METHOD OF OBTAINING ALUMINE.

nated alcali, washing the obtained precipitate, and exposing it to asufficient heat to drive off the carbonic acid. This method how-ever is imperfect, for if the alumine thus obtained be heated withcharcoal, and a diluted acid is added to the mixture, sulphuratedhidrogen gas will be liberated. It adheres to the tongue, andemits a peculiar odour when breathed upon. Sure signs that itis not pure.

It must be obvious that alumine cannot be obtained absolutelypure in this manner. For alum is a triple compound, consistingof alumine, potash, and sulphuric acid in excess. When thisexcess of acid is saturated, by adding to the solution an alcali, oreven pure alumine, a highly insoluble salt (sulphate of alumine)is produced, differing from alum only in the proportion of itsbase. When we therefore gradually add to a solution of alum,a carbonated alcali, the first effect of the alcali is, to saturate the.excess of the sulphuric acid, and the precipitate consists princi-pally of the salt which is insoluble in water. A further quantityof the alcali effects instantly a decomposition of part of the salt,which, in proportion as it takes place, becomes mixed with thealumine; and it is thus covered from the further action of thealcali. This being the case i.t is obvious that no subsequentwashing can do more than separate the sulphate of potash, andtherefore the residuum, instead of being pure alumine con-tains also a variable proportion of true sulphate of alumine;the sulphuric acid of which becoming decomposed on heating itin contact with charcoal, accounts for the sulphurated hidrogengas produced by the affusion of an acid.

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