234:  Mr.  J.  Stevenson  on  the  Chemical  and 
and  it  is  also  possible  that  an  increase  in  the  amount  o£ 
atmospheric  carbonic  acid  would  be  followed  by  an  increase 
in  the  luxuriance  of  vegetation,  and  therefore  by  an  increase 
in  the  rate  at  which  carbonic  acid  is  removed  from  the  atmo- 
sphere. Further,  a  considerable  increase  in  the  amount  of 
atmospheric  carbonic  acid  would  most  probably  cause  an 
appreciable  decrease  in  the  oxidizing  power  of  the  atmosphere. 
There  might  thus  be  a  notable  decrease  in  the  rate  of  erema- 
causis  (the  slow  decay  of  animal  and  vegetable  remains), 
which  is  a  very  important  factor  as  regards  the  addition  of 
carbonic  acid  to  the  atmosphere  ;  and  it  is  also  of  some  interest 
to  note  that  a  decrease  in  the  rate  of  eremacausis  would  be 
equivalent  to  an  increase  in  the  rate  of  the  formation  of  coal 
or  of  coal-forming  materials. 
From  the  considerations  just  mentioned,  it  is  obvious  that 
even  if  the  output  and  consumption  of  coal  should  increase 
to  a  very  great  extent,  the  effect  on  the  composition  of  the 
atmosphere  might  not  be  at  all  proportionate  to  this  increase. 
It  is  also  obvious  that  it  is  not  possible  for  us,  when  there 
are  so  many  factors  in  the  problem,  to  estimate  in  a  satisfactory 
manner  either  (l)the  amount  of  coal  that  is  likely  to  be  raised 
in  the  next  100  years  or  so,  or  (2)  the  approximate  effect  that 
this  amount  (supposing  we  could  form  an  estimate  regarding 
it)  would  have  on  the  composition  of  the  atmosphere. 
However,  I  think  that,  taking  a  general  survey  of  the 
various  factors  involved,  we  may  reasonably  conclude  that 
the  amount  of  coal  that  will  be  raised  in  the  next  twenty  years 
will  possibly  be  sufficient  to  have  an  effect  on  the  composition 
of  the  atmosphere  that  will  be  appreciable  by  chemical 
analysis  ;  and  that  in  the  next  100  or  150  years  the  amount  of 
coal  raised  and  consumed  may  cause  an  increase  in  the  amount 
of  atmospheric  carbonic  acid  that  will  have  an  effect  appreci- 
able in  other  ways,  for  example  as  regards  the  climate  or 
average  temperature  of  the  surface  of  the  earth. 
As  regards  the  question  of  the  amount  of  carbonic  acid  in 
the  atmosphere  in  former  geological  epochs,  we  may  add  here 
that  in  addition  to  inferences  which  may  be  made  from 
evidence  regarding  volcanic  activity  at  any  particular  epoch, 
it  is  possible  that  something  may  also  be  inferred  from  the 
general  character  of  the  animal  and  vegetable  life  of  the 
period.  If  the  percentage  of  atmospheric  carbonic  acid  has 
varied  very  considerably  in  different  epochs,  it  is  obvious 
that  the  percentage  in  any  particular  epoch  may  have  been 
an  important  condition  or  factor  in  determining  the  evolution 
or  survival  of  many  forms  of  animal  and  vegetable  life.  In 
this  connexion   the   observations    made    by  Dr.    Phipson  in 
