These results show that the α rays of polonium are modified in passing through matter, and that the amount of secondary rays set up varies with screens of different material. Mme Curie, using the same method, was unable to observe any such effect for the β rays of radium. The production of secondary rays by the β rays of radium is, however, readily shown by the photographic method. We have already seen (section 93) that very low velocity electrons accompany the α rays from radium or radio-tellurium spread on a metal plate. These electrons are probably liberated when the α rays escape from or impinge upon matter, and the number emitted depends upon the kind of matter used as a screen. The differences shown in the above table when the screens were interchanged are explained simply in this way. Screens employed Thickness Current in mms. observed Aluminium 0·01 Cardboard 0·005 17·9 Cardboard 0·005 Aluminium 0·01 6·7 Aluminium 0·01 Tin 0·005 150 Tin 0·005 Aluminium 0·01 126 Tin 0·005 Cardboard 0·005 13·9 Cardboard 0·005 Tin 0·005 4·4 [Illustration: Fig. 45.] =111. Secondary rays produced by= β =and= γ =rays=. An examination of the amount and character of the secondary radiation emitted by various substances, when exposed to the β and γ rays of radium, has recently been made by A. S. Eve[179]. The general experimental method employed is shown in Fig. 45. The electroscope (Fig. 45) was placed behind a lead screen 4·5 cms. thick, which stopped all the β rays and absorbed the greater proportion of the γ rays from the radium tube placed at _R_. On bringing near a plate of matter _M_, the primary rays fell upon it and some of the secondary rays, emitted in all directions, passed into the side of the electroscope, which was covered with aluminium foil of thickness ·05 mm. Before the plate _M_ was placed in position the rate of discharge of the electroscope was due to the natural leak and the γ rays from _R_, and the secondary radiation from the air. On bringing the radiator _M_ into position, the rate of discharge was much increased, and the difference between the rate of movement of the gold-leaf in the two cases was taken as a measure of the amount of secondary rays from _M_. The absorption of the secondary rays was tested by placing an aluminium plate ·85 mm. thick before the face of the electroscope.