298 Mr. Crum on the Acetates of Alumina. 



March 30, 1853. — The President in the Chair. 



Mb. David Kirkaldt was admitted a member. 



Mr. William Henderson was elected a member. 



Mr. Napier exhibited specimens of Rock Salt from Carrickfergus. 



Mr. Brjce briefly described the geological structure of the district in 

 which the salt occurs, it being obtained from the upper beds of the new 

 red sandstone, 



Mr. Paul Cameron read a paper " On the Variation of the Compass, 

 and the best means of Rectifying it ; " and " On the best means of Con- 

 structing Iron Ships, with a view to a more Correct Indication of the 

 Compass." 



April 13, 1853. — The President in the Chair. 



Mr. William Henderson was admitted a member. 

 Professor Allen Thomson described the " Anatomical Relations of the 

 Limbs of Vertebrate Animals to each other and to the Skeleton." 



April 27, 1853. — Mr. Harvey in the Chair. 

 Mr. Crum read the following paper: — 



XL. — On the Acetates and other Compounds of Alumina. 

 By Walter Crum, F.R.S. 



The salt from which most of the products described in the present 

 memoir were produced, is the tersulphate of alumina, now manufactured 

 in large quantity in the north of England, under the name of concentrated 

 alum. It is formed by the direct action of sulphuric acid upon clay, and, 

 after a certain purification, is evaporated and sold in cakes. 



The impurity which the commercial article contains in largest quantity 

 is potash alum. To separate that substance, advantage was taken of 

 its insolubility in a saturated solution of the tersulphate. The cake- 

 alum was dissolved in a quantity of boiling water insufficient for its 

 entire solution in the cold. On cooling, the excess of sulphate of alu- 

 mina, mixed with almost the whole of the potash alum, deposited, and 

 was separated by filtration through calico. The filtered solution was 

 then evaporated and cooled, when the tersulphate was found to crystal- 

 lize in granular spongy masses. It was then drained and separated as 

 much as possible from the mother liquor by strong pressure between 

 numerous folds of calico. The product was re-dissolved in boiling water, 

 and again crystallized and pressed. 



A salt was thus obtained, having little more than traces of the arseni- 



