ALU 



ALU 



M'hrn an unusual quantity of potash is 

 :ed to Jum liquor, tli' s its 



,.il form, and crystallizes in cubes. 

 This constitutes a fourth him, 



usual!) <l,s-''n;;iiish,-d by the nameofcu- 

 bie alum. It contains an excess of alkali. 

 \Vheti the potash is still further increased, 

 Chaptal li:is o the 



property f crystallizing altogether, and 

 falls down in flakes. This constitutes a 

 (if) I .t" alum, consisting of sul- 



phate of potash combined with a small 

 proportion of alumina. If three- parts of 

 alum and one of flour or sugar be melted 

 together in an iron ladle, and the mixture 

 dried till it become-, blackish and ceases 

 to swell ; if it be then pounded small, put 

 into a glass phial, and placed in a sand- 

 bath till a blue flame issues from the 

 mouth of the phial, and after burning for 

 a minute or two be allowed to cool, a sub- 

 stance is obtained, known by the name of 

 lloinberg's pyrophonis, which has the 

 property of catching fire whenever it is 

 exposed to the open air, especially if the 

 air be moist. This substance was acci- 

 dentally discovered by Homberg about 

 the beginning of the eighteenth century, 

 while he was engaged in his experiments 

 on the human f.i-ces. Me had distilled a 

 mixture of human faeces and alum till he 

 could obtain nothing more from it by 

 means of heat ; and four or five days after, 

 while he was taking the residuum out of 

 the retort, he was surprised to see it take 

 fire spontaneously. Soon after, Lemery 

 the younger discovered that honey, sugar, 

 flour, or almost any animal or vegetable 

 matter, could be substituted for human 

 fscces; and afterwards Mr. Lejoy de Su- 

 vigny shewed that several other salts con- 

 taining sulphuric acid may be substitut, ( | 

 for alum. Scheele proved that alum de- 

 prived of potash is incapable of forming 

 pyrophonis, anil that sulphate of p 

 m:i\ united for alum. And Mr. 



Proust has shew u that a number of neu- 

 tral salts, composed of vegetable acids 

 and earths, \\hendistilled by a strongfire 

 in a re! or*, leave a residuum which takes 

 fire spontaneously on exposure to the air. 

 These tarts have throv, n a great deal of 

 light on the nature of Homberg's pyro- 

 phonis, and enabled us in son.' 

 to account for its spontaneous inflamma- 

 tion. It has been ascertained, that part 

 of the sulphuric ucid is decomposed du- 

 ring the formation of the pyrophorus, and 

 of course a part of the alkaline base be- 

 comes uncombined with acid; and the 

 charcoal, which gives it its black colour, 

 is evidently divided into very minute par- 



ticles. It has bee 



ring the combustion of 'iiis a 



v of oxygen is absorbed. The in- 

 flammation is probably occasioned by the 

 charcoal; the sulplmret of potash als< 

 acts an essential part. 1'urhaps it pro- 



: sudden increase oftemix ; 

 by the absorption and solidification of wa- 

 ter from the atmosphere. 



A new process for making alum is used 

 at some works, for which we are indebt- 

 ed to Mr Sadler, which is as follows : 

 The boilers arc filled with prepared liquor 

 ofJO pennyweights, to which sulphate of 

 potaw) is added, and boiled together, un- 

 til it weighs 16 pennyweights, by whick 

 timethe. whole ofthesiiperfluous alumina 

 and the oxydc of iron is precipitated. 

 The fluid is then run into a settler, where 

 it it-mains until clear, after which it ig 

 pumped into a second boiler, and evapo- 

 rated up to 26 pennyweights, let into the 

 coolers, and left to en Hy this 



process, it is said, he gains the whole of 

 the alum atone evaporation, and from the 

 mother liquor remaining there is a pro- 

 duct, the sulphate of iron. 



ALUMINA, in chemistry, one of the 

 five proper earths. It was discovered by 

 the alchemists that alum was composed of 

 sulphuric acid and an earth, the nature 

 of which was long unknown ; but Geof- 

 froy, and afterwards Margraff, found that 

 the earth of alum is an essential ingredi- 

 ent in clays, and gives them their proper- 

 ties, hence it was called argil ,- but Mor 

 veau gave it the name of alunu'na, because, 

 it it obtained in a state oflhe greatest pu 

 rity from alum by the following process. 

 Dissolve alum in water, and add to the 

 solution ammonia as long a* any precipi- 

 tate is formed. Decant oft' the fluid part, 

 and wash the precipitate in a large quan- 

 tity of water, and then allow it to drv 

 The substance thus obtained is alumina 

 not howevi t 'fab so I uk- purity, 



forit still retains a portion of the sulphu. 

 ric acid with which it was combined \n 

 the alum. But it may be rendered tole- 

 rably pure, by dissolving (tie newly preci- 

 pitated earth in muriatic acid, evaporating 

 the solution till a drop of it in cooling de 

 posits small 'ting it l,y 



tallize. DTicen- 



tr.it ing the liquid a second time, and se- 

 parating the crystals which are a^ain 

 deposited. Hy this process, most of the 

 alum which the earth retained will be se- 

 parated in crystals. 1C the liquid be now- 

 mixed with ammonia as long as any pre- 

 cipitate ann. 



