Prof. Graham on Water as a Covstituent of Salts. 329 



salts, and to form double salts ? The salts which combme 

 together most readily are the sulphates, and to these 1 there- 

 fore turned. The result was, that in that well-known class of 

 sulphates, consisting of sulphates of magnesia, zinc, iron, man- 

 ganese, copper, nickel, and cobalt, all of which crystallize with 

 either five or seven atoms of water, one atom proved to be 

 much more strongly united to the salt than the other four or 

 six, which last generally may be expelled by a heat under the 

 boiling-point of water, while the remaining atom uniformly 

 requires a heat above 400° Fahrenheit, for its expulsion, and 

 seems to be in a manner essential to the salt. The constitu- 

 tion of crystallized sulphate of zinc, for instance, may be ex- 

 pressed thus : 



ZnSH+H« 

 We here divide the seven atoms of water — 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 we 

 may restrict the application of the name " water of crystalli- 

 zation." 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 mag- 

 nesia 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 re- 

 presented by the following formula, 



ZnS(kS)4:H^; 

 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 supersul- 

 phates, 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 hyd rated sulphuric 

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

 the 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 40° Fahrenheit. It 

 Third Series. Vol. 6. No. 35. Maj/ 1 835. 2 U 



