236      Chemical  and  Geological  History  of  the  Atmosphere. 
to  admit  of  the  long  period  o£  time  postulated  by  geologists 
as  being  necessary  for  the  deposition  of  the  observed  amount 
of  stratified  rocks,  and  probably  also  for  the  evolution  or 
sufficiently  gradual  appearance  of  changes  observed  in  the 
structure  of  animal  and  vegetable  organisms.  Both  classes 
o£  observers — the  physicists  and  the  geologists — make  use  of 
data  derived  from  recent  observations  in  order  to  calculate 
respectively  the  rate  of  cooling  of  the  earth,  and  the  rate  of 
denudation  and  deposition  of  rocks  ;  but  it  is  quite  possible 
that  in  the  course  of  past  ages  important  changes  may  have 
taken  place  in  the  rate  of  cooling  and  in  the  rate  of  denudation. 
It  is  quite  obvious,  from  what  has  been  learned  regarding  the 
high  absorptive  powers  of  carbonic  acid  for  radiant  heat, 
that  if  the  amount  of  carbonic  acid  in  the  atmosphere  was 
much  greater  in  early  times  than  it  is  now,  the  rate  of 
radiation  of  heat  from  the  earth  into  space  would  be  less  than 
it  is  at  present — assuming  the  temperature  of  the  surface  of 
the  earth  to  be  the  same  in  both  cases.  There  is  even  a 
possibility  (arising  from  the  circumstance  that  the  earth  is 
always  receiving  heat  from  the  sun  as  well  as  losing  heat  by 
radiation  into  space)  that  there  may  have  been  periods  in  the 
past  history  of  the  earth  when  the  temperature  of  the  earth 
as  a  whole  remained  constant,  or  even  increased  to  a  slight 
extent  instead  of  decreasing  at  all,  and  that  for  a  considerable 
number  of  years  at  a  time.  The  average  rate  of  the  secular 
cooling  of  the  earth  must  have  been  only  a  very  small  fraction  of 
a  degree  per  annum,  even  on  Lord  Kelvin's  estimate  of  the  age 
of  the  earth,  and  the  variations  in  the  amount  of  atmospheric 
carbonic  acid  may  have  been  sufficient  to  cause  interruptions 
of  a  fairly  long  period  in  the  process  of  cooling,  just  as  the 
succession  of  clay  and  night  and  summer  and  winter  may  be 
regarded  as  causing  frequent  interruptions  for  a  short  period 
in  the  same  process.  On  the  other  hand,  we  should  also 
observe  that  the  rate  of  denudation  may  have  varied  very 
much  in  the  course  of  seoloorical  history.  If  the  atmosphere 
was  more  extensive  in  early  times  than  it  is  now,  it  would 
carry  more  aqueous  vapour  ;  more  numerous  or  more  exten- 
sive clouds  would  form,  and  we  should  therefore  expect  the 
average  rainfall  to  be  heavier  than  it  is  now,  and  consequently 
the  rate  of  denudation  to  be  greater.  Further,  the  greater 
quantity  (by  hypothesis)  of  carbonic  acid  present  in  early 
times  and  at  various  epochs,  would  on  account  of  its  chemical 
activity  greatly  hasten  the  solution  of  limestone  and  other 
carbonates,  and  the  disintegration  of  rocks  in  general.  Also 
the  rate  of  evolution  or  the  appearance  of  important 
changes  in    the  structure   of    organisms  would  probablv  be 
