Geological  History  of  the  Atmosphere.  227 
for  the  great  variations  in  terrestrial  climate  or  in  the  tempe- 
rature of  the  surface  of  the  earth  which  are  known  to  have 
taken  place  in  past  geological  epochs.  The  principal  reason 
for  holding  the  above  view  is  the  variable  or  intermittent 
character  of  volcanic  action,  which  is  one  of  the  principal 
agencies  that  produce  or  evolve  carbonic  acid  into  the 
atmosphere. 
A  question  which  naturally  suggests  itself  at  this  stage 
is  : — What  is  the  state  of  matters  in  the  above  respect  at 
the  present  time  ?  Is  the  amount  of  atmospheric  carbonic 
acid  increasing  or  decreasing,  or  has  it  been  practically  con- 
stant in  amount  for  many  years  (or  centuries),  and  has  it 
still  a  tendency  to  remain  at  the  same  figure  ? 
At  first  sight,  it  might  be  supposed  that  a  simple  and 
possibly  sufficient  answer  to  this  question  could  be  obtained 
by  studying  the  records  of  analyses  of  the  atmosphere  made 
at  different  times,  within  the  last  hundred  years  or  so,  that 
is  to  say  since  analyses  of  the  atmosphere  were  made  at  all. 
Unfortunately,  however,  the  earlier  anabases  are  quite  un- 
reliable on  accouut  of  the  defective  analytical  methods  which 
were  used;  and  indeed  it  is  very  doubtful  if  any  analyses 
could  be  accepted  for  the  above  purpose  except  those  or 
some  of  those  which  have  been  made  within  the  last  thirty  or 
forty  years.  Further,  it  would  be  very  difficult  to  make  a 
satisfactory  comparison  of  even  the  most  reliable  analyses 
(reliable  in  so  far  as  analytical  methods  and  careful  working- 
are  concerned)  on  account  of  local  variations  in  the  amount 
of  atmospheric  carbonic  acid  and  temporary  variations  due  to 
weather  conditions  and  other  causes  ;  and  therefore  it  is  very 
doubtful  if  we  are  in  a  position  as  yet  to  infer  anything  at 
all  from  a  comparison  of  analyses  done  at  different  dates. 
It  is  possible,  however,  that  something  may  be  learned  in 
other  ways,  on  the  subject  before  us,  especially  from  a  study 
of  the  amount  of  carbonic  acid  in  the  sea.  The  reasons  for 
expecting  to  learn  something  in  this  way  are  the  solubility 
of  carbonic  acid  in  water,  the  great  depth  and  volume  of  the 
sea,  the  stillness  of  the  water  at  great  depths,  and  the  long 
time  that  would  therefore  be  most  probably  required  for 
carbonic  acid  entering  the  ocean  at  its  surface  (or  at  any 
particular  point)  to  diffuse  throughout  its  whole  extent. 
It  is  now  a  considerable  time  since  it  was  known  or  con- 
jectured that  the  sea  and  water  on  the  earth's  surface  generally 
had  a  notable  influence  on  the  carbonic  acid  of  the  atmosphere. 
In  1855  Peligot  (as  quoted  by  Prof.  Letts  and  Mr.  Blake  in 
their  important  work  on  the  carbonic  anhydride  of  the  atmo- 
sphere   referred  to  in  last  article)  pointed    out    that   water 
Q  ■>- 
