ALU 



pure alumine. Thcfe, however, whicli are more properly the 

 niineralogical tlieiii chemical charaftcrs of aliiniinc, will be 

 treated ol morcat large iiuhe rubfcqucnt miiUTalof;icalanicles. 



Pure aKimiiic, in a llate proper for chemical experiment, 

 has hitherto never been foiu.d native, and it is only of late 

 that chemills have dilcovered the method of obtaining this 

 earth fiifficiently free from foreign admixture. The me- 

 thod of Bergman and his contemporaries was to decompofe 

 a folution of purified ciyllals of alum by an excefs of car- 

 bonated potafh, or foda, and to waih the earthy precipitate 

 in repeated quantities of dillillcd water, till it came olf pcr- 

 fi clly tallelefs and pure ; a white uniform foft matter was 

 thus obtained, which was fuppofed to be carbonated alu- 

 nuiie, and this by drying in a heat below that of redncfs, 

 v.as deprived of its acid and water, and was then efteenied 

 pure alumine. The infuffieiency of this method had be- 

 gun to be fufpefted for fome time, however, particularly 

 rrom the appearance of fulphurated hydrogen, when alumine 

 thus purified was heated with charcoal, and aftcnvards 

 moillened w itli a diluted acid, and the admirable memoir of 

 Vauquelin on alum, (which has already been referred to 

 under that article) not only eftabliflied the validity of thcfe 

 fufpicions, but pointed out the method of avoiding the er- 

 rors of his predeccfTors, and thus introduced a very import- 

 ant improvement in the difficult art of chemical analyfis. 

 Alum has already been iliewn to be a triple compound of 

 alumine, potadi and fulphuric acid in excefs, and when this 

 excefs of acid is taken away, either by the addition of 

 alumine or of an alkali, an infohible fait is produced differ- 

 ing from alum only in the proportion of its earthy bafc ; now 

 the eaie with which a fait is decompofed depends very ma- 

 terially on its folubility, vvh.en, therefore, we add gradually 

 to a folution of alum a folution of carbonated alkali, the 

 fiift effeft is to neutralize the excefs of acid, and the preci- 

 pitate confifts principally of the infohible fait juft mention- 

 ed ; a further quantity of alkali, efpecially if affifled by 

 heat, will effcft the decompofition of part of the fait, but 

 in proportion as this takes place the rclidue becomes mixed 

 with the alumine, and thus is covered from the further ac- 

 tion of the alkali. This being the cafe, it is obvious that 

 no fubfequent wafliings can do more than fcparate the ful- 

 phated potaih ; and therefore the rtfidue, inttead of being 

 pure alumine, contains befides a variable proportion of 

 earthy alum, from which laft proceeds the fulphur obfer- 

 vable on heating it in a dole vcfTel with charcoal. 



The only way by which alum can be made to yield its 

 earth in a ftate of fufficient purity for delicate chemical 

 experiments, is the following. Take any quantity of Ro- 

 man alum, and diffulve it in lukewarm diililled water, filter 

 the folution, and fet it to cryHallize. When by cooling 

 and fpontancous evaporation, a fufficient portion of this 

 purified alum is depofited, take it out and rediffolve in cold 

 diililled water ; to this folution add liquid caullic ammonia, 

 a white precipitate will be thrown down, and continue the 

 gradual addition of ammonia till no farther precijiitation 

 takes place ; heat the liquor then nearly to boiling for a 

 few minutes, add more water, and throw the whole on a 

 paper filter ; in proportion as the fluid drains oft add water, 

 till it paffes through quite tafttlefs. The precipitate, while 

 yet in a pvdpy ftate, is to be removed into a fi:;(l<, and di- 

 gelled with muriatic acid till it is diffolved. The muriatic 

 folution being then concentrated by very gentle evapora- 

 tion, will at length dcpofit cryftals of alum, which are to 

 be removed, and this procefs is to be. continued till the li- 

 quor ceafts to yield anymore. Nothing now but pure alu- 

 mine remains in the folution, the potafh and fulphuric acid 

 being got rid of, at the expeiice of a Iktlc ot the alu- 



Voi. I. 



