70 Miss M. P. FitzGerald. The Origin of the [June 4, 
Prussian blue is not formed by the addition of a solution of the double 
citrate to one of potassium ferricyanide, even when acid is present, showing 
that the iron of the compound is all in the ferric state. 
It is thus seen that the double citrate presents none of the disadvantages 
pertaining to the use of other iron salts for subcutaneous or intravascular 
injection, either alone or in conjunction with a solution of ferrocyanide of 
potassium, and, moreover, when it gives in solution with the latter a distinct 
Prussian blue reaction, the additional presence of an acid, 2.¢. of extraneous 
origin, can be regarded as certain, whereas with other iron salts such a 
certainty cannot exist, as an acid may arise from the dissociation of such 
salts. For example, in a solution of ferric chloride, hydrochloric acid, arising 
from the dissociation of the salt, is present in sufficient quantity to cause an 
immediate formation of Prussian blue on the addition of potassium ferro- 
cyanide to the solution. Owing to the acid thus inherent, a similar 
formation occurs with the acetate, lactate, phosphate, sulphate (ferrous and 
ferric), nitrate, and iodide of iron, as well as with the ferrous and ferric 
ammonium sulphates, and, consequently, all such compounds are more or 
less unsuitable for the purpose and methods of an investigation such as the 
present. € 
The preparation of the double citrate used in these experiments was found 
by analysis to contain 25-7 per cent. of iron. Therefore, to ensure a solution 
of it and of the potassium ferrocyanide being so balanced that, on addition 
of a sufficient quantity of acid, all the iron of the citrate and all the Fe (CN)g 
atom groups would go out of solution as Prussian blue, it would be necessary 
to have present in solution five parts of the ferrocyanide to approximately 
every three parts by weivht of the double citrate dissulved. 
In this investigation, however, the solution used contained 1°5 per cent. of 
potassium ferrocyanide and 2°25 per cent. of ammonium ferric citrate, pro- 
portious which did not give a balanced solution as designated above. or 
the purpose of intravenous or subcutaneous injection, such balanced solutions 
are better than unbalanced ones, only, if the rate of diffusion of the two salts 
is the same through meibranes, vascular walls, and cellular structures, which 
has not been ascertained to be the case. 
ferrocyanide solutions had been standing in contact for from 36—48 hours with the 
following :— 
(1) Potassium dihydrogen phosphate. 
(2) Potassium dihydrogen phosphate and carbon dioxide. 
(3) Carbon dioxide. 
(4) Potassium dihydrogen phosphate and mono-sodium carbonate. 
The reaction was very slight in the tubes containing the latter compounds. 
