S24  Dr.  C.  G.  Barkla  on 
The  experiment  was  simply  the  following  : — . 
The  rates  of  leak  o£  an  electroscope  receiving  a  narrow 
pencil  of  primary  radiation  and  of  one  receiving  a  beam  of 
secondary  radiation  from  the  substance  experimented  upon, 
were  observed  when  no  absorbing  plates  intercepted  the 
radiations.  A  plate  of  aluminium  "01  cm.  in  thickness  was 
placed  in  front  of  the  electroscope  receiving  the  secondary 
beam,  and  the  two  rates  of  deflexion  of  the  gold-leaves  were 
again  observed.  The  percentage  reduction  of  the  ionization 
in  the  secondary  electroscope  was  found,  using  the  electro- 
scope receiving  the  pencil  of  primary  radiation  to  standardize 
the  intensity  of  the  primary.  (In  some  cases  an  electroscope 
receiving  a  secondary  beam  was  used  to  standardize  the 
intensity.) 
In  these  experiments,  no  particular  care  was  taken  to  keep 
the  character  of  the  primary  radiation  constant ;  but  in  one 
or  two  cases  several  experiments  were  made  on  a  pair  of 
metals  taken  alternately.  Bismuth  and  lead,  silver  and 
cadmium  were  treated  in  this  way.  Consequently  the  results 
can  only  be  regarded  as  approximately  true,  and  little  value 
is  attached  to  the  absolute  absorptions  obtained,  for  under 
certain  conditions  some  of  the  elements  were  found  later  to  give 
results  differing  considerably  from  these.  The  most  variable 
were  those  from  silver  to  iodine  (fig.  1).  There  was,  how- 
ever, no  indication  of  much  variation  in  the  majority  of  cases. 
The  elements  were  studied  irregularly,  and  the  discharge  in 
the  X-ray  tube  was  not  kept  more  constant  than  could  be 
done  by  ordinary  observation.  The  results,  though  incom- 
plete, and  possibly  containing  one  or  two  errors  in  detail,  are, 
I  think,  of  sufficient  interest  to  justify  their  publication  at 
this  stage  of  the  investigation. 
They  also  suggest  a  method  of  determining  atomic  weights 
by  interpolation,  for  a  small  variation  in  atomic  weight  is 
usually  accompanied  by  a  very  considerable  change  in 
absorbability  of  the  secondary  radiation.  It  appears  probable 
that  a  variation  of  atomic  weight  by  one  fourth  in  certain 
regions  would  be  detected,  but  the  possible  error  varies  con- 
siderably from  one  region  to  another. 
It  would  be  useless  to  attempt  to  calculate  from  these 
results  absorption  coefficients  for  the  different  radiations,  for 
the  primary  and  secondary  beams  consist  of  mixtures  of  rays 
differing  enormously  in  penetrating  power,  and  these  con- 
stituents produce  effects  in  the  electroscope  which  cannot 
even  be  regarded  as  approximately  proportional  to  their 
energies.  Also  the  constituent  of  the  primary  beam  that 
sets  up  in  one  metal  the  most  effective  constituent  in  the 
