ALUMINA. 307 



in a state of purity, and in regular crystals fit for 

 examination, is alum. Of this triple salt there 

 are several species ; for it consists of a certain 

 number of integrant particles of sulphate of alu- 

 mina combined with sulphate of potash, or sul- 

 phate of soda, or sulphate of ammonia, or sul- 

 phate of magnesia, and perhaps of some other 

 soluble sulphate which I have not tried. The 

 crystals of alum consist of regular octahedrons, 

 and neither the figure nor the taste of the salt is 

 altered by substituting any of the above named 

 sulphates for the others. The species of alum 

 which I chose as upon the whole of easiest ana- 

 lysis, and capable of being obtained with the 

 greatest facility, was the common alum of this 

 country, composed of sulphuric acid, alumina, 

 potash, and water, or of sulphate of alumina 

 and sulphate of potash, united together with a 

 certain quantity of water of crystallization. As 

 the analysis of this salt was attended with consi- 

 derable difficulty, but was made with the utmost 

 possible attention to precision, it will be proper 

 to give a somewhat detailed account of it. 



1. 6O875 grains of pure alum crystals were 

 dissolved in hot water, and the liquid was mixed 

 with a solution of 53 grains of chloride of barium. 

 A double decomposition immediately took place, 

 and sulphate of barytes precipitated to the bot- 

 tom : when the supernatant liquid had become 

 transparent and colourless, it was tested by sul- 



U2 



