OXALATES, NITRATES, PHOSPHATES, SULPHATES, AND CHLORIDES. 
59 
Oxide of copper . . 100’ 
Volatile matter . . 506 
]506 
Or the proportion of volatile matter in the subsait has suffered only a small reduc- 
tion, namely, from 53- 1 to 50 - 6 parts. This last subnitrate afforded drops of nitric 
acid with fumes of nitrous acid when heated in a tube, so that the subnitrate of cop- 
per retains water even at a temperature above the melting point of lead. 
The subnitrate of copper merits a careful consideration ; for the subsalts of the 
magnesian class of oxides, which can be had of a definite composition, are really 
much fewer in number than is generally supposed. What constitution ought to be 
assigned to this salt ? It will be observed that I have represented it by the singular 
formula 
HNCu 3 ; 
implying that the single atom of water which it contains is really the base of the 
salt, and that the three atoms of oxide of copper are in the place of the constitutional 
water of this nitrate of water. This opens a new view of the constitution of subsalts. 
The excess of metallic oxide which they contain may not be basic at all in certain 
cases like the present, but discharge a function in the constitution of the salt which 
has hitherto been recognised only as executed by water. For if we find water and 
oxide of copper strongly resembling each other as bases, why may not the analogy 
between them extend further, and oxide of copper be capable of discharging the 
function of constitutional water or water of crystallization in the composition of a 
salt ? Indeed the speculation that all salts whatever are neutral in composition is 
highly probable. Where the metallic oxide is in excess, as in what are called sub- 
salts, we can attribute another function than that of base to the whole or a portion 
of the metallic oxide, and thus preserve the salt neutral in composition, or according 
to its formula. To this subject I shall again recur. 
The following observation is particularly favourable to the view which we are 
taking of the constitution of subnitrate of copper. When the black oxide of copper 
is drenched with the strongest nitric acid, it is a subnitrate of copper which is formed, 
although the nitric acid may be in great excess. The black oxide is converted into 
a green powder, from which the excess of nitric acid should be drained off as well as 
possible, and the powder will be found to be in great part insoluble in water. The 
explanation seems to be, that the concentrated nitric acid employed does not contain 
the constitutional water which the neutral nitrate of copper requires, and accordingly 
that salt is not formed ; but the nitrate of water supplies itself with oxide of copper 
in the place of its deficient constitutional water ; so that the result is a nitrate of 
water with three atoms of oxide of copper attached. But when nitric acid of a spe- 
cific gravity not exceeding T42 is digested upon the same black oxide of copper, Ihe 
neutral nitrate of copper only is formed, and no subnitrate. 
