ALU 



It ia of a blui(K black colour, gcncrullv pnHiiig into t!ic 

 unn black — occurs amorphous, iii larj^c Arata. Tlic liillrc 

 of its parallel fracture is ftiiuing or even briplitly fhiuiiig, 

 with a luilre between common ami femi-mctallie : tliat of its 

 crofs fraCltue u dull, or at moll glimmeriii;:;. Fracture 

 thick and curved flaty, feldom thin (laty. Its frajjments 

 therefore are fometimes thick and fometimca thin (hiverv. 

 It feels fmooth ; is half hard ; brittle ; and but htllo 

 flallic. 



Both varieties are found in Norway, at Whitby in Eng- 

 land, in Sweden, in vSaxony, and various other provinces in 

 Germany. The alum of Great Britain and the north of 

 Europe is almofl entirely made of it, for wiiich ufe the fccond 

 variety is faid to be the bell adapted. It commonly 

 occurs in the neighbourhood of coal, and feems to differ 

 in no refped from the bituminous lliale impregnated with 

 pyTites. 



Species 6. Alum-caith — Pyritacrous clay, alnun erde 



Germ Arg'tUa alumitiarls bhiiminofa, Werner. 



AhinjariU Sweed — Tern alumineufe, I'r. — 

 TimJ'iis f'6!il, Hinig. 

 Tt lias a light or dark blackilh brown, brownifli black, 

 or blackifii grey colour. Occurs in large llrala of earthy 

 or irregularly flaty maffes. It is generally dull, but when 

 containing fcattered particles of mica, becomes occafionally 

 glimmering. Its fracture is between compaCl earthy and 

 iniperfeAly flaty. Its fragments arc partly flaky and partly 

 irregidarly blunt cornered. Its ilrtak has a feeble lullr?. 

 It is vei-y ioft, and may be rubbed to powder between the 

 fingers ; is brittle, and of veiy little tlallieity. 



When placed among burning coals, it generally blazes 

 a little ; and when moillened and expofed to the air in 

 large quantities, it heats and not unfrequently inflames. 

 From too parts of it, after torrefaftion , Klaproth obtained 

 to alum, 7.25 fulphated iron, 2.25 fulphatcd lime, and i 

 fulphated magnefia. 



It is found in alluvial and fecondary ftrata, and is inti- 

 mately connefted with bituminous wood, alum flate, and 

 coal fhale. Is ufed in the manufacture of alum in 

 Germany^ 



Lenz, Verfuch der Mineralien. — Wldenman, handbuch 

 der Mineralogie. — Lametherie, Theorle de la terre. — Berg- 

 man's Eflays — Klaproth's analytical Effays. — Ivirwan's 

 Mineralogy. 



ALUM, ManufaBure of. 

 In order to appreciate rightly the peculiar advantages or 

 difadvantages of the feveral methods of manufaAuring this 

 fait, it win be neceffaryto enter into a previous enquiiy con- 

 cerning the nature and proportions of its elements, and the 

 different chemical varieties of alum, which have hitherto 

 been confounded under the fame name. 



§ I . Anahfis and Comprjitlon of Alum. 



