Mr, Graham on the Heat disengaged in Combinations, 335 
The salt was made certainly anhydrous by exposure to 
an incipient red heat for a considerable time, and the equiva- 
lent quantity, 37°98 grains, in the state of a fine powder, was 
thrown into 1000 grains of water, It did not cake, and was dis- 
solyed completely by stirring in about one minute and a half, 
The rise of temperature in two experiments was 4°°30 and 4°"g6, 
of which the mean is 4°33. To this must be added the heat 
lost by the liquefaction and solution of the hydrate-formed, 
Rise on solution of Mg O,SO, . . *. . 4°33 
Fall from solution of MgO, SO,;+7HO 0°92 
Whole heat disengaged by Mg O,SO, . 525 
Mg O, SO,, HO. It is not easy to obtain the sulphate of 
magnesia with exactly one atom of water. The salt first 
operated upon retained, after being dried by an. oil-bath, 
at 400° to 100 sulphate of magnesia only 14°14 water, instead 
of 14°81, the single equivalent. ‘The hydrate was therefore 
21 HO, The heat evolved by the solution of 43°35 grains, 
an equivalent quantity of this hydrate in two experiments, was 
3°:06 and 3°9, of which 3°-08 may be taken as the mean. 
Another portion of the same sulphate less strongly dried, re- 
tained to 100 sulphate of magnesia 15°75 water, which is 
1;,HO. The results from the solution of 43°93 grains, the 
equivalent of this hydrate, were 3°:03, 2°°98 and 2°93, of 
which the mean is 2°°98. The mean of the two sets of expe- 
riments, or 3°°03, probably does not differ far from the truth, 
Rise on solution of MgO, SO,,HO . . 3%03 
Fall on solution of Mg O,SO.,+ 7HO. 0°92 
Whole heat disengaged by Mg O, S O,, HO 5°95 
The anhydrous salt disengaged 5°-25, while the protohydrate 
disengages 3°'95; the difference, or 1°*30, is therefore the heat 
disengaged by the combination of the first atom of water with 
sulphate of magnesia. It thus appears that of the whole heat 
evolved in the complete hydration of sulphate of magnesia, as 
nearly as possible one-fourth is due to the combination of the 
first atom of water, one-fourth of 5°25 being 1°31. 
2, Sulphate of Zinc,—The equivalent quantity of the cry- 
stallized salt, 89°59 grains, contains 39°38 water, and was 
therefore dissolved in 960°6 water, ‘The fall of temperature 
in two experiments was 1°01 and 0°'98, of which the mean 
is 1°00. ‘This sensibly exceeds the cold produced by the solu- 
tion of crystallized sulphate of magnesia, which is 0°92. The 
difference has a real foundation, and is not the consequence 
of errors in experiment; for in two other sets of observations 
on the same salts made in glass, and which may be compared 
with each other, although not with the preceding experiments, 
