422 Prof. Graham on Water as a Conslituent of Salts. 



placed over sulphuric acid ifi vacuo, without heat, were found 

 to retain only two and a quarter atomic proportions of water. 



The sulphate of magnesia and ammonia lost its six atoms of 

 water of crystallization and became anhydrous, when exposed 

 to a temperature not exceeding 270°, for one hour, having 

 previously been dried at 212°. It retained of course the atom 

 of water which is essential to the ammoniacal salts. A some- 

 what higher temperature was required to deprive the sulphate 

 of magnesia and potash of its whole water of crystallization. 



Hydrated Sulphate of Lime : CaS H + H. 



The only crystalline hydrate of sulphate of lime, which is 

 known, contains two atoms of water. It occurs native in [the 

 form of] gypsum and selenite. Pounded selenite loses little or 

 nothing in the open air at 212°. Water begins to escape at a 

 temperature not much higher, but is not completely expelled 

 by any degree of heat under 270°. That hydrated sulphate of 

 lime may contain an atom of saline water, is indicated by the 

 existence of a double saltof sulphate of lime with sulphate of 

 soda, constituting the mineral Glauberite. I succeeded in ob- 

 taining a definite compound of sulphate of lime with one atom 

 of water, by drying pounded selenite, at 212°, in vacuo over 

 sulphuric acid*. The salt which had been so dried at 212° 

 did not form a coherent mass, like stucco, when made into a 

 paste with water. The affinity of sulphate of lime for the sa- 

 line atom of water appears to be feeble, as the salt can be 

 made quite anhydrous under 300° ; and consequently the sul- 



* It has subsequently been observed, that the water is reduced under 

 one atomic proportion, by a protracted exposure to the same tempera- 

 ture. 



