1910.4 Hydrochloric Acid in the Gastric Tubules. 69 
of the hydrochloric acid, and by some modification of technique or change 
of salt the Prussian blue reaction in particular might still be of service 
in the solution of the problem. 
This opinion has received its justification, and results of a positive nature 
have been obtained by the substitution of the double citrate of iron and 
ammonia for the lactate of iron as used by Bernard in conjunction with 
potassium ferrocyanide. 
For the purpose to be accomplished, and the methods to be employed, the 
double citrate of iron and ammoniawas found to bea salt free from objectionable 
_ qualities inherent in the lactate and other salts of iron previously employed in 
Claude Bernard’s method. In its officinal preparation, and to meet the 
requirements of the pharmacopcela, the salt is generally made by dissolving 
freshly precipitated ferric hydrate in citric acid. Ammonia is added until 
the neutral point is reached, and the solution is then evaporated to the 
consistency of a syrup. In this condition it is spread out in thin layers on 
poreelain plates, and completely dried. The resulting scale-like residue is 
not uniform in its composition, but as a rule does not contain less than 
18 per cent. of iron, and may contain considerably more. The salt gives 
either a very feeble reaction to blue litmus paper or no reaction at all. The 
reaction of the sample used in the experiments was very faintly alkaline to 
red litmus paper ; and in aqueous solution the salt possessed a sweetish taste 
and very faintly astringent properties. 
If an aqueous solution of the double citrate is added to one of potassium 
ferrocyanide a formation of Prussian blue does not occur, nor will it have 
developed at the end of 24 hours. But if hydrochloric acid is added to the 
mixture, sufficient in quantity to make it of 0°03645 per cent. strength, 
which is a concentration less than that existing in the gastric juice, the 
Prussian blue reaction begins to develop and becomes more marked at the 
end of two hours, the colour of the mixture at the outset being greenish. 
If the acid added corresponds to 0:0729 per cent. strength, the formation of 
Prussian blue begins very quickly, and if it is of greater strength, ze. 0-1 per 
cent., the reaction is both immediate and intense. , 
The addition of sodium dihydrogen phosphate, or of disodium hydrogen 
phosphate, however concentrated, to a mixture of the double citrate and 
potassium ferrocyanide in solution will not produce Prussian blue, even 
during the course of 24 hours. Neither will carbon dioxide produce the 
reaction, even when present in such a mixture under considerable tension.* 
* The addition of the corresponding phosphates of potassium, or of mono-sodium 
carbonate, also gives a negative result. A series of experiments showed further that a 
Prussian blue reaction developed only after the ammonium ferric citrate and potassium 
