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K h i ~ isilicxr gel containing cobalt nitrate i owed to set, and is then overlaid by a solution of aodiiirn sulfide, black rings of cobalt sulfide are fomied in profusion. The first rings are very narrow and very numerous, the later rings beconie gradudly broader, lcss numerous per unit of lcngth xud spaced iurther apart. A suitable silica gel is rriade by pouring ro8 cc. uf 1.060 coinrnerciiil sodium silicate solution into 33 cc. nuter to whicli 3.5 PC. glseial acetic acid and 0.70 granis cobalt nitrate crystals lmve been added previously. At once aft,er mixing the two solutions, the resulting mixture, which is still pink, is poured into the special tubes clescribrd below, each one i-6 ceiving 2 2 cc. TIie height of the liquid in the tubcs is i jo nini from the bottom. The gel sets in about 3 minutes, and is overlaid a t once with a 3$& sodium sulfide (3% Sa,S) solution. Rings iorrn a t once and continue to iorm over a period of days. A portion of such a tube is shown in the sccoinpanying photograph. Such was thc firvt procedure. It had t.he deieet of involving a collapse of the upper part of the silica gel, as the solvent action of the alkaline sulfide took plscc. To the rate of d v a n c e of the black front, it u-as nticesiiary to allow ior the distance collapsed, which incrcnsed from day to day; thc first rings were destroyed and so withdrawn iroin observations; t.he distance of the top oi the gel to any point in the gri ehanged constantsly;there were several other disturbing results so that comparative values over several
