4 
226  Mr.  J.  Stevenson  on  the  Chemical  and 
that  the  value  of  the  external  penetrating  radiation  is  ex- 
ceptionally high  in  the  place  where  I  worked  ;  this  fact  may 
account  for  some  part  of  the  discrepancy. 
The  other  research  is  that  by  Eve  *.  He  concludes  from 
his  experiments  that  much  the  larger  part  of  the  ionization 
in  a  closed  vessel  is  due  to  the  radium  emanation  in  the  air 
enclosed.  This  explanation  seems  to  me  self- inconsistent ; 
for  surely  if  the  quantity  of  emanation  in  the  small  volume 
of  air  inside  the  vessel  causes  an  effect  so  large,  the  far 
larger  quantity  outside  the  vessel  must  have  an  appreciable 
effect,  even  though  the  wTalls  are  so  thick  as  to  cut  out  all 
a  rays.  In  fact,  I  do  not  see  how  Mr.  Eve  would  explain 
Cooke's  result  on  cutting  down  the  ionization  by  thick  screens 
— a  result  which  all  who  have  tried  it  have  confirmed. 
Mr.  Eve  confines  his  remarks  to  large  vessels,  and  would 
probably  not  suggest  that  my  experiments  were  vitiated  by 
this  cause.  But  I  have  tried  directly  what  proportion  of 
the  ionization  can  be  attributed  to  the  enclosed  emanation. 
This  is  simplv  done  by  storing  a  volume  of  air  for  a  time 
(24  days)  sufficient  to  allow  the  emanation  to  die  away. 
I  have  never  found  a  decrease  of  more  than  5  per  cent, 
between  the  2nd  and  the  24th  day,  and  usually  the  change 
was  much  smaller.  Again,  the  presence  of  emanation  in  my 
boxes  would  give  a  term  proportional  to  the  volume,  and  the 
curve  would  tend  to  bend  away  from  the  horizontal  axis  ; 
the  curves  drawn  showT  no  sign  of  this.  I  have  therefore 
omitted  all  consideration  of  the  emanation  in  the  arguments 
of  §7. 
XIX.  The  Chemical  and  Geological  History  of  the  Atmosphere. 
By  John  Stevenson,  M.A.,  F.I.C.f 
IV. 
Observations  on  the  question  as  to  whether  the  Amount  of  Car- 
bonic Acid  in  the  Atmosphere  is  at  present  increasing  or 
decreasing  ;  edso  Note  on  the  Bate  of  the  Secular  Cooling  of 
the  Earth. 
IN  the  last  article  of  this  series  (Philosophical  Magazine, 
Jan.  1905,  p.  88)  we  saw*  that  there  is  good  reason  to 
believe  that  the  amount  of  atmospheric  carbonic  acid  has 
varied  within  wide  limits  during  geological  history,  limits 
probably  wide  enough,  when  regard  is  taken  of  the  high 
absorptive  power  of  carbonic  acid  for  radiant  heat,  to  account 
*  Phil.  Mag.  July  1905. 
|  Communicated  by  the  Author. 
