54 
PROFESSOR GRAHAM ON THE CONSTITUTION OF 
Oxalate of Chromium and Potash , of Peroxide of Iron and Potash, of Peroxide of Iron 
and Soda, 8$c. 
Cr'Cr CC 3 + 3 K CC + H 6 . 
Fe Fe CC 3 + 3 K CC + H 6 . 
Fe Fe CC 3 + 3 Na CC + H 10 . 
This group of salts has not hitherto been submitted to analysis, although they 
occupy the same important position among the oxalates which the alums do among 
the sulphates. 
13. Oxalate of Chromium and Potash. 
This remarkable salt was first described by Dr. Gregory, and its optical properties 
have been made the subject of a memoir by Sir David Brewster*. It is easily pre- 
pared by the following process, which is Dr. Gregory’s, with the proportions slightly 
altered, from a knowledge of the composition of the salt. One part of bichromate 
of potash, two parts binoxolafe of potash, and two parts hydrated oxalic acid, are 
dissolved together in hot water. There is a copious evolution of carbonic acid gas, 
arising from the deoxidation of the chromic acid, and nothing fixed remains except 
the salt in question ; of which a pretty concentrated solution crystallizes upon cooling 
in prismatic crystals, which are black by reflected light, but of a splendid blue colour 
by transmitted light, when sufficiently thin to be translucent. 
This salt, strongly dried without decomposition, was found to lose 1F67 per cent, 
of water. 
The oxide of chromium cannot be precipitated from it completely by means of an 
alkaline carbonate, and it is very remarkable that only a small portion of the oxalic 
acid is thrown down from this salt by chloride of calcium. 
To determine the proportion of oxalic acid, the salt was heated in strong sulphuric 
acid, and the gases allowed to escape through a tube containing chloride of calcium. 
15T9 grains of the crystals lost 6*71 grains by this treatment, which loss is the weight 
of the oxalic acid. Hence this salt contains 44-17 per cent, of oxalic acid. 
When this double oxalate is ignited, carbonic oxide escapes, and the residuary salt 
is a mixture of chromate and carbonate of potash, which is entirely soluble in water, 
and contains no oxide of chromium. In four experiments the fused residuary salt 
amounted to 0'5458, 0-5411, 0'5454, and 0"5425 of the weight of the crystals operated 
upon, while it should be 0"5433, provided this residuary salt is a mixture of two 
atoms chromate and one atom carbonate of potash, and that the composition of the 
crystals is as follows : 
* Philosophical Transactions, 1835. 
