ALU 



ALU 



\Vhen an unusual quantity of potash is 

 added to alum liquor, the salt loses its 

 usual form and crystallizes into cubes. 

 This constitutes a fourth variety of alum, 

 usually distinguished by the name of cu- 

 bic alum. It contains an excess of alkali. 

 When the potash is still further increased, 

 Chaptal has observed, jhe salt loses the 

 property of crystallizing altogether, and 

 falls down in flakes. This constitutes a 

 fifth variety of olurn, 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 becomes 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 

 Homberg's pyrophorus, 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 Hombergabout the 

 the beginning of the eighteenth century, 

 while he was engaged in his experiments 

 on the human faeces. He 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 

 faeces ; and afterwards Mr. Lejoy de Su- 

 vigny showed that several other salts con- 

 taining sulphuric acid may be substituted 

 for alum. Scheele proved that alum de- 

 prived of potash is incapable of forming 

 pyrophorus, and that sulphate of potash 

 may be substituted for alum. And Mr. 

 Proust has shewn that a number of neu- 

 tral salts, composed of vegetable acids 

 and earths, when distilled by a strong fire 

 in a retort, leave a residuum which takes 

 fire spontaneously on exposure to the air. 

 These facts have thrown a great deal of 

 light on the nature of Homberg's pyro- 

 phorus, and enabled us in some measure 

 to account for its spontaneous inflamma- 

 ti6n. It has been ascertained, that part 

 of the sulphuric acid 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 been ascertained, that du- 

 ring the combustion of the pyrophorus a 

 quantity of oxygen is absorbed. The in- 

 flammation is probably occasioned by the 

 charcoal ; the sulphuret of potash also 

 acts an essential part. Perhaps it produ- 

 ces a sudden increase of temperature, by 

 the absorption and solidification of water 

 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 are filled with prepared li- 

 quor of 10 pennyweights, to which sul- 

 phate of potash is added, and boiled toge- 

 ther until it weighs 16 pennyweights, by 

 which time the whole of the superfluous 

 alumina and the oxyde of iron is precipi- 

 tated. The fluid is then run into a settler, 

 where it remains until clear, after which 

 it is pumped into a second boiler, and eva- 

 porated up to 26 pennyweights, let into 

 the coolers, and left to crystallize. By this 

 process, it is said, he gains the whole of 

 the alum at one 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 Geoffrey, 

 and afterwards Margraflf, found that the 

 earth of alum is an essential ingredient 

 in clays, and gives them their properties, 

 hence it was called argil but Morveau. 

 gave it the name of alumina, because it 

 is obtained in a state of the greatest pu- 

 rity from alum by the following process. 

 Dissolve alum in water, and add to the 

 solution ammonia as long as any precipi- 

 tate is formed. Decant off the fluid part, 

 and wash the precipitate in a large quan- 

 tity of water, and then allow it to dry, 

 The substance thus obtained is alumina ; 

 not however in a state of absolute purity, 

 for it still retains a portion of the sulphu- 

 ric acid with which it was combined in 

 the alum. But it may be rendered tole- 

 rably pure, by dissolving the newly preci- 

 pitated earth in muriatic acid, evaporating 

 the solution till a drop of it in cooling de- 

 posits small crystals,' setting it by to crys- 

 tallize, separating the crystals, concen- 

 trating the liquid a second time, and se- 

 parating the crystals which are again de- 

 posited. By this process, most of the 

 alum which the earth retained will be se- 

 parated in crystals. If the liquid be now 

 mixed with ammonia as long as any preci- 

 pitate appears, this precipitate, washed 

 and dried, will be air. mm a nearly pure 