ALU 



mine in the crjrflals, the linuor !> therefiv? tl be dilnled 

 with water ; and ammonia fully fufficient for the deeompo. 

 fition of the muriated alutniiic being then added, the proceft 

 ot filtration and edulcomtiun is to be (rone through ai be- 

 fore, and the rcfult will be pure ahimir.c. Ou account of 

 the length of thib method, and the polTibility that even 

 after all a very minute proportion of fulphated potadi may ftill 

 remain, it liaj been the pratlice of late with X'auquclin and 

 Hertholitt to procure their pure aluininc fiom luch of the 

 natund clays as contain only fdex and alumine. hy dirertion 

 in muriatic acid and decoinj)ufition of the folution by am- 

 monia. 



Pure alumine, obtained by the above mcthodi, it opacjur, 

 of a fnow white colour, a fmooth lomcwhat unMuoun icci, 

 has no linell, even wlien breathed upon, or moiflciied wiiK 

 warm water, nor any proper taftc ; when placed upon the 

 tongue, however, it abfoibs ;J1 the moifture with which it 

 finds itfelf in contaii, and thus occafions a peculiar fenfa- 

 tion of allringency. It is readily difTufiblc, and remain* 

 for a long time fufpendcd in water, but appears to be to- 

 tally infoluble in this fluid. Its fpecific gravity i? varioutly 

 cilimated, according to the degree of deliccatiun, by Berg- 

 man it is reckoned 1. 30 J, while Kirwan allows it as much 

 as 2.0. After being thoroughly dried in a heat jiill not 

 luflicient to dellroy its plallicity, it is capable of ahforbing 

 2 J times its weight of water, without allowing any to 

 dropout, and the water thus mixed is retained more ob» 

 llinately at the ufiial atmofpheiic temperature by alumine 

 than by any other earth ; a freezing cold however caufeB 

 this earth to contrac^t remarkably, and thus fquce/e out a 

 large proportion of its water. 



Alumine is the only earth that poneffes the property of 

 plallicity, or of being kneaded up with water into a foft 

 dudilc paile, capable of being formed by the hand or the 

 potter's wheel intcf any fliape that may be required ; the 

 plallicity the-efore of all the natural clays ij owing to their 

 aluminous part ; nor is this property dellroyed even by a 

 veiy large adiiiixture of other earths ; in the finer kinds of 

 pottery fearcely a fourth of the whole mafs is pure alunune, 

 and yet its plallicity is unimpaired. If a piece of tempered 

 clay is dried gently in the air, it retains its form, but 

 becomes quite brittle ; its former ductility may, however, be 

 rellored by again kneading it with water. If expofed to a 

 red heat it hardens, contrails in all its diibenfions, become* 

 more Cumpaft, and of greater fpccilic gravity, and is no 

 longer plailic, nor can this property be rclVored to it by 

 any other means than by folution and precipitation ; hence 

 bricks or pottery ware, after having been baked, if pi.unded 

 ever io fine, are no more capable of forming a palle wiih water. 



The action of caloric on alumine is accompanied by fome 

 interefting phenomsna which deferve mentioning. If the 

 purell phillic alumine is expofed to a low red he-at, it becomes 

 of a bluiih black colour, efpecially on the infide, as is mani- 

 fell by breaking a piece acrofs that has been thus hrated ; 

 as foon as this colour is perceived the plailicity is dellroyed, 

 a fad that renders it probable Unit this property of alumine 

 depends on fomething elfc than mere water. 13y a further 

 iucreale of the heat with acccfs of air, the carboiiacrotis to- 

 louring matter is burnt out, ami the alumine acquires a ic- 

 fplendent white colour, becoming at the fame time harder, 

 deiifcr, and of lefs bulk : all thcfe changes .idvance in gra- 

 dual progreflion in proportion to the heat ; and after it ha* 

 thus experienced the full effect of our moll powrerful fur- 

 naces, it will be found to be fo hard as to give fire with 

 ilcel, and r'c'uccd to nearly out half of its original bulk. 

 Upon this lalt property is founded the ufe of IViJ^cwcod's 

 PYROMETER, for mcafuring the higher degrees of heat. The 

 5 K deiTcafa 



