Beam balances were constructed with a photoelectromagnetic negative feedback coupling which made it possible to measure directly the resulting force of molecular attraction between plane and spherical surfaces of two plates separated by a narrow gap, as a function of the width of this gap. The width of the gap was determined from the diameter of Newton's rings. Using a simple equation, it was then possible to determine the energy of interaction per unit area of infinite plane surfaces of the same nature as a function of the width of the gap.Measurements were made for the pairs: quartz-quartz, mixed thallium halide-mixed thallium halide, and chromium-quartz in a range of gap widths from 0·07 to 0·3 microns. In all cases, within the limits of experimental error, the results agree with the values derived from the equations of the theory of molecular attraction of macro bodies developed by E. M. Lifshitz. At the same time, the London-Hamaker equation, which assumes the additivity of molecular interaction, is shown to be inapplicable.In the measurements, special attention was paid to the removal of surface electrical charges by means of ionization of the air, and also to the removal of dust particles and to decreasing the effect of vibrations and the viscosity of the air on the measurements.The discrepancy between the present results for quartz and those of Overbreek and Sparnaay, who obtained forces 5000 times greater than the values corresponding to the Lifshitz equations, is apparently explained by the incomplete removal of surface charges in their measurements.