ALU 



ALU 



decreafe of bulk is in part occafionej by the expulfion of earths. For further particulars fee the falts under their rC' 

 the hill particles of water ; but from the augmented fi)eeific fpeftive acids. 



gravity of the alumine, it is plain that an aftual eondenfation 

 or approximation of moleculx takes place, as is obfervable in 

 various other porous luhftances previous to {ufion. Whether 

 any artificial heat is able to bring this earth into real fafion 

 is as yet dubious ; for though Lavoifier, by means of a 

 blow-pipe charged with ox\gen gas, reduced a piece of alu- 

 mine to a pafly femi-fluid ilate, yet it is probable, as the 

 earth was obtained from alum, that a minute portion of pot- 

 afh might flill be contained in it, and thus ait as a flux. 



Alumine has a ftrong affinity for metallic oxyds, tfpecially 

 the oxyd of iron ; hence arifes the difficulty, and indeed al- 

 irioft impuflibility, of obtaining alum free from iron in the 

 great way, becaufe all natural clays and aluminous ores con- 

 tain more or lefs of this metal. The only way of accurately 

 feparating thefe two fubllances is by digeilion in cauftic 

 potafh or foda, which will dilTolve the earth, but not the oxyd. 



Thefe two fubitancee are alfo capable of atting on each 

 other in the dry way at high temperatures ; and fome im- 

 portant experiments on this iV.ojeft are recorded by Achard 

 and Kirwan, from which it appears, that when the propor- 

 tion of alumine exceeds that of the oxyd of iron, the mix- 

 ture is in all cafes veiy difficultly fulible ; that when the pro- 

 portions of the ingredients are equal, and efpecially when 



■\mmonia has not yet been obferved to exert any aftion 

 on pure aluminous earth ; but both potalh and foda, when 

 caultic, will difiolve it without any difficulty. This may be 

 done by evaporating to drynefs, and igniting in a filver cru- 

 cible, a mixture of cauftic alkali and alumine, and then 

 hxiviating the mafs, or merely by boiling fome frelh preci- 

 pitated ahimine in a watery folution of tte alkali. This 

 alkalized alumme has of late been recommended as a pre- 

 ferable mordant to common alum in the fixing of thofe co- 

 lours that are injured by the prefence of fulphuric acid. To 

 feparate alumine from its folution in cauilic alkali, it is necef- 

 fary to add nitric or muriatic acid in fufficient quantity to 

 neutralize the alkali and diffolve the alumine, and then to 

 precipitate the earth by cauftic ammonia. 



The aftion of barytes on alumine is analogous to that of 

 the alkalies, yet prefents fome peculiar charafters. When 

 a folutien of cauftic barytes in water is added to a liquid 

 muriat of alumine, the firft effecl is the appearance of a 

 precipitate, owing to the decompofition of the fait by 

 the barytes ; if this laft, however, is added in excefs, the 

 alumine is rediiTolved by it, and the liquor becomes clear. 



Again, if equal parts of newly precipitated alumine and 

 cauftic baiytes are boiled together in a quantity of diftilled 



the iron predominates, the refult, after expnfure to a heat of water fufficient to take up the barytes, about half the mix- 



about 160° Wedgewood, is a dark-coloured vitreous fiag. 



The attniftioa too that fubhfts between alumine and ve- 

 getable or animal colouring matter, is fingularly powerful. 

 Thus, if, to a water)' infufion of cochineal or madder, a few 

 drops of a folution of alum are added, a decompofition 

 fhortly takes place, and the whole of the tinging particles 

 tinite, and are precipitated together with the aluminous bafe 

 of the earthy fait, leaving the fupernatant liquor wholly co 



ture will be difTolved, and upon analyfis the infoluble refidue 

 will be found to confilt of alumine, with a fmall proportion 

 of barytes, while the folution will confift of much barytes 

 and a little alumine. By adding to the hquor fome muriatic 

 acid, to engage the excefs of barytes,, a flocculent precipitate 

 will be depofited, confifting of the two earths, nearly in the 

 propcntion of the original inloluble refidue. Hence it ap- 

 pears that alumine combines with barytes into a fait which 



lourlefs. Fugitive colours alfo, by this combination, be- is infoluble in mere water, but is capable of being rendeied 



come of fufficient permanence to refift for a long time the " 



changbs to which they are fubjcil : hence is explained the 

 preparation of the I-,ake J>!^ments, and the theory of Mor- 

 dants in the art of Dying. 



