348 Mr. Graham on the Heat disengaged in Combinations. 
salt is 5°04, while the fall on the solution of its constituents 
dissolved separately is 1°°51 for the sulphate of potash, and 
0°67 for the hydrated sulphate of copper, making together 
2°°18, which is less by 0°:86 than the former. The fall on 
the solution of the crystallized double sulphate of zinc and 
potash approaches more closely to the united falls of its con- 
stituents dissolved separately, the former being 2°°60 and the 
latter 1° + 1°51 = 2°51. The fall on the solution of the 
crystallized double sulphate of magnesia and potash is 2°30; 
the united falls of its constituent salts 0°92 + 1°51 = 2°43. 
No perceptible change of temperature was observed when the 
solutions of a pair of these salts are mixed to form the double 
salt; which is in accordance with the conclusion of Dr. An- 
drews, that no heat is evolved in the combination of salts. 
I have not, however, succeeded in obtaining any direct 
proof of the formation of the double sulphates on mixture. 
To a solution of 77°97 grains, or one equivalent, of crystal- 
lized sulphate of copper in 1000 grains of water, 41°41 grains, 
one equivalent, of sulphate of ammonia dried at 234° were 
added and dissolved. ‘The fall on the solution of the last salt 
was 0°°56, or the same as when the salt is dissolved in pure 
water. No change took place in the colour of the solution of 
the copper salt. The last salt was selected for this experi- 
ment because it appears more disposed to form double salts 
than even the sulphate of potash. 
In certain cases, a double salt is formed on using a bisul- 
phate, while it is not with the neutral sulphate; as in the 
formation of sulphate of zinc and soda, from sulphate of zinc 
and bisulphate of soda, but not from sulphate of zinc and neu- 
tral sulphate of soda. ‘To a solution of 85:23 grains, or the 
equivalent, of crystallized bisulphate of potash in 1000 grains 
of water, 89°59 grains, or the equivalent, of crystallized sul- 
phate of zinc were added and dissolved, with a fall of 1°00, 
or the same as in pure water. Toa similar solution of bi- 
sulphate of potash, 77°35 grains, or one equivalent, of crystal- 
lized sulphate of magnesia were added and dissolved, with a 
fall of 0°-86; the same fall also as on the solution of the lat- 
ter salt in pure water. Yet the double salts crystallized out 
readily from botli of these solutions. 
I have formerly represented the anhydrous sulphate of 
magnesia and potash as corresponding with the protohydrated 
sulphate of magnesia. Now both these salts assume six atoms 
of water, and the heat then disengaged by the two salts is 
nearly the same :— Heat of hydration. 
Mg 0; S0,4 KR On8.0; sie <p ewes eee 
Me Oy 8.054 Dean oni) rae eye ty SRS 
