290 On the Proportion of Water in the Magnesian Sulphates. 



Theory of Theory of 

 Experiment. 6HO. 7HO (Pierre). 



Water 24-89 24*03 27*32 



Sulphate of zinc and potash 75*11 75*97 72*68 



100* 100* 100* 



The experiment obviously indicates six and not seven equi- 

 valents of water. The slight excess of 0*86 per cent, of water 

 is not more than is usually found in crystallized salts, arising 

 from the difficulty of divesting them entirely of water me- 

 chanically interposed between the plates of the crystals. The 

 peculiarly high disposition of this particular class of salts to 

 retain mechanical water, has been noted by Mitscherlich, my- 

 self, and almost every one else who has made them the sub- 

 ject of investigation. It has probably been the cause of the 

 error into which M. Pierre has fallen, in over-estimating their 

 proportion of water. 



Although it is scarcely necessary to extend the inquiry to the 

 other double salts of the class, which being isomorphous with 

 the last have necessarily the same proportion of water, still I 

 may be allowed to avail myself of a series of five analyses of 

 the double sulphate of copper and potash lately executed in 

 the laboratory of my friend Prof. Fownes, and which he has 

 kindly communicated to me. 



These experiments all concur in proving that six equiva- 

 lents is the proportion of water in the double sulphate of 

 copper and potash, and not seven equivalents. 



Although M. Pierre gives seven atoms of water to the double 

 sulphate of magnesia and potash, he adds, near the end of his 

 paper, as if to qualify the statement, that when he communi- 

 cated his results to M. Balard, that chemist informed him that 

 the double sulphate of magnesia and potash contained no 

 more than six equivalents of water, and was therefore consist- 

 ent with the views of Mr. Graham. 



With reference to the single atom of water strongly retained 

 by the magnesian sulphates, an experiment was made on sul- 

 phate of zinc. The crystallized salt dried for several days at 

 212°, in the same circumstances as those in which the double 

 sulphate of zinc and potash became anhydrous, still retained 

 water. The heat being continued for three or four days after 



