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HISTORICAL SURVEY 33
development of dry plates of this kind was commonly effected with pyrogallic acid. This substance is obtained by dry distillation of the gall-nut. It has a powerfully reducing action — that is, it precipitates metallic silver from silver solutions, exactly as green vitriol does.
But pyrogallic acid alone is not able to bring out an image on such an exposed dry plate, because another substance is necessary to yield the silver. This substance, viz., a solution of silver, is found on the plates themselves when they are wet. But in the case of these dry plates, the silver salt has been washed off ; therefore mixed solutions of pyrogallic acid and nitrate of silver had to be employed as developer. Finely divided silver is precipitated, adheres to the exposed places, and thus brings out the image. Nevertheless, dry plates did not at first give such beautiful and secure results as moist plates.
Experiments of Carey Lea and others. — The first step towards perfecting dry plates was taken by Carey Lea. He found that the preparation of dry plates could be essentially simplified by producing the sensitive salts of silver in the collodion itself. He also found that bromide of silver gave more sensitive dry plates than iodide of silver, when a different developer was used. His mode of preparing dry plates depended, therefore, on the production of bromide of silver in collodion. He dissolved bromine salts in collodion, added a solution of silver, and thus produced silver bromide, which remained suspended in the glutinous medium, forming what was termed the emulsion. The superfluous salts were removed by washing with pure water, after the mixture had been left to dry in shallow saucers. This film, after being again dried, was dissolved in alcoholic ether, and gave a pure emulsion which dried instantly on the plates. The sensitiveness of these dry plates was about one-fourth as great as that of a wet iodized collodion plate.
The process of developing these dry plates, however, c