Mr Graham on Water as a Constituent of Salts. 305 



saline atom remains ; and to this compound six atoms of water 

 are attached in the common crystals. These crystals, placed 

 over sulphuric acid in vacuo, thermometer 68°, were found to lose 

 six atoms water, retaining only one. Exposed to the air at 212°, 

 the crystals likewise readily effloresced down to one atom ; and 

 the sulphate of zinc is known to be deposited from a boiling so- 

 lution, in crystalline grains, containing one atom of water. On 

 the other hand, the sulphate of zinc was found to retain this 

 single atom of water at the high temperature of 410° Fahrenheit, 

 but to lose it, and become anhydrous, at a temperature not ex- 

 ceeding 460°. In all such cases, the hydrated salt was heated 

 in a tube receiver, by means of an oil or solder-bath, of which the 

 temperature was observed by a thermometer. However strongly 

 it has been heated, without being decomposed, the sulphate of 

 zinc always regains this atom of water when moistened, slaking 

 with the evolution of heat. Common sulphate of zinc is there- 

 fore " sulphate of zinc with saline water ;" and the true or ab- 

 solute sulphate of zinc is unknown to us in the crystalline form, 

 or in a soluble state. But we may continue to designate the salt 

 we possess as sulphate of zinc, as the name is attended with no 

 dubiety. 



Sulphate of Zinc with Sulphate of Potash : ZnS(KS)+H 6 . Sul- 

 phate of Zinc and Potash. 



In this well-known double salt, we have sulphate of potash 

 substituted for the saline water of sulphate of zinc, and the six 

 atoms of water of crystallization remain. It is readily formed, 

 on mixing together solutions of sulphate of zinc and sulphate of 

 potash, in atomic proportions. It is formed likewise, and sepa- 

 rates by crystallization, when the sulphate of zinc is added to the 

 bisulphate of potash ; and, in that case, an interesting double de- 

 composition occurs. 



VOL. XIII. PART T. Q, q 



