ALU 



tion of fiiperfluous arid. In order to Jtteniiiiic tins, fon.e 

 tincn'rtallizable aluininous fiilj li.it was niixtd with acidulous 

 fu'.phat of potalli, and aflordcd as great an abundance oi 

 alum as wlitii the neutral fislphat of potaih was made ufc of. 

 Hence, no doubt can remain concerning tlie influence and 

 particular mode of atlion cxercifcd by potaih and ammonia 

 in the manufadurc of alu.n. 



The experiments of Bergman ard of fevcral other chc- 

 iti'ids afcertaiiicd, that when a foUnion of commcm alum is 

 boiled with a cjuanlity of pure alumine, this laft combines 

 with it, and forms a peculiar fait infoluble in water, known 

 bv the name of ne\itial aluminous fulphat, or alum faturatcd 

 with its own earth. To this faft was added another of equal 

 importance, by Vauquelin, namely, that the earthy fait thus 

 precipitated retains its potafh or ammonia, for by digellion 

 in dilute fulphuric acid, it is diflolved, and affords oftahedral 

 cryllals of alum ; it even ai)pears from the memoir of this 

 pliilofopher quilted above, that the prefence of one of the 

 two alkalies is neceffary to the formation of this neutralized 

 alum. To an uncryllalli/able lolution of fulphated alun.ine 

 perfeftly free from alkali, he added fome pure alumine, and 

 found that a part of it was diflolved to the complete fatura- 

 tion of the aeid, but that no precipitation took place ; hav- 

 ing then added a few drops of fulphat of potafh, a precipitate 

 was depofited fhorlly after, poUefUng all the properties of 

 the foregoing faturatcd alum. Hence is cflabllflied the ne- 

 cciTity of^ fulphated potafh or ammonia, to enable alum, by 

 combining with a larger proportion of its bafe, to pafs to the 

 earthy Hate. 



The alum of commerce always contains fulphat of potafh 

 either alone or mixed with fulphated ammonia, and as it is 

 often of confequence to the manufacturer to know the abfo- 

 lute and relative proportions of thefc falts, the following me- 

 thod of analyfis may be had recourfe to. Firft, let a fmall 

 piece of the alum be reduced to powder, and mingled with 

 a folution of cauitic potafli in fufficient quantity to decom- 

 pofe it entirely : if then, upon gently heating, it gives out 

 an ammoniacal odour, as is generally the cafe, this indi- 

 cates the prefence of fulphated ammonia. Havmg obtained 

 this indication, let two or three hundred grains of the alum 

 be diffolved in diflilled water and put into a tubulated retort, 

 and then add quick-lime, equal in weight to the fait : by 

 making this mixture boil for about twenty minutes, the 

 whole of the ammonia will be expelled, and may be con- 

 denfed by cold water in the receiver, or a Woulfe's appara- 

 tus : this ammoniacal liquor, being then carefully laturated 

 with fulphuric acid and cryftallized, will fhew the quantity 

 ©f fulphated ammonia. The refidue in the retort being 

 mixed with warm water and filtered, a clear liquor will be 

 obtained, containing the fulphat of potafh, with fome fele- 

 iiite ; this latter will be precipitated by boiling and evapora- 

 tion, and the remaining fluid will then depofit the fulphat of 

 potafh in a cryftalline form. When the previous affay does 

 not indicate the prefence of ammonia, the alum is to be 

 docompofed by cauflic ammonia, the precipitate is to be well 

 wafhed, and the liquors being added together, are to be 

 gently evaporated to drynefs ; the fait thus obtained is to 

 be heated in a crucible till it ceafes to exhale white vapours 

 tif ammoniacal fulphat, and the refulue is fulphat of potafh. 



§ 2. ManuJaBure of Alum from ths Jalinc-eartby ores. 



