Synthetic retarders having thiol radical, gave typical Langmuir isotherms in adsorption on photographic emulsions. Samples having no thiol grouping can be adsorbed, but the order of amount adsorbed is one tenth of the former. In spite of the difference of the adsorption properties, adsorbed amount necessary to retard the after-ripening process to the same order is almost equal. On some assumptions, area of domain encircled by adsorbed molecules was computed from experimental data. If the mechanism of chemical ripening by reduction is an autocatalytic one, this domain may represent sensitivity specks of maximum size in this experimental emulsion.
Estimation of this size gave the area of 700-2000 A², which is filled of 40-270 270 atoms of silver.

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1. The theory of SM silver potential in the silver mercaptide and silver sulfide equilibrium and its significance in photographic emulsions are discussed.
2. The SM silver potential of nine thiourea derivatives have been measured in various pH at 50°C.
3. Fog density and m. u. d. sensitivity about nine thiourea derivatives have been measured in twelve kinds of emulsions of which pH and pAg changed widely, are increased when their SM silver potential less noble than the St silver potential of control emulsion.
4. Then the authors have proposed that SM silver potential is adequate to measure of “Lability” of compounds containing sulfur atoms.

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