Geological  History  oftlie  Atmosphere.  233 
acid  produced  by  the  burning  of  the  coal  remained  in  the 
atmosphere,  the  amount  of  atmospheric  carbonic  acid  would 
be  rather  more  than  doubled  (as  compared  with  its  present 
amount).  In  110  years  (1906-2015)  the  total  amount  o£ 
coal  raised  would  be  1531  X  109  tons,  equal  to  1911  times  the 
present  annual  output,  and  sufficient  to  yield  4195  X  109  tons 
of  carbonic  acid,  or  twice  the  amount  of  our  present  atmo- 
spheric carbonic  acid.  The  amount  present  in  the  atmosphere 
would  then  (on  the  supposition  mentioned  above)  be  three 
times  its  present  quantity,  and  in  volume  would  amount  to 
0*084  per  cent,  of  the  whole  volume  of  the  atmosphere.  In 
150  years  (1906-2055)  the  total  quantity  of  coal  raised  would 
be  7426  x  109  tons,  or  about  9283  times  the  present  annual 
output, and  sufficient  to  yield  21783  x  109tons  of  carbonic  acid, 
or  rather  more  than  9  times  the  total  amount  of  atmospheric 
carbonic  acid  at  present.  The  quantity  present  in  the  atmo- 
sphere then  would,  on  the  conditions  assumed  above,  be  fully 
10  times  its  present  amount,  or  about  0*28  per  cent,  of  the 
whole  atmosphere  by  volume.  By  this  time,  if  not  even  at 
a  much  earlier  stage,  the  amount  of  carbonic  acid  in  the 
atmosphere  would  have  a  serious  effect  on  human  and  animal 
respiration,  and  it  would  therefore  be  of  little  use  to  continue 
further  the  calculations  at  the  above  rate  of  increase  (4  per 
cent,  per  annum).  Of  course,  even  apart  from  the  effect  on 
the  atmosphere,  it  is  not  at  all  likely  that  the  4  per  cent, 
rate  of  increase  or  anything  very  nearly  approaching  it  will 
be  maintained  for  150  years,  or  perhaps  even  for  a  much 
shorter  period.  Considerations  regarding  the  rate  of  increase 
of  the  population  of  the  world  and  the  probable  amount  of 
coal  required  for  human  consumption,  and  possibly  also 
considerations  regarding  practical  difficulties  connected  with 
the  raising  of  the  coal,  obviously  give  the  negative  to  such  a 
supposition. 
At  the  same  time,  it  is  quite  possible  that  the  4  per  cent, 
annual  rate  of  increase  may  be  maintained  for  the  next  20  or 
the  next  50  years,  and  therefore  a  very  great  increase  in  the 
annual  output  of  coal  is  quite  possible.  Even  if  the  average 
rate  of  increase  should  fall  to  2  per  cent,  or  so  per  annum, 
the  increase  in  the  output  in  the  course  of  50  years  would 
still  be  very  great,  and  would  probably  be  sufficient  to  have 
an  appreciable  effect  on  the  composition  of  the  atmosphere. 
We  must  of  course  also  bear  in  mind  that  we  cannot  expect 
all  the  carbonic  acid,  or  perhaps  even  a  largo  proportion  of 
the  carbonic  acid  produced  by  the  burning  of  the  coal,  to 
remain  as  a  permanent  addition  to  the  atmosphere.  A  large 
proportion  of  it  will  most  likely  be  absorbed   by  the   sea  ; 
