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RELATIVE MASSES OF PHOTO-SILVER 175
On the other hand, the fogging effect of excess of sulphiding agent might be owing to the probability of adlineation of stray silver atoms increasing with the mass, and particularly with the concentration (i.e., size) of silver sulphide. The importance of the competition between sensitizing specks, as proposed by Sheppard, Trevelli and Loveland, and further developed by Sheppard (28) , does not appear to be diminished by the foregoing considerations. The actual question of the relative amounts of silver and silver sulphide in sensitivity specks and in latent image specks respectively can only be attacked to a limited extent by chemical analysis, but this should be capable of deciding in certain cases whether the silver is far from equivalent to sulphur. Extension of Hilsch and Pohl's optical method to silver halide crystals containing silver and silver sulphide may also yield results of importance.
The considerations advanced in respect of the relative masses of "sensitivity specks" and "photo-silver" in the "latent image," if correct, lead to certain interesting conclusions. Apparently, in highspeed plates the "latent image" should be actually composed much more of silver sulphide than of metallic silver, although developability would be owing to accretion of this latter element. Further, the "nucleus bareing" action described by Luppo-Cramer might consist as much (or more) in a reorientation or displacement of silver atoms (to form a lattice nucleus) as in actual "laying bare" or digging out of embedded nuclei. Also, the element of contingency or probability in photographic sensitizing is even more emphasized than when nucleus size was the sole variable considered. Size of nucleus becomes a contingent condition for securing sensitivity, rather than a necessary and sufficient condition; the probability of active silver lattices being formed increases with the size of the sensitivity nucleus.
Conclusion. — Recent determinations and calculations indicate that the number of atoms of photo-silver formed per gram of silver halide in latent image formation is too small to agree with previous "critical size" theories of nucleation. It is suggested that "aspect," or orientation of silver atoms to initiate a silver space lattice is a more essential function of sensitizing specks than mere size.
Bibliography
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(3) Sheppard. S. F..: Phot. Korr., 1922. 58-59. 76.
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(12) Jones. L. A., and Schoen. A. L.: J. Opt. Soc. Amer.. 1923. 7. 213.
(13) Sheppard. S. E.. Trevelli. A. P. H.. and Loveland. R. P.: ,/. Franklin Inst.. 10 25. 200. 5 1