Mf. Graham on the Heat disengaged in Combinations. 331 
grains of oil of vitriol, the equivalent of the protohydrate 
being 613°5. It was weighed in ai exceedingly thin and 
light glass spherule, which was afterwards broken in the 
water, and the acid diffused through the latter. ‘The greater 
portion of the heat is disengaged in the first two or three 
seconds after mixture; or its evolution is almost instanta- 
neous. ‘Tc avoid the loss of heat by communication to the 
air, during the short time that must elapse before the ther- 
mometer in the liquid becomes stationary, the crucible, water 
and stitrer were previously cooled down so far below the 
teniperature of the air, as the liquid was expected, from a 
preliminary experiment, to risé on the addition of the acid. 
The crucible was also placed withiti a glass jar containing 
tow, to impede the passage of heat by conduction. I am in- 
debted for several valuable hints on the mode of conducting 
such experiments, to the papers of Dr. Andrews* and Profes- 
sor Hess+; who have preceded me in similat investigations. 
The rise of temperature in a preliminary experiment, in 
which the water and crucible were not previously cooled, was 
8°78 R. In two other experiments in which the crucible and 
water weré previously cooled before the addition of the acid, 
the rise was 3°88 and 3°°85. The mean of the last results, 
or 3°°86, may therefore be taken as the heat disengaged in the 
hydration of an equivalent of the protohydrate of sulphuric 
acid. No perceptible change of temperature occurred on 
dilating further with water the products of these experiments. 
2. HO,SO;+ HO. This is the crystallizable hydrate 
of sulphuric acid, of density 1°78. 36°3 grains, the equivalent 
quantity, were mixed with 1000 gtains of water, as in the pre- 
ceding case. The rise of temperature in three experiments 
was 2°40, 2°°36 and 2°40; of which the mean is 2°39. 
The dilution of this hydrate gives occasion to the disen- 
gagement of 1°47 less heat than the preceding hydrate. It 
appears, therefore; that in the dilution of the first hydrate or 
of sulphate of water; 1°47 is due to the combination of the 
first’ atom of water, with which it forms the erystallizable 
hydrate, and 2°39 to combination with all the rest, making 
together 3°86. 
8. HO,SO,+2HO0O. This is the hydrate of sulphuric 
acid in the formation of which the greatest contraction is ob- 
served to occur. With 41'93 grains, or one equivalent, the 
rise of temperature on dilution was in three experiments con 
ducted as before, 1°88, 1°86 and 1°85, of which the mean 
is 1°°86. The difference between the heat evolved by the 
present and the immediately preceding hydrate is 0°°53, which 
(* See Phil. Mag, S. 3. vol. xix. p. 183.] [t 7b. p.19.] 
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