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Molecular attractions are measured between pairs of flat quartz plates and between a flat and a spherically curved plate. Considerable precautions are taken against spurious electric charges, dust and gel particles which might interfere with the measurements. Silicone oil is used for damping. Distances between the flat plates have been varied from 5000-9500 A, and from 940-5000 A for the plate and sphere combination. Attraction forces varied from 0.002-0.3 dyne. The results agree with the presence of retarded van der Waals' forces (Casimir and Polder, Lifshitz). If the force per unit area between flat plates is represented by F = --B/d4, the value of B = 1-2x 10-19 erg cm, is in good agreement with existing theories, and with the previous experimental results obtained by Derjaguin and Abrikosova and by Kitchener and Prosser. An explanation is suggested why earlier measurements by Overbeek and Sparnaay using a similar method led to much stronger attractions.