Rays emitted by Substances exposed to y Mays. 637 



lead, and bismuth. The mercury screen consisted of a little 

 box of thin sheet zinc filled with mercury, the inside surface 

 of the box being covered with paraffin-wax to prevent the 

 mercury coming into contact with the zinc. The dimensions 

 of the box were : 4'8 cm. long, 1*3 cm. broad, and 5*5 cm. 

 high. 



That the amount of secondary cathode radiation from a 

 substance depends on the nature of the y rays to which it is 

 exposed, can be conveniently demonstrated as follows : — The 

 difference in the amount of radiation from iron and aluminium, 

 and lead and aluminium, is determined with a screen of 

 copper, and the former difference divided by the latter, A 

 value of this ratio is then found, using a screen of lead. The 

 whole process may be repeated several times and the means 

 taken. Each ratio can be obtained with ease correct to five 

 per cent. In this way, using screens of copper and lead 

 respectively, 1*8 and 2'4 cm. thick, the writer obtained the 

 corresponding ratios 6*4 and 4*2. 



It will be well, before stating the principal experimental 

 results obtained, to give some numbers from which an idea 

 of the relative magnitude of the quantities measured, and 

 the probable accuracy of the determinations, can be obtained. 

 When a copper screen 1*8 cm. thick was used (see Table V.), 

 and the radiating substance was aluminium, a leak of 1792 

 divisions in 40 seconds was obtained, and with lead as the 

 radiating substance a leak of 1850 divisions in 10 seconds 



Table V. 





Lead 

 screen 

 2 mm. 



thick. 



Bismuth 

 screen 



1*4 cm. 

 thick. 



Lead 

 screen 

 1-3 cm. 



thick. 



Mercury 

 screen 

 1'4 cm. 



thick. 



Tin 

 screen 

 1'3 cm. 



thick. 



Zinc 

 screen 

 20 cm. 

 thick. 



Copper 

 screen 

 1-8 cm. 

 thick. 



Iron 

 screen 

 2-5 cm. 



thick. 



C-Al ... 



—155 



-138 



-126 



-135 



- 83 



- 64 



- 62 



- 83 



S-Al ... 



+ 18 



+ 26 



+ 45 



+ 34 



4- 27 



4- 28 



4- 16 



+ 26 



Fe-Al ... 



158 



189 



181 



188 



160 



127 



125 



122 



Ni-Al ... 



176 



225 



227 



218 



185 



156 



156 



160 



Cu-Al... 



198 



239 



263 



230 



191 



158 



156 



167 



Zn-Al... 



216 



271 



272 



270 



222 



189 



185 



202 



Sn-Al ... 



408 



465 



442 



456 



389 



365 



374 



408 



Pb-Al... 



800 



800 



800 



800 



800 



800 



800 



800 



Phil. Mag. S. 6. Vol. 14. No. 83. Nov. 1907, 



2 U 



