Mr Granam on Water as a Constituent of Salts. 3138 
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, zine, &c. 
Anhydrous 
Salt. 
Selenite, dried for ten days in open \ 
air at 212, . . . 17.07 4.27 
Do. dried in vacuo at 212, . . 17.61 3.04 
Sulphate of lime with one atom of 
water (by theory), 
Do. with two atoms of water (by 
theory),. 2... 1... 
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- 
SS DS) Bene OS 
* 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. Rr 
