

 NEITHER POTASH NOR AMMONIA. 449 



Thus, there are no fewer than five species of 

 compound salts, having each the form of the 

 regular octahedron, and containing each the 

 same number of atoms of water. These are 



1. Ammoniacal alum, 



2. Potash alum, 



3. Soda alum, 



4. Ammonio-persesquisulphate of iron, 



5. Potash-persesquisulphate of iron. 



Were we to confine our views to the first three 

 species, we would be tempted to conclude with 

 M. Mitcherlich, that sulphate of ammonia, sul- 

 phate of potash, and sulphate of soda are isomor- 

 phous salts ; while in the fourth and fifth spe- 

 cies the alumina is replaced by peroxide of iron. 

 And Mitcherlich has shown that alumina and 

 peroxide of iron are isomorphous substances. 



In consequence of this last circumstance, it is 

 obvious that alum is very likely to contain per- 

 oxide of iron, unless very great care indeed be 

 taken in preparing it. The only certain way of 

 obtaining it quite free from iron would be to 

 dissolve clay free from iron in sulphuric acid, 

 and to crystallize the solution by adding the re- 

 quisite quantity of sulphate of ammonia and sul- 

 phate of potash. I am satisfied that at the pre- 

 sent price of sulphuric acid, such a manufactory 

 might be established with advantage. 



6. Calcareo-acetate of copper. This is an ex- 

 ceedingly beautiful salt of a fine deep blue co- copper, 



VOL. II. " F f 