The only place where this kind of ore is found in fufficient 

 abundance to be worth working, is at the Solfatara, a few 

 miles from Naples. The Solfatara, called by the ancients 

 Forum Vulcanl, Cnmpi Lcucogd, is a fmall plain, at the top of 

 a hill, covered with a white foil, and exhaling fulphureous 



ALU 



vapours which, during the night, emit a pale blue lambfnt 

 light : the ground, even at the furface, is contidcrably warm, 

 proceeding, no doubt, from iubtcrranean lire. It has con- 

 tinued in nearly the fame ilate from the age of Pliny to the 

 prefent time, and is celebrated by this author in his Natural 

 Hiflory (lib. xxxv. ch. 50.) for its fulphur, hut not for its 

 alum, as the Abbo Mazeas affirms. On the contrary, by 

 his ominion of the Campi Leucogci, when mentioning the 

 various places fro.n which alum was then procured, it is plain 

 that the cftablifhment of the ahun works of the Solfatara is 

 of more recent origin. The white clayey foil of this plain, 

 being conflantly penetrated by fulphureous vapours, and the 

 extuilations during the night being for the nioll part mixed 

 with the dew, and thus returned upon the furface, caufe it 

 to be covered with a light filiiie efHorefcence. This, to- 

 gether with the earth to which it adheres, is daily collected 

 and diilributed into leaden cauldrons, fo as to fill about two- 

 thirds of their capacity ; v.ater is then added, till it Hands 

 about three or lour inches above the furface of the clay, and 

 this, in a few hours, by the afliilance of the natural heat of 

 the ground in which the cauldrons are fet almofl up to the 

 brim, txtrails the alum diflufed through the clay, and de- 

 poilts it in rough cryllals on its furface. Thefe cryftals 

 being taken out and waflied in the mother liquor, are put 

 with frelTi water into other boilers, and again diflolved as be- 

 fore, by the natural heat of the ground ; the folution is 

 then run through a filter into large wooden coolers, and in 

 a day or two aflords a large quantity of pure colourlefs cry- 

 llals. Hence it appears that the alimi exifls ready formed 

 in the earth of the Solfatara, and the whole of the manu- 

 faftiu'ing part is reduced merely to lixiviation and purifica- 

 tion. The proportion of fait muff neceflanly be very vari- 

 able, thofe parts that are expofed to the rain, and that lie 

 above the general level, will contain the lealt. A fpecimen 

 that was analyzed by Bergman yielded eight per cent, of 

 alum. The Abbe Mazeas, from fix pounds of the earth., 

 procured, by lixiviation, two pounds and a half of cryflals, 

 or about 41 per cent. The alum itfelt has not yet been 

 analyzed ; it feems probable, however, that its alkaline part 

 is entirely potafli. 



§ 3. ManufaSure of Jlhimfrom alumfione. 



It is at La Tolfa, not far from Givita Vecchia, i-n the 

 Roman Hate, that the manufacture of alum from this fpecies 

 of ore is principally carried on. All the alum known in ' 

 commerce by the name of Roman alum is thus prepared, as 

 well as the Levant or Smyrna alum. 



The ore of La Tolfa forms veins of confiderable hardnefs, 

 which are feparated by means of blafling from the reft of the 

 rock ; the pieces thus obtained are brought to the calcining 

 oven, which is inerely a hole dug in a riling ground, four or 

 five feet in diameter, and from five to fix in depth, with a 

 lateral gallery, communicating with the open air, and the ' 

 bottom of the furnace. The bottom being covered with * 

 faggots of brufli-wood, the pieces of ore are fi<ilfulJy ■ 

 laid over ihem, fo as to form a kind of hollow vault, between 

 the interlHces of which is an ample pafTage for the fmoke. 

 As foon as the fire is kindled and the flame begins to appear 

 between the flones, a workman is at hand to regulate the . 

 combuftion, that it may be neither too great nor too feeble ;• 

 in the courfe of fi-om three to five hours the fmoke begins 

 to decreafe, and the fire burns brightly; this is allowed to go 

 on till the fmell of burning fulphur begins to be prevalent, 

 which is a fign that tlie ore is fufficiently roafted. The fire 

 is now raked out, and the ilones are left to cool. The 

 fign of this firll procefs being well coududed, is, that 



