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



shale exposed to the air in the mine, which has always a tempe- 

 rature of from 60° to 70° (Fahrenheit) for several years. By 

 this treatment the ore becomes gradually covered with a white 

 efflorescence of sulphate of iron and alumina, having an appear- 

 ance like asbestus. The following are analyses of this hair salt, 

 which I made at Dr. R. D. Thomson's laboratory at St. Thomas's 

 Hospital under his superintendence ; and here I take the oppor- 

 tunity of thanking him for the kind assistance he afforded me in 

 these and subsequent analyses. A trace of potash is present. 



These analyses show this salt to be composed of 1 atom ter- 

 sulphate of alumina united to 4 atoms protosulphate of iron and 

 34 atoms of water, or Al 2 O 3 3S0 3 + 4FeO S0 3 + 34HO. 



The decomposed ore was then lixiviated, and the black residue 

 thrown upon the hill, as it was termed, or mass of previously 

 lixiviated shale. This hill, by further oxidation under the influ- 

 ence of the atmosphere, afforded another product of alum. The 

 lixivium was then evaporated, and the copperas allowed to cry- 

 stallize out. To the solution from these crystals a salt of potash 

 or ammonia was added, when alum was deposited. These cry- 

 stals, after being washed, were roached in the ordinary manner. 



As the demand for alum increased, this process was found 

 too tedious ; the fresh or slightly decomposed ore was therefore 

 taken and calcined, thus accelerating the natural process of de- 

 composition. To ascertain the composition of the fresh shale I 

 analysed it, and the results are as follows : — 

 Specific gravity 2'54. 



25 grs. fused with carbonate of soda gave — 



Per cent. 

 Silica . . . 

 Alumina 

 Peroxide of iron 

 Carbonate of lime 



100 grs. dried till it ceased to lose weight at 300° F. lost 

 2 '02 per cent, water. 



25 grs. deflagrated with pure nitre and carbonate of soda, and 

 the filtered solution acidulated and precipitated by Ba CI, gave — 



10-41 FeO = 809 Fe 



