Mr Graham on Water as a Constituent of Salts. 313 



known, contains two atoms of water. It occurs native in gyp- 

 sum 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 

 salt of sulphate of lime with sulphate of soda, constituting the 

 mineral Glauberite. I succeeded in obtaining a definite com- 

 pound 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 saline atom of water appears 

 to be feeble, as the salt can be made quite anhydrous under 300° ; 

 and consequently the sulphate of lime has much less disposition 

 to form double salts than the sulphates of magnesia, zinc, &c. 





Anhydrous 

 Salt. 



Water. 



Anhydrous 

 Salt. 



Water. 



Selenite, dried for ten days in open ) 

 air at 212°, j 



Do. dried in vacuo at 212°, . . . 



Sulphate of lime with one atom of \ 

 water (by theory), ... j 



Do. with two atoms of water (by ) 

 theory), j 



17.07 

 17.61 



4.27 

 3.04 



100 

 100 

 100 



100 



25.01 

 14.72 

 13.13 



26.26 



In drying gypsum, to make plaster of Paris, a third or a fourth 

 of the water of the salt is allowed to remain, by which it sets more 

 strongly. But the salt may be made quite anhydrous, I find, and 

 yet retain the power of recombining with two atoms of water, if 

 dried at a temperature not exceeding 270° F. ; although the hy- 



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

 proportion, by a protracted exposure to the same temperature. 



VOL. XIII. PART I. R r 