In the direft way fulphur appears to contraft no union 

 with alumine ; and the hepatic gas that is feparated by an 

 acid from alum, after having been heated with charcoal, is 

 no longer a decifive evidence of fulphuret of alumine, fince 

 the difcovery of the neceffity of potaffi to the very conftitu- 

 lion of common alum. 



Upon the gaffeous fubftances alumine has not been ob- 

 ferved to produce any change, although Humboldt has 

 publifhed (Annales de Chimie, vol. xxix.) a long and plau- 

 fible memoir, to fnew that alumine abforbs the oxygen of 

 the atmofp'iere, and hence produces an important effeft in 

 the economy of vegetation. It is true, indeed, that many 

 natural clays will deoxygenate atmofpheric air ; but this is 

 folely owing to the carbonaceous matter and oxyd of iron 

 that they contain, it having been proved by accurate expe- 

 riments, inltituted for this purpofe by Theod. Sauflure and 

 others, that pure alumine has no effeft whatever on oxygen 

 gas or atmofpheric air, 



foluble therein by the affiftance of barytes. In the dry 

 way, at about 150° Wedgewood, any mixtures of the two 

 earths in which the alumine preponderates remain pulveru- 

 lent ; but when the barytes is three or four times as much 

 as the alumine, the powder concretes into a hard mafs, 

 without, however, ihewing any figns of fufion. In order 

 to decompofe barytic alumine, diffialve the whole in mihiatic 

 acid, and add cauftic ammonia ; the alumine alone will be 

 precipitated. 



Strontian produces the fame eff'eft on alumine as barytes, 

 but more feebly ; the aAion of thefe two fubftances in the 

 dry way, on each other, has not yet been the fubjeft of 

 experiment. 



It appears highly probable that lime has a fimilar affinity 

 for alumine, as the reft of the alkaline earths poftefs ; the 

 only experiment, however, upon the fubjeft, is one of Mor- 

 veau's ; he mixed equal parts of muriat of alumine and mu- 

 riat of hme in folution, and immediately a precipitate took 

 place, which was infoluble by an excefs of acid ; this has 

 been fince repeated by Darracq, a pupil of Vauquelin, 

 without effeft, the liquor remaining perfectly limpid ; hence 

 it is probable that the alumine of Morveau was not quite 



All the acids are capable, in particular cireumftances, of free from fulphuric acid, and that the infoluble precipitate 



combining with alumine ; but thefe combinations are not 

 accompliftied with the fame eafe as thofe between the acids 

 and alkahne earths. The ftronger mineral acids will take up 

 alumine from clay by digeftion at a boihng heat, but the ve- 

 getable and other weaker acids will not readily effeft a folu- 

 tion, except the alumine is prefented to them recently preci- 

 pitated by an alkali from fulphuric, nitric, or muriatic acid, 



was merely felenite. In the dry way lime and alumine m 

 any proportions are infufible, except by means of a blow- 

 pipe, charged with oxygen gas. 



The aftion of magnefia on alumine is not yet fully afcer- 

 tained : it appears, however, from Mr. Chenevix's experi- 

 ments, that the ammoniaco-magnefian triple falts, are formed 

 with difficulty, when alumine is prefent, and that magnefia 



All the aluminous falts are decompofed with precipitation of prevents, in a great meafure, the folubihty of alumine ia 

 the earth by the cauftic or carbonated alkalies, or alkaline the cauftic fixed alkalies. This combination of the two 



earths 



