80 



placeable by a salt, and not by an alkaline base. To develop this 

 new function of water, in the case of the sulphates, was the object of 

 the present communication. In that well-known class of sulphates, 

 consisting of sulphates of magnesia, zinc, iron, manganese, copper, 

 nickel, and cobalt, all of which crystallize with either five or seven 

 atoms of water, one atom proved to be much more strongly attached 

 to the salt than the other four or six, which last generally may be ex- 

 pelled by a heat under the boiling point of water, while the last atom 

 uniformly requires a heat above 400" Fahrenheit for its expulsion, 

 and seems to be in a manner essential to the salt. The constitution 

 of crystallized sulphate of zinc, for instance, may be expressed thus : 



SZH + H6. 



The seven atoms of water are here divided into one atom, which is 

 essential to the constitution of the salt, as we know it, and six atoms 

 which are not so; and to this last quantity the name " water of crys- 

 tallization" may be restricted. Now, in the double sulphate of zinc 

 and potash, the single atom of water 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 this 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 following formula: — 



BZ(SK) + H«; 

 which differs only from the previous formula in having the sign of 



sulphate of potash (SK) 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 : — 



SH(SK), 



with or without water of crystallization in addition. There is like- 

 wise a provision in the constitution of hydrated sulphuric acid for the 

 production of such a double salt, as in the case of sulphate of zinc. 

 Hydrated sulphuric acid of specific gravity 1-78 contains two atoms 

 of water, and is capable of crystallizing at a temperature so high as 



