198 Mr. Croft on a new Oxalate of Chromium and Potash, 



crystallize any better by spontaneous evaporation than out of 

 a very concentrated solution; it seems however to form more 

 regularly in warm air, as in summer. The best crystals are 

 generally formed on the surface of the solutions : they are very 

 minute, in the form of triangular plates ; when the crystals 

 form a mass at the bottom of the liquid the plates are thicker, 

 but their form is indistinguishable. The salt is of a deep red 

 colour by reflected as well as by transmitted light; the solu- 

 tion is green, or even black (when concentrated) by reflected 

 and red by transmitted light. The solution when at a 

 boiling temperature remains red, as is seen best by candle- 

 light : the same is the case with the solution of the black salt, 

 which shows that the purple oxide of chromium contained in 

 these salts is not converted by a boiling heat into its green 

 modification; the purple oxide must, however, as is well known, 

 be first brought into combination with the oxalic acid, for the 

 black salt can never be obtained by dissolving green oxide of 

 chromium in binoxalate of potash. 



A solution of caustic potash added to a solution of the red 

 salt turns it bright green, but causes no precipitate until boiled, 

 when the greater part of the oxide of chromium is thrown 

 down. Carbonates of the alkalies partly change the colour 

 in the same manner, but do not precipitate the oxide so readily. 

 Ammonia causes no precipitate, nor does chloride of calcium, 

 owing to the formation of Dingler's oxalate of chromium and 

 lime; when ammonia is added a green precipitate containing 

 oxide of chromium is formed. 



This salt contains a large quantity of water of crystalliza- 

 tion, which can only be driven out by a strong heat, as is also 

 the case with the black salt (Graham). It loses about 15-16 

 per cent, at 100° cent., and 19 per cent, at 200° cent. The 

 last portions of water can only be driven out at 300° cent. 

 Near this point the salt begins to be decomposed, and conse- 

 quently the determination of the water is rendered somewhat 

 difficult. per cent. 



0-9986 gramme of salt lost 0*2638 water = 26*42 



0-7481 0*1965 ... = 26'27 



0-8971 0-2532 ... = 28'22 



The determinations of the oxide of chromium and the po- 

 tash were performed in the following manner. The salt was 

 heated red-hot : in this operation great care must be taken, 

 for the salt possesses the curious property of decomposing 

 with considerable violence (without explosion) into a green 

 powder, which unless the heat is applied very gradually, is 

 forced out of the crucible, and the analysis is thus lost. When 

 the temperature is raised gradually the crystals retain their 



