216 _ Prof. Davy on some further applications of 
The same occurs, as far as the conversion of the ferrocyanide 
into ferrideyanide, when an acidified solution of the former salt 
is brought in contact with a solution of the permanganate of 
potash, which is instantly decolorized by the reducing action of 
the ferrocyanide of potassium, which is thereby converted into 
the ferridcyanide, and this decoloration of the permanganate 
continues as long as any of the ferrocyanide remains in the mix- 
ture. 
Again, if a solution of the ferrocyanide of potassium, acidified 
strongly with either hydrochloric or suiphuric acid, be brought 
in contact with a solution of the bichromate of potash, the same 
change of the ferrocyanide into the ferridcyanide immediately 
takes place. 
The first reaction has been long known, and 1s the means em- 
ployed at present for obtaining the ferridcyanide or red prussiate 
of potash for manufacturing and other purposes; the second re- 
action has been more recently discovered; but I am not aware 
that the third, in the case of the bichromate, is generally known, 
or that the changes which occur in the reaction have been pre- 
viously studied. 
From experiments which I made, it would appear that when a 
solution of ferrocyanide of potassium, acidified with hydrochloric 
acid, was mixed with one of the bichromate of potash, the follow- 
ing reaction was produced, viz. 6(K? Fe Cy?) + KO, 2 Cr O® 
+7HC]=3(K? Fe? Cy®) + 4 KC1+ Cr? C+ 7HO; for, amongst 
other facts, I may observe that when I mixed together solutions 
of the two salts in the proportions corresponding to 6 equiva- 
lents of the ferrocyanide of potassium to 1 of the bichromate 
of potash (as indicated in the above formula), acidifying the 
mixture with hydrochloric acid, I found that the whole of the 
ferrocyanide was converted into the ferrideyanide, and that any 
quantity less than that proportion of the bichromate of potash 
left more or less of the ferrocyanide unchanged. The same 
results followed the use of sulphuric acid; and it appears that a 
similar reaction occurs with this acid as with hydrochloric acid, 
with the exception that in this case the 4 equivalents of chloride 
of potassium and the 1 equivalent of sesquichloride of chromium 
are replaced by 4 equivalents of sulphate of potash and 1 of the 
sesquisulphate of chromium. 
The proportion of either acid used, provided there is enough 
to strongly acidify the mixture, does not appear to affect the re- 
action; for I obtaimed precisely the same results where a very 
large amount of acid was employed as where the quantity neces- 
sary Only to strongly acidify the mixture had been added. 
On these three reactions which I have noticed, may be based 
the means of employing the ferrocyanide of potassium in several 
