It has been recognised since the middle of the eighteenth century that one of the most fundamental characteristics of living organisms is their capacity to oxidise substances incapable of oxidation at ordinary temperatures; but no qualitative estimation of this power of oxidation was carried out until the time of Ehrlich, whose classical experiments on the injection of methylene blue into the intact animal revealed the fact that certain organs seemed to have a higher reducing power than others. Later, much histo-chemical work was done. Numerous observers studied the effect of staining tissues and cells in reagents which indicated by their colour whether they were reduced or oxidised. Several such indicators were used by the earlier workers, especially alphanaphthol and pyronin and alpha-naphthol and gentian violet; but the two chief methods were the intracellular formation of an indophenol, introduced by Röhmann and Spitzer in 1895 (20), and the oxidation of the leucobase of methylene blue, introduced by Unna in 1911 (23). In the latter case the cell was placed in a solution of the completely reduced dye, and the conclusion was that wherever the blue colour appeared, there the cell had been able to oxidise it. The indophenol method depended on the actual formation and precipitation of a dye in the cell by an oxidative condensation. The original reactants used were dimethylparaphenylenediamine and alpha-naphthol, but various later workers modified this by using other phenols and other aromatic amines, so that indophenols of different colours were produced.
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