The identity of the earthy bafe of alum with pure clay, 

 was firfl afcertained by GeofTroy and Hellot, and the fuc- 

 cefTive experiments of Pott, Margraaf, and Macquer, upon 

 the fame fubjeft, put an end to the controverfy concerning 

 the nature of aluminous earth, which has ever fince been 

 univerfally received as the fame with pure clay or aluminc, 

 according to the reformed nomenclature. The acid in 

 alum has always been confidered as the fulphuric, and the 

 only qucftion among chemifls on this head is whether the 

 acid is necelTarily in excefs. A folution of alum reddens 

 litmus paper, and exhibits other properties of an uncom- 

 bined acid ; but on the other hand it is contended by 

 Morveau,that cryftallization and edulcoration would cfltclu- 



ALU 



«Ily feparale any fuch excef% and thorefj:c lli»t the ch.mjje 

 of vegetable culoiii-s it, uot aw uncquivm-al proof of fuuer- 

 abuudant acid. Rtfcrvin^r tlic cunfidrration of tliin and limi- 

 lar cafea till wc come to treat of the article !..\Tt'« ati<i!«i it 

 is fufTicient to obfcrve here, as iiidccd Bergman ha< clr«ily 

 (liewii, that the acid rxilh in alum witli two vcr)- different 

 degrees of affinity. By the ac\ion of iron filingj on a folu- 

 tion of alum, all the figns of uiicoinbincd or hxitly adhcr- 

 iiig acid arc dcftroyed, fulphat^;d iron is produced, and m 

 wliite earthy precipitate lakes place, confilling of the alum 

 deprived of a fmall portion of its acid, but Aill retaining 

 the greater part, as may be proved by the furtiicr dccum- 

 pofition of it by a caullic alkali ; and to this fupcrabtin- 

 dant or (lightly combined acid, is entirely owing the tadc, 

 the foltibility, and moll of the other external cliaraAcrs of 

 the fait. 



The component parts of alum, according to Bergman, 

 are 3S fulphuric acid, iS alumine, and 44 water of cryltaU 

 lizatioti. Obfci-ving, however, that thofc folutioiis, which 

 contained a great excefs of fulphuric acid could not be 

 brought to ciyllallization by the addition of lime, foda or 

 barytes ; but only by mcang of potafh or ammonia ; finding 

 alio lulphat of potalh in many fpccict of alum, he appears 

 often induced to believe that the alum of commerce is a triple 

 fait confiding of ftilphuric acid, alumine and pourti. The 

 fubjedl remained in this (late of uncert.iinty till it came under 

 the notice of the moll eminent analWl of modem times, the 

 accurate and indefatigable Vauquelin, to whofe admirable 

 Memoir on the combinations of alumine with fulphuiic acid, 

 we are indebted for the final illuftnition of a quelliun of equal 

 importance to the chcmiH and manufaflurer. 



In order to afcertain the component parts of alum, and 

 to determine tlie iitcefTity and peculiar .agency of alkalies iu 

 its preparation, he diffolved in pure fulphuric acid fome alu- 

 mine equally pure ; the folution was evaporated feveral 

 times to dryncfs to drive off the excefs of acid, and the dry 

 and pulverulent relidue being then re-diflblved in water, was 

 brought by evaporation to various dates of fpecilic granty 

 for the purpofe of crydallization ; but, notwithlbnding every 

 precaution, a foft magma, confiding of crydalline flakes, was 

 all that could be procured. The folution, which had thus 

 condantly rcfufed of itfclf to afford crydallized alum, began 

 to depolit fome immediately on the addition of a few drops 

 of potafh, and by gradually adding the alkali, drop by drop 

 as the depoljtion of alum ceafed, the whole was converted 

 into pure alum, without the fmallcfl mixture of fulphated 

 potafh. 



Another portion of the fame pure alumiaous fulphat wai 

 mixed with carbonated foda, but without obtaining any cry- 

 ftals. Nor were lime or barytes more cfTicatious. 



Hence it appears plainly that the ufe of potafh is not 

 merely to engage the excefs of acid, othenvife foda, barytes 

 and lime, ought to have produced the fume cffeft. Again, 

 if potalb and ammonia unite only to the fuperabundant acid, 

 the fulphats of potafh and ammonia (hould occafion no change 

 in the pure aluminous fulphat ; bnt, on the contrary, if they 

 form an effential condiment part of alum, then they (liould 

 produce the fame effefts when combined with fulphiu-ic acid, 

 as when pure. To afcertain this, a folution of fulphatcd 

 aluminc was mixed with a few drops of fulphat «f potafh, 

 the immediate effeft of which was the produftion of oAahe- 

 dral cryilals of alum. Sulphat of ammonia produced the 

 fame refult. 



It might ilill, perhaps, be objefted that the a^ion of thefe 

 falts, as they arc remarkably greedy of fulphuric arid, deter- 

 mined the cryftalliz.-ttion of the alum, by the (imple abi'orp- 



5 I 2 tiun 



