Geological  History  of  the  Atmosphere.  2  29 
quite  possible  that  the  normal  or  ordinary  state  o£  matters  as 
regards  the  balancing  of  these  agencies  may  be  notably 
different  in  the  cases  of  the  sea  and  land  respectively.  Also 
it  is  obvious  that  we  ought  to  know  with  a  high  degree  o  f 
certainty  the  amount  o£  free  carbonic  acid  in  sea-water  tha  t 
corresponds  to  the  ordinary  percentage  in  the  atmosphere,  and 
questions  regarding  analytical  methods  would  have  to  bo 
very  carefully  considered.  In  this  connexion  it  may  ba 
noted  that  Jacobson  many  years  ago  observed  that  the  free 
or  loose  carbonic  acid  of  sea-water  is  not  all  driven  off  by 
boiling,  even  for  a  considerable  time,  but  that  some  of  it 
remains  until  only  the  dry  residue  is  left. 
As  it  is  therefore  obvious  that  the  method  of  enquiry 
suggested  involves  the  consideration  of  so  many  complicated 
and  rather  obscure  questions,  I  will  not  pursue  it  further, 
but  may  remark  in  passing  that  it  is  quite  possible  that  an 
amount  of  information  on  the  points  mentioned  and  others 
pertinent  to  the  subject  may  yet  be  obtained  sufficient  to  allow 
of  a  satisfactory  inference  being  made. 
There  is,  however,  another  and  a  simpler  way  by  which 
we  may  arrive  at  some  idea  as  to  how  the  amount  of  free  or 
loose  carbonic  acid  in  sea-water  compares  with  that  in  the 
atmosphere  ;  and  that  is  to  compare  the  average  percentage 
of  carbonic  acid  in  sea  air  with  the  average  percentage  in 
country  or  continental  air.  There  must  always  be  an  inter- 
change going  on  between  the  sea  and  the  atmosphere  above 
it ;  and  if  we  should  find  that  sea  air  contains  more  (or  less) 
carbonic  acid  than  country  air,  then  we  may  conclude  that 
the  sea  is  giving  off  more  (or  less)  carbonic  acid  than  it  takes 
in  from  the  atmosphere,  and  that  it  is  therefore  richer,  or 
poorer,  as  the  case  may  be,  in  free  (or  loose)  carbonic  acid 
than  the  atmosphere,  relatively  to  other  gases  present  in  the 
atmosphere,  and  after  allowance  has  been  made  for  solubility 
or  other  considerations  of  a  similar  nature. 
There  is  a  fair  amount  of  evidence  available  on  this  point, 
as  there  are  recorded  analyses  of  the  air  at  a  considerable 
number  of  places  both  by  sea  and  land  wThich  are  fairly 
comparable  with  each  other  ;  and  the  general  inference  which 
is  obtained  by  comparing  them  is  that  sea  air  is  poorer  in 
carbonic  acid  than  country  air.  There  is  no  doubt  a  con- 
siderable amount  of  discrepancy  between  the  results  obtained 
by  different  observers,  the  difference  between  the  percentage 
of  carbonic  acid  in  sea  air  and  country  air  being  very  slight 
according  to  some  observers  and  very  oreat  according  to 
others.  Professor  T.  E.  Thorpe,  as  the  result  of  a  Large 
number  of  analyses  done  in  1866,  found  that  the  air  of  the 
