The possibility that the rock-wall of a vein acts as a dialyzer, allowing some dissolved substances to penetrate it while confining other substances to the vein, was suggested by G. F. Becker. • Recently W. Lindgren and F. L. Ransome 2 have discussed the subject in detail in connection with Cripple Creek gold deposits. On the experimental side, F. H. King and later L. J. Briggs, a filtering very dilute soil solutions (containing potassium and stadium carbonates, sulphates, nitrates, and

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... es) through a porcelain tube, observed an increase in the concentration of the unfiltered portion and a decrease in the concentration of the flitrate. "This would follow from the tube behaving as an imperfect semi-permeable membrane, allowing the water to pass through the filter walls more readily than the dissolved salt. "4 The work to be described in the present paper was carried out with an ordinary small Pasteur-Chamberland. filter-tube enclosed in a stoppered glass cylinder to prevent evaporation. To wash the tube several liters of the ferric sulphate solution were passed through before observations were begun. A solution of ferric sulphate of about •Aooo molecular concentration (Ca 0.562 g. Fe2(SO4)a, 9H20 in one liter) was filtered through the Pasteur tube under pressure of one or two feet of water. Oxide of iron began to separate owing to hydrolysis soon after the solution was made up and this substance formed an adherent coating on the filter and became in fact a