90 
Proceedings of the Royal Society 
include the case of homologous series, and the essential difference 
between this series and x, <f> • x, <f> 2) • x. d> 3 • x <£ n) • x is pointed 
out. 
2. Notice of an Easy Method of Preparing Nitroprussides. 
By Dr Lyon Playfair, F.R.S. 
Since I described the method of making nitroprussides by the 
action of nitric acid on yellow prusside of potassium, Hadow 
(Chem. Soc. Journal, 1866) has given a more productive process, 
by treating potassic ferridcyanide with an equivalent quantity of 
mercuric chloride, of sodic nitrite, and of acetic acid. Hadow 
contends that N 2 0 3 substitutes Cy 2 in this reaction, instead of NO 
as supposed by me. I am still of opinion that the formula of 
nitroprussides is FeCy 5 NOM 2 . For if an equivalent quantity of 
an acid he added to a solution of potassic ferridcyanide, and a 
stream of NO be passed through this solution, at a temperature of 
between 70° C and 80° C, nitroprusside is slowly but abundantly 
formed, with the evolution of hydrocyanic acid. 
FeCy 6 K 2 H + NO = FeCy 5 NOK 2 + HCy. 
In these experiments every care was taken to remove all traces 
of a higher oxides of nitrogen. But this process is not productive, 
and is only interesting as showing that NO can substitute Cy. 
After various experiments, I found the following simple process 
to yield excellent results : — Equal weights of potassic ferridcyanide 
and of crystallised tartaric acid are dissolved separately in the 
smallest quantity of cold water. One-half of the solution of tartaric 
acid is then mixed with the solution of ferridcyanide and briskly 
stirred, the clear liquid being decanted from the precipitated cream 
of tartar. A standard solution of sodic nitrite* having been de- 
termined beforehand, a quantity, corresponding to one equivalent 
for each equivalent of the potassic ferridcyanide used, is now divided 
into two parts. One-half of this solution is added to the cold mix- 
ture as above described, and immediately after the remaining half 
* I prefer for this purpose sodic nitrite, made by fusing lead with sodic 
nitrate, lixiviating with water, and then adding H 2 S0 4 cautiously, till the 
lead is precipitated. 
