In these experiments the material subject to bombardment was placed immediately in front of the source of _α_ particles and observations on the ejected particles were made on a zinc sulphide screen placed in a direct line a few centimetres away. Using radium C as a source of _α_ rays, the ranges of penetration, expressed in terms of centimetres of air, were all in these cases greater than the range of free nuclei (30 cms. in air) set in motion in hydrogen by the _α_ particles. By inserting absorbing screens of 30 cms. air equivalent in front of the zinc sulphide screen the results were quite independent of the presence of either free or combined hydrogen as an impurity in the bombarded materials. Some of the lighter elements were examined for absorptions less than this, but, in general, the number of H particles due to hydrogen contamination of the source and the materials was so large that no confidence could be placed in the results. In such experiments many scintillations can be observed, but it is very difficult to decide whether these can be ascribed in part to an actual disintegration of the material under examination. The presence of long-range particles of the _α_ ray type from the source of radium C still further complicates the question, since in general the number of such particles is large compared with the disintegration effect we usually observe. To overcome these difficulties, inherent in the direct method of observation, Dr. Chadwick and I have devised a simple method by which we can observe with certainty the disintegration of an element when the ejected particles have a range of only 7 cms. in air. This method is based on the assumption, verified in our previous experiments, that the disintegration particles are emitted in all directions relative to the incident rays. A powerful beam of _α_ rays falls on the material to be examined and the liberated particles are observed at an average angle of 90° to the direction of the incident _α_ particles. By means of screens it is arranged that no _α_ particles can fall directly on the zinc sulphide screen. This method has many advantages. We can now detect particles of range more than 7 cms. with the same certainty as particles of range above 30 cms. in our previous experiments, for the presence of hydrogen in the bombarded material has no effect. This can be shown at once by bombarding a screen of paraffin wax, when no particles are observed on the zinc sulphide screen. On account of the very great reduction in number of H nuclei or _α_ particles by scattering through 90°, the results are quite independent of H nuclei from the source or of the long-range _α_ particles. The latter are just detectable under our experimental conditions when a heavy element like gold is used as scattering material, but are inappreciable for the lighter elements. A slight modification of the arrangement enables us to examine gases as well as solids.