1. Salts of 5:6-benz-4-carboline and its derivatives precipitate colloidal gum benzoin and other negatively charged lyophobic sols at comparatively low concentrations, but at considerably higher concentrations they do not bring about flocculation of these colloids but confer a positive charge on the colloidal particles. These effects are similar to those observed with many dyestuffs.

2. The effect of the simultaneous presence of benzcarboline salts and of gelatin of pH 4·6 and pH 7·0 has been examined. The presence of small quantities of gelatin appears markedly to decrease the adsorption of benzcarboline ions.

3. The effect of various benzcarboline salts on the electro-capillary curve of mercury has been determined. In presence of small concentrations (M/20000) of these salts the maximum of the electro-capillary curve occurs to the right of that of the primitive (negative polarisation of the mercury), and the greatest fall in surface tension occurs to the left of the maximum of the primitive (positive polarisation of the mercury).

4. These results indicate high adsorption of benzcarboline ions even on a positively charged surface. The possible cause of this unexpected result is discussed.

The authors wish to express their indebtedness for a grant from Imperial Chemical Industries, Ltd., which defrayed in part the cost of the apparatus employed in the electro-capillary experiments. They also wish to express their thanks to Mr W. Leeper and Mr W. Spragg for valuable assistance.