314 Mr Granam 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, : : - P ‘ CaSH+H 
Gypsum dried at 212°, . : ‘ CaSH 
Anhydrous gypsum (dried at 270°), CaS— 
Anhydrite, : é ; i 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 zine, &c. by MgS—, ZnS—, &c.; the absolute sulphates 
of water, magnesia, zinc, &c., HS, Mg, 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. 
