Page:Radio-activity.djvu/162

 ionize the gas when their velocity is reduced below a certain value. From observations of the relative ionization produced in gases by the [Greek: alpha] and [Greek: beta] rays, Strutt suggested in 1901 that the [Greek: alpha] rays might consist of positively charged bodies projected with great velocity. Sir William Crookes, in 1902, advanced the same hypothesis. From a study of the [Greek: alpha] rays of polonium Mme. Curie in 1900 suggested the probability that these rays consisted of bodies, projected with great velocity, which lost their energy by passing through matter.

The writer was led independently to the same view by a mass of indirect evidence which received an explanation only on the hypothesis that the rays consisted of matter projected with great velocity. Preliminary experiments with radium of activity 1000 showed that it was very difficult to determine the magnetic deviation of the [Greek: alpha] rays. When the rays were passed through slits sufficiently narrow to enable a minute deviation of the rays to be detected, the ionizing effect of the issuing rays was too small to be measured with certainty. It was not until radium of activity 19,000 was obtained that it was possible to detect the deviation of these rays in an intense magnetic field. How small the magnetic deviation is may be judged from the fact that the [Greek: alpha] rays, projected at right angles to a magnetic field of 10,000 units, describe the arc of a circle of about 39 cms. radius, while under the same conditions the cathode rays produced in a vacuum tube would describe a circle of about ·01 cm. radius. It is therefore not surprising that the [Greek: alpha] rays were for some time thought to be non-deviable in a magnetic field.

89. Magnetic deviation of the [Greek: alpha] rays. The general method employed to detect the magnetic deviation of the [Greek: alpha] rays was to allow the rays to pass through narrow slits and to observe whether the rate of discharge of an electroscope, due to the issuing rays, was altered by the application of a strong magnetic field. Fig. 32 shows the general arrangement of the experiment. The