OXALATES, NITRATES, PHOSPHATES, SULPHATES, AND CHLORIDES. 
53 
zinc, &c., the oxalate of magnesia or of zinc precipitates, while the quadroxalate of 
potash is formed, and remains in solution or crystallizes, being very sparingly soluble, 
according to circumstances. When binoxalate of potash is digested upon magnesia 
or upon oxide of zinc, a portion of the oxide is dissolved, but is quickly deposited 
again as an insoluble oxalate, and no double salt formed. But one member at least 
of the magnesian class of oxides, namely, oxide of copper, is dissolved by the binoxa- 
lates of the alkalies, and forms double salts, which were discovered and carefully 
examined by M. Vogel of Bayreuth. 
12. Oxalate of Copper and Potash. 
KCC + CuCCH 2 ; 
and also 
KCC + CuCCH 2 + H 2 . 
The binoxalate of potash, when considerably diluted, and digested with heat upon 
the oxide of copper, dissolves it easily, and a double salt of sparing solubility crystal- 
lizes, presenting itself generally in two forms, one of which contains two and the other 
four atoms of water, according to the analyses of M. Vogel, which I have repeated 
and confirmed so far as the water is concerned. The crystals containing four atoms 
of water soon become opake by exposure to the air, and lose two atoms of water by 
efflorescence. 
Binoxalate of ammonia likewise dissolves oxide of copper, and does so still more 
readily than the binoxalate of potash, which may depend upon the circumstance that 
the resulting double salt of ammonia is considerably more soluble in water than the 
double salt of potash. The oxalate of copper and ammonia crystallized in plates of 
a blue colour, and seemed to affect one form only. Of these plates, 9'38 grains were 
readily decomposed by heat, and left 2-37 grains black oxide of copper, or 25 27 per 
cent., which is almost exactly the proportion of that of the oxide of copper, which a 
salt of two atoms water should contain, namely, 25 - 37 per cent. This salt loses water 
readily at 212° Fahr. ; and of the 11'52 per cent, which it is supposed to have on the 
theory of its containing two atoms of water, 1T46 per cent, escaped by the exposure 
of the salt to that temperature. M. Vogel describes two other double oxalates of 
copper and ammonia; but it is evident that they contain ammonia and not oxide of 
ammonium ; so that they do not come under our consideration at present. 
It is to be remarked that the oxalate of copper and potash is represented above by 
a formula quite analogous to that of binoxalate of potash, oxide of copper being 
simply substituted for basic water. We have oxalate of potash in both cases, to 
which there is attached oxalate of copper with two atoms of water in the one formula, 
and oxalate of water with two atoms of water in the other. It is to be remembered 
that in the case of the sulphates, the double sulphate of copper and potash was shown 
to have a similar analogy in constitution to the bisulphate of potash. 
