puflibic fo at to pe- 

 netrate mto all the pores of the wood and plaller; three more 

 fucccffive layers were then laid on, and the laft was pohbrd 

 fo as to prefcnt an uniform, even, folid face. In order to 

 prevent the wood-work of the citling from warping, it u'at 

 covered on the outfide with a thick coating of cement, and a 

 light roof of tiles was laid over the whole. By fubltituling 

 this cement for a lining of lead, a vail faving was cffcfttd in 

 the firft expcnce ; and it has been found, by long experience, 

 to require much fewer repairs than even had ilfclf. 



The clay o\ight to he of the purcft kind, fueh as pipe-ebvi 

 that it may contain ueither lime nor magnefia, and as little 

 as poflible of iron. It is to be tempered with water, and 

 made into balls five or fix inches in diameter; thcfc being 

 dried in the fun, arc afterwards calcined in a furnace j the 

 firft effeil of the heat is to blacken them, but foon after they 

 become red hot, the carbonaceous matter which caufcs the 

 blacknefs is burnt out. Being thus withdrawn from the fire 

 and cooled, they arc broken down into fuiall fragmenti, 

 and fpread on the floor of the chamber. In this ftate they 

 are expofed to the vapour of fulphuric acid from the cora- 

 builion of iulphur and nitre ; and in a few days the pieces 

 arc obferved to crack and open, and to be penetratca witl( 

 llender faline crjllals. The earth being at length covered 

 with efflorefcences, it is removed from the chamber, and ex- 

 pofed to the air under fheltcr of a Ihed, that tlie acid may 

 obtain its higheft degree of oxygenation, and become tho- 

 roughly united with the earth. It is now lixiviated, and the 

 liquor contains, in fiilution, little elfe than acidulous fulphat 

 of nlumine : this being boiled down to the proper coufiftcncc, 

 a folution of fulphated potafli (being the refidue in the poti 

 of combuliion from which the fulphuric acid was produced 

 in the chamber, and confifting of tlu; alkaline bafe of the 

 nitre combined with fome of the fulphuric a«id) is poured in, 

 and the liquor being then transferred into a large vat, per- 

 fect cr)'ll;js of alum are ftiortly depofited, whicli are after- 

 wards relincd in the ufual manner. 



The advantages of this procefs arc numerous. It may be 

 canied on whenever a fupply of proper clay can be had. The 

 fpace taken up by the works is mucli lefs extenfive thao 

 what is required according to the common methods. The 

 whole maiuifacture is perfonned in at moft one-third of the 

 time ufually nccelTary. A large quantity of fuel is laved. 

 The extraneous falts in the mother-water are fewer; an im- 

 portant ufe is made of the refidual fulphat of potafh ; and 

 hiftly, the alum itfclf is much purer, and almofl equally well 

 adapted to fix the dehcate dyes as that of La" Tolfa, the 

 commercial price of wliich is generally about double that of 

 the Englifh alum. 



§ 6. Bruttfwlck jllum. 



The dilute r«d colour of the roch alum, and the flefh-co- 

 loured efflorefcences with which its crj'ftals are covered, being 

 itsdiflinguilhing charader among the tnerchantt, occaiioncd 

 two brothers of the name of Gravenhorft to manufadure, 

 fome years ago, a fpurious imitation of it at Brunfwick. Wc 

 know not whether the manufaiiture is ftill carried on or not : 

 but if it if, the public will be benefited by the communi- 

 cation oi 30 eafy method «f dctedtiug the cotuilerfcit, more 



efpecially 



