PROFESSOR GRAHAM ON THE CONSTITUTION OF 
combinations of sulphuric acid with salts. Chromic acid, which is isomorphous with 
sulphuric, forms combinations which I consider as analogous to these. With the 
neutral or yellow chromate of potash it forms the red chromate of potash, and with 
chloride of potassium it forms M. Peligot’s salt ; which differ only from M. Rose’s 
corresponding combinations of sulphuric acid, in being more permanent. The supe- 
rior stability of these chromic acid combinations unquestionably depends upon the 
little affinity for water which their acid possesses, while the affinity of sulphuric acid 
for water is very great. Hence we may suppose that the red chromate of potash is 
not a direct combination of two atoms of chromic acid with one atom of potash, but 
a combination of one atom of chromic acid with one atom of yellow chromate of 
potash ; and it may be represented as follows : 
(K Cr) Cr. 
The red chromate of potash will thus belong to a new order of combinations, dif- 
fering essentially from proper salts, which contain an oxide as base. This salt, there- 
fore, cannot be adduced as militating against the law that “all salts are neutral in 
composition”; the only known exceptions to which law are, I believe, afforded by 
the anormal classes of phosphates, phosphites, and arseniates. 
I have devoted much time to the examination of subsulphates of the magnesian 
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class of oxides, particularly of the subsulphate of zinc and the subsulphate of copper. 
These salts were generally formed by the partial precipitation of sulphate of zinc or 
sulphate of copper by means of caustic potash. They have both a disposition to carry 
down sulphate of potash, which is never entirely removed from them by washing ; 
while one of them, the subsulphate of zinc, is itself decomposed by washing. When 
most successfully prepared, they were found to contain four atoms of metallic oxide 
to one atom of acid, (instead of three atoms oxide, as M. Berzelius supposed,) to- 
gether with four atoms of water. I have not hitherto been able to form a distinct 
idea of their constitution, or to decide between different views which may be taken 
of it. But the force with which water is retained in these subsalts is very remarkable. 
The subsulphate of copper loses no portion of its four atoms of water at 212°, and I 
have not been able to reduce the quantity of water retained by this salt so low as one 
atomic proportion, even at the melting point of lead. 
The constitution of the subsulphate of copper appears to be changed when it is 
made anhydrous by heat. In the progress of the desiccation of the salt, its colour 
passes from a dull blue to an olive green, and it finally becomes of a chocolate brown, 
and is then anhydrous. Water poured upon the brown matter comes off of a blue 
colour, dissolving out a considerable portion of the soluble sulphate of copper. It 
appears, therefore, that the water originally present in the subsulphate must discharge 
some important function in its constitution, the subsalt being obviously decomposed 
when made anhydrous. 
The Alums form a most important class of the sulphates, but I have never had it 
in my power to compare their constitution with that of the sulphate of alumina itself. 
