406 On the Composition of Plate-sulphate of Potash. 



The density of the double sulphate is also greater than that of 

 sulphate of potash. The speciiie gravity of its crystals, finely 

 pulverized, taken in oil of turpentine at 59° F., was found to be 

 2-6682. After being fused, its specific gravity was 2"6708. 

 The specific gra\aty of sulphate of potash, taken in the same 

 way and at the same time, was in the crystallized state 2'6438 ; 

 and in the vitreous or fused state, 2-6567. Dr. Lyon Playfair 

 gives 2'644 as the specific gravity of sulphate of potash. 



There is also a marked difference in the solubility of this salt 

 in cold and boiling water, as compared with that of sulphate of 

 potash. 100 parts of a saturated solution of KO SO^, at 217^ 

 contain 22'5 parts of salt, or 100 parts of water at that tempera- 

 ture dissolve 29 parts ; 100 parts of the solution of plate-salt at 

 217°'5 contain 28'9 parts, or 100 water will dissolve nearly 40'8 

 parts. 



It is, I think, more than probable that certain specimens 

 which have been figured as sulphate of potash, were crystals of 

 this double salt. I have crystallized sulphate of potash under 

 various circumstances, but have never obtained crystals in the 

 forms figured by Phillips*, Brooke t, and others, unless a salt of 

 soda has been present in the solution, causing, as I believe, the 

 formation of the double salt. Both upon the large and small 

 scale, sulphate of potash was deposited either in rhomboidal 

 prisms or in hexahedral prisms, with bipyramidal summits, or in 

 bipyramidal dodecahedrons. 



In conclusion, I may mention that I have examined the suc- 

 cessive crops of crystals obtained by recrystallizing the plate-sul- 

 phate. Several pounds of the salt were dissolved in water, and 

 treated as usual, the crystallizing operations being continued to 

 the 11th crop. The results were as follows: — 



\st Crop. Sulphate of potash with a little plate-salt. 



2nd Crop. Plate-salt, with short prisms of sulphate of soda. 



Zrd Crop. Chiefly sulphate of soda ; crystals large and bold. 



4//j Crop. Layer of plate-salt with sulphate of soda above. 



^th, Qth and 7th Crops. Sulphate of soda. 



Stli, Qth and lOth Crops. Almost wholly plate-salt. 



llth Crop. Sulphate of soda. 



It is therefore quite manifest, that, with care, this salt can be 

 produced at pleasure by recrystallization from the commercial 

 salt, or by crystallizing a mixture of sulphate of potash and sul- 

 phate of soda in equal atomic proportions. 



* Anu. Phil. 1822. t Ibid. 1824. 



