Geological  History  of  the  Atmosphere.  231 
to  the  effect  that  inland  or  country  air  is  richer  in  carbonic 
acid  than  sea  air,  and  that  therefore,  on  the  hypothesis  of  the 
regulating  influence  of  the  sea  and  its  reminiscent  character, 
the  amount  of  carbonic  acid  in  the  atmosphere  at  present  is, 
or  quite  recently  was,  increasing. 
It  should^  however,  be  added  that  in  making  this  inference 
I  am  assuming  that  the  amount  of  telluric  carbonic  acid 
evolved  into  the  sea  is  and  usually  has  been  less  than  the 
amount  evolved  on  land.  If  this  were  not  so,  the  inference, 
if  any,  that  could  be  drawn  from  a  comparison  of  the  analysis 
of  sea  and  country  air  would  be  different  from  that  just  given. 
However,  the  assumption  seems  reasonable  in  view  of  our 
knowledge  of  the  subject.  I  am  therefore  still  inclined  to 
infer,  from  the  evidence  brought  forward,  that  the  amount  of 
carbonic  acid  in  the  atmosphere  is  increasing  ;  and  in  any 
case  the  points  referred  to  in  the  course  of  the  above  discussion 
are  of  considerable  interest  in  themselves. 
The  (probable)  increase  in  the  amount  of  atmospheric 
carbonic  acid  is  most  probably  due  to  the  operation  of  natural 
forces,  though  it  is  just  possible  that  the  large  quantity  of 
coal  which  is  now  raised  and  consumed  may  have  an  appre- 
ciable effect  on  the  amount  of  atmospheric  carbonic  acid. 
This  in  itself  is  a  subject  of  very  considerable  interest.  It 
was  the  subject  of  Lord  Kelvin's  address  at  Toronto  in  1897 
(referred  to  at  the  beginning  of  my  first  article),  in  which  he 
expressed  the  opinion  that  possibly  there  might  be  enough 
coal  in  the  world  to  use  up  all  the  oxygen  of  the  atmosphere, 
and  therefore  that  statistician s,  when  making  estimates  re- 
garding the  future  output  of  coal  or  the  total  available  coal- 
supply  of  the  world,  should  take  into  account  the  effect  which 
the  burning  of  the  coal  might  have  on  the  composition  of  the 
atmosphere. 
The  quantity  of  coal  annually  raised  at  present  may  be 
taken  in  round  numbers  as  800,000,000  tons.  Assuming 
that  it  contains  on  the  average  80  per  cent,  of  carbon,  the 
combustion  of  this  amount  will  yield  nearly  2347  x  10u  tons 
of  carbonic  acid,  a  quantity  which  is  almost  exactly  t>\>  of 
the  total  amount  of  carbonic  acid  annually  removed  from  the 
atmosphere  by  the  growth  of  vegetation,  as  roughly  estimated 
in  my  last  article  ;  and  quite  possibly  it  is  as  great  a  quantity, 
or  even  greater,  than  the  amount  of  carbonic  acid  which  is 
now  annually  contributed  to  the  atmosphere  from  telluric  or 
volcanic  sources.  At  the  same  time,  it  is  only  about  the  -.y1  0 
part  of  the  amount  of  carbonic  acid  which  is  present  in  the 
atmosphere,  and  it  is  obvious  that  it  would  take  several  years* 
production  and  consumption   of   coal   at  this  rate  to  have  an 
