„ 0·100 1000 „ 0·200 1480 Mica 0·025 520 Glass 0·155 1130 Platinum 0·030 1310 Copper 0·085 1740 Lead 0·130 2610 If _e_/_m_ is a constant for all the rays, the value of _HR_ is proportional to the velocity of the rays, and it follows from the table that the velocity of the rays which just produce an effect on the plate through ·13 mms. of lead is about 7 times that of the rays which just produce an impression through ·01 mm. of aluminium. It will be shown, however, in section 82, that _e_/_m_ is not a constant for all speeds, but decreases with increase of velocity of the rays. The difference in velocity between the rays is in consequence not as great as this calculation would indicate. On examination of the rays from uranium, Becquerel found that the radiation is not as complex as that from radium, but consists wholly of rays for which the value of _HR_ is about 2000. =78. Examination of the β rays by the electric method.= The presence of easily deviable rays given off from an active substance can most readily be shown by the photographic method, but it is necessary, in addition, to show that the penetrating rays which produce the ionization in the gas are the same as those which cause the photographic action. This can be conveniently tested in an arrangement similar to that shown in Fig. 25. [Illustration: Fig. 25.] The radio-active matter _A_ is placed on a lead block _B´´_ between the two parallel lead plates _BB´_. The rays pass between the parallel plates and ionize the gas between the plates _PP´_ of the testing vessel. The magnetic field is applied at right angles to the plane of the paper. The dotted rectangle _EEEE_ represents the position of the pole piece. If a compound of radium or thorium is under investigation, a stream of air is required to prevent the diffusion of the radio-active emanations into the testing vessel. When a layer of uranium, thorium or radium compound is placed at _A_, the ionization in the testing vessel is due mainly to the action of the α and β rays. The α rays are cut off by adding a layer of aluminium ·01 cm. thick over the active material. When the layer of active matter is not more than a few millimetres thick, the ionization due to the γ rays is small compared with that produced by the β rays, and may be neglected. On the application of a magnetic field at right angles to the mean direction of the rays, the ionization in the testing vessel due to the rays steadily decreases as the strength of the field increases, and in a strong field it is reduced to a very small fraction of its original value. In this case the rays are bent so that none of them enter the testing vessel.