670 Mr. A. 8. Eve on the Secondary Radiation 
electrometer. The secondary rays could be intercepted 
by an aluminium screen, so as to test their absorption. 
Fig. 1. 
Pradiatfer 
Apparatus.—In the present experiment (fig. 1) an electro- 
scope of square section was used ; one face was removed and 
replaced by aluminium foil, ‘05 mm. thick. The radium 
(25 mgs.) was placed within a small glass test-tube inside a 
second test-tube. The glass therefore cut off the a rays and 
some of the low velocity 8 rays. The electroscope was 
screened to a great extent from the radium by two blocks of 
lead, each 4°5 ems. thick. Thick plates of various substances, 
used us radiators, were placed at a distance of 20 cms. from 
the radium, and 12 ems. from the thin face of the electro- 
scope. Screens of wood, aluminium, glass, brass, and lead 
could be placed in front of the electroscope in order to measure 
the degree of penetration of the secondary rays. The appa- 
ratus was subsequently moditied (fig. 2), to determine the 
Fie. 2. 
Lead 
L. 
——_—_—Ea=EEEEEE ee” 
Aadaror 
secondary radiation from liquids. The primary rays were 
screened from the electroscope by the same lead blocks as 
before, but placed horizontally, and raised from the table. 
