ME. T. GEAH-M\I ON LIQUID DIFFUSION APPLIED TO ANALYSIS. 
211 
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 produced by a minute addition of nitric, hydrochloric, and sulphuric acids 
in the cold, and of oxalic and tartaric acids with the aid of a slight heat. It is remark- 
able that acetic acid does not pectize the ferrocyanide of copper and many other colloids. 
Sulphate of potash, sulphate of copper, and metallic salts generally appear to pectize the 
red liquid. The oxalate of ammonia, if any is present, remains in solution. 
Xeiitral 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 resembling a colloid. The precipitate dissolves readily with the aid of a gentle 
heat, in one-sixth of its weight of oxalic acid, gbing the well-known solution of prussian 
blue, used as an ink. Prussian blue is equally soluble in the oxalate and binoxalate of 
potash. ’SATien 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 incompletely, 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 quan- 
tities of the alkaline salts for precipitation. 
Ferridcyanide of Iron. — The blue precipitate from the ferridcyanide of potassium and 
a protosalt of iron is soluble in oxalic acid and the binoxalate of potash, but not in the 
neutral oxalates. This blue liquid is quite incapable of passing through the dialyser, 
and is equally colloidal with ordinary prussian blue. So also is basic prussian blue 
prepared by the spontaneous oxidation of precipitated ferrocyanide of protoxide of iron. 
This last colloid might probably be purified with advantage upon the dialyser. 
The ammonio-tarti'ate of iron, ammonio-citrate of iron, and similar pharmaceutical 
preparations are chiefly colloidal matters. 
Sucrate of Copper. — The deep blue liquid obtained by adding potash to a mixed solu- 
tion of chloride of copper and sugar appears to contain a colloidal substance. Placed 
on a dialyser for four days, tlie blue liquid became green, and no longer contained either 
potassium or chlorine ; it in fact consisted of oxide of copper united with twice its 
weight of sugar. The external liquid remained colourless, and gave no indication of 
copper when tested with sulphuretted hydrogen. The colloidal solution of sucrate of 
copper was sensitive in the extreme to poetizing agents. Salts and acids generally gave 
a bluish-green precipitate ; even acetic acid had that effect. The precipitate, or pectous 
sucrate, after being well-washed, consisted of oxide of copper with about half its weight 
of sugar, and is therefore a subsucrate. AVhen the green liquid is heated strongly, it 
gives a bluish-green precipitate, and does not allow the copper to be readily reduced to 
the state of suboxide. The subsucrate of copper possesses considerable vivacity of colour, 
and* might be used as a pigment. A solution, of sucrate of copper absorbs carbonic acid 
from the air with great aridity. 
2g2.- 
