724 Proceedings of Royal Society of Edinburgh. [sess. 
The blue salt (a) crystallises in the same system as the corre- 
sponding potassium compound, and is indistinguishable in appear- 
ance from it ; the red salt (b) is stated to be more freely soluble 
in water than is the corresponding potassium compound, but 
resembles it closely in all other respects. 
Subsequently to these researches of Werner on the Chromoxalates, 
Retgers * published his important papers on the relations of 
density and composition in various isomorphous mixtures. 
From theoretical considerations he showed that by plotting the 
specific volume against the composition, expressed in the percentage 
of one of the constituents, a straight line should be obtained if 
mixtures alone were present and no definite compounds were 
formed. The experimental results which he obtained in numerous 
cases of isomorphism were in complete accordance with this deduc- 
tion, and he came finally to the conclusion that two truly isomorphous 
compounds cannot unite together to give a definite compound.! 
In view of Rutgers* results, Rosenheim J assumed that Werner’s 
compounds should be considered as isomorphous mixtures of the 
potassium and ammonium salts. As, however, this has not been 
generally recognised, and as direct experimental results had not 
been obtained, I thought it advisable to apply Retgers’ method of 
investigation to the blue chromoxalates, which form well-defined 
crystals. To obtain the material for this purpose, following 
Gregory’s original method, I mixed solutions of different pro- 
portions of potassium bichromate, potassium and ammonium 
hydrogen oxalates, and oxalic acid, as indicated below : — 
Mixture. 
K 2 Cr 2 0 7 . 
khc 2 o 4 . 
nh 4 hc 2 o 4 . 
h 2 c 2 o 4 . 
Ratio 
K : NH 4 . 
1 
1 
4 
5 
6 ... 
2 
1 
3 
1 
5 
5 : 1 
3 
1 
H 
n 
5 
9 . 3 
2 • 2 
4 
1 
2 
2 
5 
4 : 2 
5 
1 
4 
5 
2 : 4 
6 
(NH 4 ) 2 Cr 2 0 7 .l) 
4 
5 
... : 6 
* Zeit. fur phys. Chem., 3, p. 497, etc. 
t Ibid., 15, p. 528. 
+ Zeit. fur anorg. Cliem., 11, p. 314, footnote. 
