Silver nitrate
There are two additional points relevant to Reade's erroneous claim: he was discussing the use of Gallic acid with silver nitrate. Silver nitrate is not a halide and unlike the chloride and fluoride of silver it has not the potential to develop the latent image. In addition Reade failed to understand or to make a distinction between tannic acid and gallic acid, referring to either "tincture, infusion of, or a decoction of galls" and gallic acid as though all were interchangeable. Any of these solutions would contain little more than 3% gallic acid, which is relatively slow acting. Tannic acid, on the other hand, which constitutes between 60 and 79% is fast acting. The result being that it would immediately act upon any gelatine present to render it insoluble; hence its use, since time immemorial, to tan leather which is a strategic material (i.e.,for solder's boots, and harness to attach guns to gun-carriages etc.). Talbot would have known of this group of organic compounds and there is evidence that he had experimented with gallic acid (2-3-4 tri-hydronitrobenzoic acid) since 1835 at the latest. First synthesised by Carl William Scheele in 1786 whose studies were widely known (earlier, in fact if you reference his experiments with secret writing). Reade's images darkened quickly because the tannic acid component of the"extract of galls" has the power to spontaneously reduce silver nitrate to its metallic state.