21 5 
the compounds of chromium. 
heat it so as to expel a considerable portion of the water 
of crystallization, we obtain a greenish-black matter quite 
tasteless, and similar to charcoal in its appearance. If this 
powder be digested in water it gradually dissolves, and the 
solution has the usual colour and taste of a solution of oxalate 
of chromium. It reddens vegetable blues. 
A quantity of oxalic acid crystals, containing 45 grains of 
real acid, was dissolved in water and digested for several 
weeks over an excess of hydrated oxide. The quantity of 
oxide dissolved was exactly 45 grains, and the weight of the 
oxalate after evaporation to dryness was 120 grains. Hence, 
the constituents were, 
Oxalic acid - - 45 or 5 - or 1^ atom. 
Green oxide - - 45 or 5 - or 1 atom. 
Water - - - so or 3.333 or 3 atoms. 
120 
This oxalate was very nearly though not quite neutral. 
We see here the difficulty in obtaining exact chemical com- 
pounds of bodies which are not capable of crystallizing. 
Probably, I might have rendered the salt neutral by means 
of a slight admixture of ammonia. 
10. Tartrate of Chromium. 
Tartaric acid gradually dissolves hydrated oxide of chro- 
mium, and forms a dark blue-coloured liquid, having the 
usual sweet acidulous taste which characterises these salts. 
When the liquid is evaporated to dryness, a black, brittle, 
tasteless powder remains, which is tartrate of chromium. 
To determine the composition of this salt, 148 grains of 
