Our measurements of proton-proton scattering during 1936 established the existence of large nuclear forces between protons at close distances, in addition to the usual Coulomb forces. These nuclear forces give rise to scattering in excess of the Coulomb prediction, appreciable at 700 kilovolts and very marked at 900 kv. In the energy region between 200 kv and 600 kv the amount of scattering to be expected on the basis of attractive nuclear forces is radically different from that to be expected on the hypothesis of repulsive nuclear forces since in the first case a decrease and in the second an increase, with respect to the classical scattering, is predicted. We have accordingly extended our measurements to include this energy region, and have obtained results which can only be explained by the assumption of a strong attractive force of short range acting in addition to the repulsive Coulomb forces between the particles. The magnitude of the nuclear force required is in approximate quantitative agreement with that deduced from our results of 1936, experimental difficulties due to the low residual energies of the scattered particles, and the very small numbers of counts in some regions of voltage and angle (as low as three percent of the Coulomb prediction) preventing a high precision in these results below 600 kv.