222 Prof. Davy on some further applications of 
The reaction of the bichromate of potash on the ferrocyanide 
might be employed in the valuation of the ferrocyanide of 
potassium and other ferrocyanides—having previously,in the case 
of those which were insoluble, converted them into the ferro- 
cyanide of potassium by boiling them with caustic potash, and 
separating the insoluble oxides by filtration. 
It might also be employed for the valuation of the commercial 
red prussiate of potash, which is now to some extent employed 
as a bleaching agent in calico-printing, and which consists of 
varying quantities of ferro- and ferrid-cyanide of potassium 
together with chloride of potassium. By ascertaining first how 
much a given quantity of the sample requires of a standard solu- 
tion of bichromate of potash to convert the ferrocyanide present 
into ferridcyanide, the per-centage of that substance would be 
known ; and then by taking another portion of the sample and 
converting the ferridcyanide it contained, by reducing agents, 
such as the sulphites of soda and potash, &c., into the ferrocyanide, 
and finally determining the amount of bichromate necessary to 
bring the whole of the ferrocyanide then present into the state 
of ferridcyanide, the difference in the two results would indicate 
the proportion of ferridcyanide originally present in the sample. 
The last application of ferrocyanide of potassium which I shall 
notice in the present communication, is its employment as a 
reducing agent. It has long been known that the cyanide of 
potassium possesses most powerful reducing properties, and has 
been very usefully employed for that purpose in the reduction of 
different metallic salts under various circumstances; but I am 
not aware that the ferrocyanide of potassium has been proposed 
or used for similar purposes: at least, I have referred to a great 
number of analytical and general chemical works, and in none 
of them is this salt recommended as a reducing agent, though 
the cyanide is so much extolled for that purpose. According 
to my experiments, the ferrocyanide is a far more convenient 
reducing agent than the cyanide, and may be substituted for it 
in many cases of reduction with the best results, as it possesses 
many unquestionable advantages over that salt for this purpose. 
Thus the ferrocyanide does not deliquesce and decompose when 
exposed to the air, whereas the cyanide rapidly absorbs moisture, 
and, unless kept in very well-stoppered bottles, becomes quite wet, 
and in this state quickly decomposes; and this deliquescence on the 
part of the cyanide is often a source of much inconvenience in its 
use as a reducing agent, owing to the almost unavoidable absorp- 
tion of more or less moisture which takes place in mixing it with 
the substance to be reduced, and during the introduction of the 
mixture into the reducing tube. The ferrocyanide, on the other 
hand, in a thoroughly dried and finely powdered state, can be 
