Relationship between chemical structure and the fog inhibiting action was examined with heterocyclic mercapto derivatives and some sulfur-containg heterocyclic compounds such as thiazole, rhodanine, isorhodanine, thiazolidine-carboxylic acid, imidazole, pyridine, quinoline, pyrimidine, thiazoloquinoline and quinine derivatives, possessing affinity to silver or silver ion. For such purposes, introduction or blocking of an active grop, removal of an injurious group and comparison with other heterocyclic derivaties as the control were carried out.
Fog inhibition effect was judged by the emulsion test in which
(1) the maximum value of relative fog, at the time of ripening, i. e. fog at the time of sample addition/fog by the control gelatine, was chosen and from which the fog inhibitory power of the sample was found from the sensitivity at the time and the ripening conditions; and (2) the comparison of curves obtained by plotting fog at the time of ripening and sensitivity on the ordinate and abscissa, respectively, with that of the control gelatine.
The results obtained may be summarized as follows:
i) The majority of compounds which possess SH group or a skeleton which can form SH group by ring cleavage can act as sensitizor or fog inhibitor.(Example: Thiazole, isorhodanine, thiazlidine-carboxylic acid, mercaptoimidazole, mercaptopyrimidine, etc.). It is assumed that a similar fact can be established with corresponding selenium compounds.
ii) Many of the compounds which possess active NH group in the molecule and possess affinity to silver ion show fog inhibiting acion.(Example: Thiazolidine-carboxylic acid, mercaptoimidazole, mercaptopyrimidine, etc.).
iii) The silver salts of SH group are assumed to be stablized when they form intramolecular salt with the neighboring COOH group or ≡N radical and often show desensitive fog inhibiting action.(Example: Thiazolidine-carboxylic acid, aminopyridine, etc.).
iv) The inhibiting action of these compounds disapper when the active hydrogen in the active center, SH or NH group, is blocked or its chelation, ability decreased by the change of neighboring groups responsible for the chelation.
v) The fog inhibiting action of desoxyquinine does not belong to any of the above classifications. It seems necessary, therefore, to consider other and more complicated factors for the appearaance of such an action, such as the solubility, shape of the molecule and adsorption of the crystal lattice of silver bromide.

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