420 COMPOUND SALTS 



stitution of an atom of sulphate of ammonia for 

 an atom of sulphate of potash. 



5. Ammonio-trisulphate of alumina. This 

 salt may be obtained by precipitating a, boiling 

 solution of ammoniacal alum by an alkali. It is 

 an insoluble powder, which I have not particu- 

 larly examined. 



g. Ammonio-persulpliate of iron. This salt 



persulphate r r 



of iron. was first prepared by Mr. Cooper, who described 

 it as a simple persulphate of iron ; but for our 

 first knowledge of its constituents and its pro- 

 perties we are indebted to Dr. Forchhammer. 

 As soon as I became acquainted with Dr. Forch- 

 hammer's experiments,* I examined some of 

 Mr. Cooper's crystals, which he had been kind 

 enough to send me several years ago, and found 

 them to contain abundance of ammonia, and to 

 be in fact the very same triple salt as that de- 

 scribed by Dr. Forchhammer. 



This salt is easily obtained by mixing toge- 

 ther solutions of persesquisulphate of iron and sul- 

 phate of ammonia in the proportion of two inte- 

 grant particles of the former to one of the lat- 

 ter, concentrating the mixture somewhat, and 

 setting it aside. The double salt is deposited 

 in very regular crystals ; it is transparent and 

 colourless, and is crystallized in octahedrons 

 precisely similar to those of alum. It is soluble 



* 



v Annals of Philosophy (second series) V. 4-09. 



