Anomalous Liesegang stratifications produced by the action of light

The investigation to be described in the present paper was originally begun with the intention of studying the Liesegang phenomenon under conditions more strictly defined than have been observed hitherto, and more particularly with definite volume ratios of the reacting solutions. In all the experiments recorded in the literature the amount of solution placed on a given, or unknown, quantity of gel has been arbitrary and is generally not even stated. A quantitative treatment of the phenomenon is impossiblea priori as long as this ratio is unknown, and would probably become simplest if one, or both, volumes were made—practically—infinite. In the course of the preliminary studies for such experiments an extremely striking anomaly, not observed so far, was discovered, which appears to deserve being put on record before the main investigation is completed. The reaction chosen for this work was the formation of lead chromate in agar gel by the interaction of lead acetate (dissolved in the gel) with potassium or sodium dichromate, or with potassium chromate. This reaction, in very low concentrations, produces extremely fine and numerous stratifications, which were first obtained by L. J. de Whalley; the low concentrations and the cheapness of the ingredients also made it eminently suitable for work involving the use of large volumes. Soon after its discovery the reaction was employed by the author and the results discussed in their bearing on some controversial points, in the paper quoted above. A normal specimen is illustrated in fig. 1 (Plate 8); in this, as in all other specimens photographed, the excess of soluble dichromate present in the gel has been removed by prolonged diffusion, so as to show the lead chromate strata as distinctly as possible.