The conception of the nucleus atom had its origin in 1911 in order to explain the scattering of an _α_ particle through a large angle as the result of a single collision. The observation that the _α_ particle is in some cases deflected through more than a right angle as the result of an encounter with a single atom first brought to light the intense forces that exist close to the nucleus. Geiger and Marsden showed that the number of particles scattered through different angles was in close accord with the simple theory which supposed that, for the distance involved, the _α_ particle and nucleus behaved like charged points, repelling each other according to the law of the inverse square. The accuracy of this law has been independently verified by Chadwick, so that we are now certain that in a region close to the nucleus the ordinary laws of force are valid. These scattering experiments also gave us the first idea as to the probable dimensions of the nuclei of heavy atoms, for it is to be anticipated that the law of the inverse square must break down if the _α_ particle approaches closely to or actually enters the nuclear structure. This variation in the law of force would show itself by a difference between the observed and calculated numbers of _α_ particles scattered through large angles. Geiger and Marsden, however, observed no certain variation even when the _α_ particles of range about 4 cms. were scattered through 100° by a gold nucleus. In such an encounter, the closest distance of approach of the _α_ particle to the center of the nucleus is about 5 x 10^-12 cm., so that it would appear that the radius of the gold nucleus, assumed spherical, could not be much greater than this value. There is another argument, based on radioactive data, which gives a similar value for the dimensions of the radius of a heavy atom. The _α_ particle escaping from the nucleus increases in energy as it passes through the repulsive field of the nucleus. To fix a minimum limit, suppose the _α_ particle from uranium, which is the slowest of all _α_ particles expelled from a nucleus, gains all its energy from the electrostatic field. It can be calculated on these data that the radius of the uranium nucleus cannot be less than 6 x 10^-12 cm. This is based on the assumption that the forces outside the nucleus are repulsive and purely electrostatic. If, as seems not unlikely, there also exist close to the nucleus strong attractive forces, varying more rapidly than an inverse square law, the actual dimensions may be less than the value calculated above.