300 Mr Graham on Water as a Constituent of Salts. 



tion." Now, in the double sulphate of zinc and potash, the single 

 atom of water in question pertaining to the sulphate of zinc is 

 replaced by an atom of sulphate of potash, and the six atoms of 

 water of crystallization remain. Sulphate of magnesia combines 

 with sulphate of potash after the same manner, and so do all the 

 other salts of the class. The constitution of the crystallized 

 sulphate of zinc and potash, which may be taken as the type of 

 this family of double salts, is therefore represented by the fol- 

 lowing formula, 



ZnS(KS)+H c ; 

 which differs only from the previous formula in having the sign 

 of sulphate of potash (KS) substituted for the sign (H) of the 

 essential atom of water. 



From a contemporaneous examination of the supersulphates, 

 the conclusion proved to be inevitable, that they also are double 

 salts ; that the bisulphate of potash, for instance, is a sulphate of 

 water and potash, and that its formula is as follows, 



HS(KS), 

 with or without water of crystallization in addition. There is 

 likewise a provision in the constitution of hydrated sulphuric 

 acid for the production of such a double salt, as in the case of 

 the sulphate of zinc. Hydrated sulphuric acid of specific gra- 

 vity 1 .78 contains two atoms of water, and is capable of crystal* 

 lizing at a temperature so high as 40° Fahrenheit. It is the only 

 known crystallizable hydrate of sulphuric acid. It may be repre- 

 sented by the formula, 



HSH, 



ZnSH, 



which may be compared with that of sulphate of zinc placed be- 

 low it. This second atom of water present in hydrated suphuric 

 acid, is replaceable by sulphate of potash, a salt ; and the bisul- 

 phate of potash results from the substitution. But the first 

 atom of water in the acid hydrate can be replaced only by an 



