64 Miss M. P. FitzGerald. The Origin of the [June 4, 
The Prussian blue method was again resorted to by Sehrwald in 1889.* 
He criticised the experiment of Claude Bernard, and stated that in it the 
chemical premise was wrong, as the lactate of iron and potassium ferro- 
cyanide had been employed for the production of Prussian blue in the 
presence of an acid; and such a result could not follow, since the iron in 
the lactate acts as a divalent metal, and therefore the salt belongs to the 
ferrous oxide compounds, which will not form Prussian blue with potassium 
ferrocyanide either in the presence or in the absence of an acid, a ferric salt. 
being necessary for such a formation. He also says that if Claude Bernard 
obtained Prussian blue in the stomach, notwithstanding his chemical error, 
this would be due to the inherent tendency of ferrous salts to oxidise and 
therefore change to ferric salts, which with potassium ferrocyanide will form 
Prussian blue in an acid solution. Through this tendency, which would 
naturally persist in the body, the blue colour could appear in the acid 
parietal cells. 
Sehrwald, therefore, used lactate of iron together with potassium ferricyanide, 
which, in an acid solution, forms Turnbull’s blue (Prussian blue). To avoid 
the possibility of the reaction being prevented by one or other of the salts under- 
going change in the body, he abandoned the injection method, and placed 
small sections of a fresh stomach, first, into a solution of lactate of iron for 
one day, and thence, after washing quickly, into a solution of potassium 
ferricyanide. 
As Lepine before him, Sehrwald had observed the acid reaction of different. 
iron salts in solution, including the lactate, and in such cases, without the 
further addition of an acid, that the formation of Prussian blue would occur 
when solutions of an iron salt and potassium ferricyanide were brought 
together. Taking this fact into consideration, he argued that if the cells 
were alkaline the acidity of the reagent would be neutralised in part and the 
cells would be uncoloured, and that if this alkalinity exceeded a certain 
degree the cells must remain so, but if acid, they would become intensely 
blue in colour. 
As the result of his experiment, he found that the parietal cells had become 
deep blue in colour, while the chief cells were absolutely colourless. He 
therefore concluded that the parietal cells were less alkaline than the 
chief cells—the alkaline reaction being demonstrated by their colourless 
condition—and that the intensity of the blue colour in the parietal cells 
pointed to their being neutral if not acid in reaction, and strongly 
supported the view of their forming the acid. 
Sehrwald also observed a blue colour on the surface of the mucosa, and in 
* Sehrwald, ‘Miinchener Med. Wochenschr.,’ 1889, p. 177. 
