Prof. Graham on Water as a Constituent of Salts, 423 



phate of lime has much less disposition to form double salts 

 than the sulphates of magnesia, zinc, &c. 



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 sels more strongly. But the salt may be made quite anhy- 

 drous, I find, and yet retain the power of recombining with 

 two atoms of water, if dried at a temperature not exceeding 

 270° Fahr. ; although the hydrate which results on slaking in 

 the last case is rather pulverulent. When gypsum has been 

 dried at a higher temperature, as at 300° or 400° Fahr., 

 it refuses entirely to combine with water, and is technically 

 called burnt stucco. The anhydrous sulphate of lime which 

 occurs in nature exhibits the same indifference to water. In 

 anhydrite we have, I believe, the true or absolute sulphate of 

 lime in a crystallized state. The body which results from ex- 

 posing hydrated sulphate of lime to 270°, although composed of 

 nothing but sulphuric acid and lime, should be viewed as the 

 debris of the hydrated sulphate of lime, and not confounded 

 with the absolute sulphate of lime, which last has no disposi- 

 tion to combine with water. The first, which we may call 

 " anhydrous gypsum," is an imperfect body. We know sul- 

 phate of lime in four states, which may be expressed symbo- 

 lically as follows: 



Gypsum CaSH + H 



Gypsum dried at 212° Ca'SH 



Anhydrous gypsum (dried at 270°) CaS — 



Anhydrite CaS 



Here we distinguish the imperfect body, anhydrous gypsum, 

 from anhydrite, by placing the minus sign after the former. 

 In the same manner, concentrated sulphuric acid, or oil of 



vitriol, may be represented by HS— ; anhydrous sulphate of 



