Science . ds explanation is briefly: Under certainconditions supersaturated solutions areformed, and when solid crystals or nuclei 1 Liesegang, Zeit f. Phys. Chemie, 1907, 59, 444. For details see Ostwald-Fischer, Theoreticaland Applied Colloid Chemistry, Wiley and Sons,New York, 1918. 3 Ostwald, Lehrbueh der Allg. Chemie (2aufl.), XL, 778. iBechold, Zeit f. Phys. Chemie, 1905, 52, 185. 5 Hatschek, E., Kolloid Zeitschrift, 1911, 9, 97;1912, 10, 124. July 22, 1921] SCIENCE 79 of precipitation are excluded, this supersatur-ated condition may persist for a considerabletime without the spontaneous
Science . ds explanation is briefly: Under certainconditions supersaturated solutions areformed, and when solid crystals or nuclei 1 Liesegang, Zeit f. Phys. Chemie, 1907, 59, 444. For details see Ostwald-Fischer, Theoreticaland Applied Colloid Chemistry, Wiley and Sons,New York, 1918. 3 Ostwald, Lehrbueh der Allg. Chemie (2aufl.), XL, 778. iBechold, Zeit f. Phys. Chemie, 1905, 52, 185. 5 Hatschek, E., Kolloid Zeitschrift, 1911, 9, 97;1912, 10, 124. July 22, 1921] SCIENCE 79 of precipitation are excluded, this supersatur-ated condition may persist for a considerabletime without the spontaneous development ofa solid phase. Such solutions he calls meta-stahle. By the diffusion of the silver salt intothe chromate gelatine, a solution is formedwhich in relation to the silver salt is super-saturated. A precipitate is formed only afterthe metastable limit has been exceeded. ThisjDreeipitate occurs naturally in zones con-centric with the drop. On the precipitatethat is formed the silver supersaturated. The siimllcr lijfure {A} sliowa the sphere ofinfluence of the silver nitrate. The larger figure(B) shows the efEeot of the smaller when includedin the larger. Note that the smaller sphere hasremoved the chromate so that the rings in thelarger are interrupted. chromate in the region lasts until all the sol-uble silver is precipitated. Then the silversalt wanders out over the ring into thechromate gelation until a new supersaturatedregion is formed and the precipitation processis repeated. The main objection to Ostwalds explana-tion is that a supersaturated condition hasbeen shown unnecessary for ring formation;also that there are other factors involved inthe ring formation.* Since no explanationhas been accepted, I wish to present one whichseems adequate. The chromate in the gelatine is relativelyfixed and diffuses very slowly; when AgNOjis added, there is an immediate formation ofsilver chromate not only under the silver so- lution but there is a sphere of influence whichcan
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