310 EARTHS PROPER. 



with carbonate of soda and carbonate of ammo- 

 nia. Six experiments made in this way gave the 

 mean quantity of alumina from 6O87-5 grains of 

 alum, equal to 6'746 grains ; but the true quan- 

 tity is obviously*6*7<5 grains : because this small 

 addition is necessary to complete the weight of 

 the alum. 



From the preceding detail it is evident that 

 alum is composed of 



Sulphuric acid 20 



Alumina . 6-75 



Potash . 6 



Water . 28-125 



60-875 



One integrant particle of the sulphuric acid is 



united to the potash, and three to the alumina, 



It is obvious from this, that 6*75 must represent 



Atomic the weight of three atoms alumina. Consequent- 



2-25. " ly, the weight of alumina is 6 -r = 2-25. 



The composition of alum may be stated some- 

 what differently. For it is obviously composed 

 of 



3 atoms sulphate of alumina = 21-7^ 

 1 atom sulphate of potash =11 

 25 atoms water . . = 28-125 



CO-875 



The analysis of alum just given was made by 



