314 Mr Graham on Water as a Constituent of Salts. 



drate which results on slaking in the last case is rather pulveru- 

 lent. When gypsum has been dried at a higher temperature, as 

 at 300° or 400° F., 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 exposing hydrated sulphate of lime to 270°, although com- 

 posed of nothing but sulphuric acid and lime, should be viewed 

 as the debris of the hydrated sulphate of lime, and not confound- 

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

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

 hydrous gypsum," is an imperfect body. We know sulphate of 

 lime in four states, which may be expressed symbolically as fol- 

 lows : 



Gypsum, CaSH+H 



Gypsum dried at 212°, . . . CaSH 



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 magnesia, sul- 

 phate of zinc, &c. by MgS — , ZnS — , &c. ; the absolute sulphates 



of water, magnesia, zinc, &c, HS, MgS, ZnS, &c, being unknown 

 to us. 



The view which is given in this paper of the constitution of 

 the sulphates, must not be hastily generalized and applied to 

 other classes of salts. From investigations not yet completed, I 

 am satisfied that each class of salts has its peculiarities, which 

 must be studied before the law of the class can be laid down. 



