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PSEUDO-PHOTOGRAPHIC EFFECTS 73
passing through glass, mica, or aluminium foil even of extreme thinness, and it is not affected by an electric field. If the resin is dissolved in such an inactive liquid as alcohol, it renders the liquid active. He therefore came to the conclusion that the result could not be due to any radioactive products, but very probably was brought about by vapour given off.
Action of Metals. — A large number of experiments of a similar nature have been carried out by S. Saeland,1 in which the action of various metals upon photographic plates were tested. The surface of the metal was first of all cleaned with emery paper and then placed in contact with, or very near to, a photographic plate for a considerable time.
A photographic effect was then observed. The effect was greatest with magnesium, next with aluminium, then with zinc. As the metal surface becomes changed by exposure to the action of the air, so it gradually loses its activity in this respect, and it is worth noting that the more active the metal when clean, the sooner does it lose its power. When the surface is again cleaned, the activity is brought back to its maximum value.
Not only did the pure metals which were tested behave in this way, but several amalgams, in particular those of zinc, behaved in a similar manner.
That this effect is in all probability brought about by chemical action taking place between the metal and the sensitive film is clearly indicated by the following experimental results . If some time elapses between the exposure of the plate and its development, the blackening is much more intense, and a rise in temperature produces a similar result.
If the pressure of the air is diminished the effect of the metal becomes less and less, until in a vacuum it has no apparent effect. When dry hydrogen gas is used in the place of air the metal is without effect upon the plate ;
1 Ann. d. Physik., 26, 5, pp. 899-917, 1908.