422 Mr. J. Wilson's Researches into the Alum Manufacture. 



appears from a letter to the late Charles Mackintosh, Esq., 

 published in his memoir, in which the process followed at Haen- 

 setter is thus described: — "Their process of alum making is 

 nearly as follows : the schistus, which has something the appear- 

 ance of our undecomposed ore, lies under a stratum of limestone 

 about 2 feet from the surface of the earth, and upon being quarried 

 is built into kilns in the following manner : first, a layer of fir or 

 rather pine-leaves or branches intermixed with birch ; second, a 

 layer of schistus which has been used as fuel in the evaporating 

 or roasting boilers ; third, a layer of raw schistus, and so on in 

 rotation as before. After burning, the schistus is put into 

 steeps, which are nearly the same as ours at Hurlet ; water is 

 poured upon it, the same water being poured upon five steeps 

 before it is fit for use. The liquor is then run into settling cis- 

 terns, after remaining some time in which it is next put into the 

 first boilers for evaporation. These boilers, which are of lead, 

 are constructed in the following manner : — Six of them, 4 feet 

 diameter each, are built in one storey, in two rows, with three 

 fires under them ; they are made of lead, open above and arched 

 in the bottom, which is of cast iron. From these boilers the 

 liquor is run into wooden coolers which are very small, 3 feet 

 long and 2^ feet broad, with three wooden partitions in each. 

 The crystallized alum is afterwards washed and roached exactly 

 in the same manner as it is with us. Their roaching-casks are 

 about twice as large as ours, and require to stand for about three 

 weeks. From this it appears that the small quantity of alkali 

 formed from burning the schistus with wood is sufficient for 

 their purpose." 



The Kinekulle slate in Dr. Thomson's museum at St. Thomas's 

 Hospital, I found to have the following composition : — 



Specific gvavitv, 2"42. 



Silica 52-28' 



Alumina 16 - 64 



Protoxide of iron . . . 6'96 



Iron .1 361 



Sulphur J 4 - 13 • 



Potash 7-98 



Lime 1*53 



Magnesia 1*10 



Water 1-40 



Carbonaceous matter . 437 



Too 



The calcined Swedish ore generally has a brown colour, but 

 when it assumes a red appearance it is found to be not so pro- 

 ductive in alum. . 



