CONTAINING POTASH. 433 



quently, contain just 20 grains of sulphuric acid. 

 I precipitated a solution of 64'15 grains of the 

 salt by ammonia, and the peroxide of iron se- 

 parated, after being washed, dried, and ^exposed 

 to a red heat, weighed almost exactly 10 grains. 

 The residual liquid being evaporated to dryness, 

 and the salt remaining exposed to a red heat, 

 left ll grains of sulphate of potash ; but it was 

 not quite free from iron. I conclude from this 

 analysis, (which was only once performed) that 

 the constituents of the salt are 



2 atoms persesquisulphate of iron 25 

 1 atom sulphate of potash 1 1 



25 atoms water . . 28-125 



64-125 



This is the fourth compound salt which we have 

 met with having the regular octahedral form of 

 alum ; and all of them contain just 5 atoms of 

 water. 



6. Potash-persulphate of iron. During the 

 crystallization of the last described salt, I fre-inm, 

 quently got, in the first place, a deposition at 

 the bottom of the vessel consisting of a greenish 

 brown matter, obviously crystalline, though the 

 shape was not sufficiently regular to admit of 

 measurement. When this crust was dried in the 

 open air, it had a fine greenish yellow colour, 

 and was composed of grains seemingly cubical, 

 often having the solid angles rounded off, so as 



Vor,, II. E e 



