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RELATIVE MASSES OF PHOTO-SIL\ rEB 173
sion that this "Ursilber" was metallic silver and not silver sulphide was based entirely on solubility in persulphate, a criterion which I cannot regard as definitive for dispersions of silver sulphide so finely divided as would be in question. In any case, since we can demonstrate (a) presence in gelatin of "labile" (silver sulphide forming) sulphide sufficient to give Weigert and Liihr's "Ursilber" values, (b) presence on silver bromide grains of comparable amounts of silver sulphide, (c) removal from gelatin of comparable amounts of "labile" sulphur — it would seem that any demonstration of such quantities of metallic silver requires also analytical evidence of the absence of silver sulphide. We are proceeding at present to a comparison of Weigert and Liihr's silver determination with our method for determining "labile" sulphide and silver sulphide.
This question, viz., to what extent silver or silver sulphide forms the substance of "sensitizing specks" in a given case is, however, rather beside the present point, which is the discrepancy, on the basis of the data available, between the masses of "sensitizing specks" and of photo-silver respectively. This can be expressed broadly as:
Mass per 1 gram Molecules
AgBr in grams, per molecules AgBr.
Photo-silver 1 0-" to 1 0-7 1:1 0'° to 1 07
Sensitivity specks 10-fllol0-4 I:105to2xl04
A concentration of photo-silver of 1 0-9 grams per 1 gram AgBr means only 50 to 60 atoms of silver in a grain of 1^2x0.1m. Even supposing all these silver atoms present in a surface layer 2 molecules thick of ca. 107 molecules AgBr, the proportion of photo-silver atoms is extremely low. Any chance aggregate out of this would be very small compared with:
(a) The number of silver atoms supposed necessary to be aggregated to a particle large enough to form a developable nucleus — according to Zsigmondy and Svedberg around 300 atoms;
(b) The mass of silver sulphide sensitizing specks — the mass of photo-silver being estimated at about 10-5 to 10-2 of these.
If these principal conclusions prove correct they throw considerable doubt upon the hypothesis that the necessary and sufficient condition for developability of a silver halide grain is the presence of a silver nucleus of sufficient size. It is true that if the presence be assumed of silver "sensitivity specks" of size a few atoms below the critical limit (cf. argument of Eggert and Noddack already noted) the hypothesis can be formally saved. But if the sensitivity speck consists principally or entirely of silver sulphide, how can the addition or absorption of a very few silver atoms evolve a nucleus large enough for developability?
It has been shown recently by the writer and W. Vanselow (21) that silver sulphide nuclei can replace silver nuclei very effectively in catalyzing the thermal decomposition of silver oxalate. The reaction AgsCoO, > 2 Ag + 2 C02
proceeds rapidly at the interface Ag,S: Ag,C,Ot, but naturally replaces this by the interface Ag: Ag,C.O,. Since the total interface grows with time of reaction, the course is still autocatalytic, but the