Radiation  from  Ordinary  Materials.  221 
direction  of  the  imposed   field   would  hardly  influence  the 
penetration  of  the  rays  or  the  ionization  in  the  box. 
When  the  experiment  was  made,  it  was  found  that  the 
change  in  the  ionization  when  the  direction  of  the  field  was 
reversed  could  be  barely  detected  by  the  apparatus,  which 
would  have  shown  a  change  of  2  per  cent,  with  certainty  ; 
but  the  current  seemed  slightly  greater  when  the  window 
was  charged  negatively.  This  would  indicate  that  the  rays 
were  positively  charged.  They  certainly  cannot  carry  a 
negative  charge,   but   may  be   without   a    charge  of   either 
§  14.  One  more  experiment  mast  be  described  before  we 
proceed  to  the  discussion  of  the  results.  It  was  directed  to 
ascertaining  what  proportion,  if  any,  of  the  activity  of  lead, 
the  most  active  metal,  could  be  attributed  to  the  presence  of 
radium  diffused  throughout  its  substance  as  an  impurity. 
For  this  purpose  1500  grammes  of  the  metal  used  in  the 
experiments  (lead  (1))  were  dissolved  in  nitric  acid,  and  the 
solution  thoroughly  boiled  to  expel  any  emanation  ;  it  was 
then  placed  in  a  bottle  of  which  it  rilled  all  but  500  c.c. 
The  air  in  the  bottle  could  be  displaced  into  a  testing-vessel 
of  the  usual  form,  the  electrode  of  which  was  connected  to 
to  the  compensating  apparatus  by  which  a  change  of  1  per 
cent,  in  the  ionization  could  be  detected.  When  air  was 
drawn  through  the  solution  into  the  testing-vessel,  no  change 
could  be  noted.  The  solution  was  then  allowed  to  stand  for 
twenty-four  days  to  allow  the  emanation  to  attain  radioactive 
equilibrium.  At  the  end  of  this  time  the  500  c.c.  of  air  were 
displaced  into  the  testing-vessel,  but  no  change  in  the  current 
could  be  detected  ;  it  did  not  increase  by  1  per  cent.,  nor 
did  it  decrease  by  that  amount  when  fresh  air  was  subse- 
quently introduced,  free  from  emanation. 
From  the  data  of  §  11  we  can  calculate  easily  the  increase 
in  the  ionization  which  would  have  been  observed  if  the 
whole  activity  of  the  lead  had  been  due  to  the  presence  of 
radium.  If  A  is  the  number  of  ions  produced  per  sec.  by 
the  rays  from  one  gramme  of  any  substance  when  totally 
absorbed  in  air,  s  =  '00016aA  (§  7,  eqn.  4).  For  lead, 
s=  260  and  A=l'35xl0'';  the  1500  grammes  would  have 
produced  2  x  108  ions.  If  this  ionization  had  been  caused  by 
radium  in  radioactive  equilibrium,  one-fourth  of  it  would 
have  been  clue  to  the  emanation.  Hence  the  emanation 
alone  would  have  produced  '5  X  10s  ions  per  sec.  The  normal 
current  through  the  testing- vessel  corresponded  to  something 
less  than  10"'  ions  per  sec.  ;  the  emanation  would  have  in- 
creased the  leak  500-fold.     Since  the  observed  change  was 
