Mr. T. Graham on Liquid Diffusion applied to Analysis. 303 



was observed by Pean de Saint-Gilles in his metaperoxide of 

 iron. 



When a red solution, such as that described, was dialysed, 

 the oxalate of ammonia came away in a gradual manner; 30*6 

 per cent, of the oxalate of ammonia were found in the colourless 

 diffusate of the first twenty-four hours, 31 per cent, of the same 

 salt in the diffusate of the next three days, and 18'2 per cent. 

 in the diffusates of the following seven days, making altogether 

 79*8 per cent., or four-fifths, of the whole oxalate of ammonia 

 originally introduced. A small portion of the ammoniacal salt 

 is retained with force, as might be expected from a ferrocyanide. 

 Although the diffusate appeared colourless, it was found to con- 

 tain a little oxide of copper, narnety, 0*041 gramme (of which 

 0*022 gramme diffused out in the first twenty-four hours), from 

 2 grammes of ferrocyanide of copper placed in the dialyser. 



The liquid ferrocyanide of copper, both before and after being 

 dialysed, may be heated without change, but it is pectized by 

 foreign substances with extreme facility. This effect is pro- 

 duced by a minute addition of nitric, hydrochloric, and sul- 

 phuric acids in the cold, and of oxalic and tartaric acids with 

 the aid of a slight heat. It is remarkable that acetic acid does 

 not pectize the ferrocyanide of copper and many other colloids. 

 Sulphate of potash, sulphate of copper, and metallic salts gene- 

 rally appear to pectize the red liquid. The oxalate of ammonia, 

 if any is present, remains in solution. 



Neutral Prussian Blue. — The blue precipitate from perchloride 

 of iron and ferrocyanide of potassium, or ferrocyanic acid, is a 

 bulky hydrate, which dries up into gummy masses, so far resem- 

 bling a colloid. The precipitate dissolves readily with the aid 

 of a gentle heat, in one-sixth of its weight of oxalic acid, giving 

 the well-known solution of prussian blue, used as an ink. Prussian 

 blue is equally soluble in the oxalate and binoxalate of potash. 

 When the solution of prussian blue in oxalic acid was placed on 

 the dialyser, no colouring matter came through, but 28 per cent, 

 of the oxalic acid, diffused away in the first twenty-four hours, 

 accompanied by traces of peroxide of iron. The oxalic acid 

 appears to leave the colloidal solution very slowly and incom- 

 pletely, 8 per cent, diffusing away in the second twenty-four 

 hours, 11 per cent, in the next four days, and 2 per cent, in 

 the following six days. The colloidal solution of prussian blue 

 was pectized by small additions of sulphate of zinc and several 

 other metallic salts, but required larger quantities of the alkaline 

 salts for precipitation. 



Ferridcyanide of Iron. — The blue precipitate from the ferrid- 

 cyanide of potassium and a protosalt of iron is soluble in oxalic 

 acid and the binoxalate of potash, but not in the neutral oxalates. 



