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DRY PLATES, FILMS AND PAPERS 179
cessive formation by the action of light of sub-haloids from the normal haloid by removal of one atom of the halogen at a time.
The normal haloid is considered by him to be of the form Ag2„H2tt and not AgH, thus the formula of silver bromide according to his theory becomes Ag8Brs.
A Peculiar Effect of Chemicals upon Normal Latent Image. — The following peculiar effect of chemicals upon the normal latent image has been noted by E. Demole.1 If a plate has been exposed under a negative and then immersed for some minutes in a 1 per cent, solution of potassium ferricyanide, then rinsed and developed in a hydro quinone developer, great over-exposure is then found to have no effect at all in accelerating the development or as regards the final result. Further, when the development is carried out under the action of the light of a candle instead of that from the red lamp a negative is obtained instead of a positive. Such a result can be produced even when only very short exposures are given, although a longer exposure renders the result obtained more vigorous.
Thus he found that a Lumiere plate (blue label) was sufficiently exposed for ordinary development by 1 second exposure under a negative to an arc lamp at 50 cms. distance. When ferricyanide and candle-light were used, exposures ranging from 1 to 170 seconds were found to give negatives, but with 180 seconds the plate showed a positive which became a negative, with 7 minutes a positive only but not clear in the whites, while with 14 minutes' exposure it produced a perfect positive. He explains this action on the assumption that an oxybromide is formed which is comparatively difficult to reduce by the
Koninh. ATcad. Weiensch. Amsterdam Proc, 11, pp. 2-29, 1908; Konink. Akad. Weiensch. Amsterdam Proc., 11, pp. 730-747, 1909; Zcitschr. Wiss. Phot., 6, pp. 438-442, Dec. 1908.
For further work on solarization, see article by G. A. Perley, Joum. Phys. Chem., 13, pp. 630-658, Nov. 1909.
1 Comptes Iiendus, 144, pp. 565-567, March 1907.