Beckerf  has  measured  the  absorption  for  various  gases  of  very  fast  cathode

rays,  due  to  a  fall  of  potential  of  about  35,000  volts,  and  having  a  velocity,

therefore,  of  about  10^®  cm.  per  second.    The  values  he  obtained  for  the

x^atio  X/p,  where  X  is  the  coefficient  of  absorption  and  p  the  density,  ranged

between  1000  and  3000.     The  values  obtained  for  the  same  ratio,  using

"Cranium  ^-rays,  which   according  to   Becquerel  have  a  velocity  of  about

X  -6  X  10^®  cm.  per  second,  ranged  from  about  4  to  10.     The  difference  is  thus

^Considerably  more  than  can  be  accounted  for  by  the  mere  difference  in  velocity

o^  the  rays  in  the  two  cjwes.
The  "  absorption  "  of  the  cathode  rays  is  always  measured  by  finding  the

^^ling   off  in   intensity  of   the   rays  crossing  a  small  fixed  area  (namely,

tilie    aperture   of  the   Faraday  cylinder   used  in   the  measurements)  when

Ararious  thicknesses  of  the  absorbing  substance  are  interposed  in  the  path

of   the  beam,  the  original  beam  being  always  a  nearly  parallel  pencil  of

i^ays.      It  will  thus  be   seen   that  what  is  actually  measured   in    these

experiments  corresponds  not  to  the  "  absorption  "  of  the  ^S-rays,  but  to  the

quantity  which  we  have  called  scattering.
Comparing  the  results  of  Becker  for  the  cathode  rays  with  the  values

♦  Mcaelland,  *  Roy.  Dublin  Soc.  Trans.,'  vol.  9,  part  IV,  p.  41,  1907.
t    'Ann.  der  Phys.,'  vol.  17,  p.  381,  1905.
206        Scattering  of  the  jS-raysfrom  Uranium  by  Matter.
