232  Mr.  J.  Stevenson  on  the  Chemical  and 
appreciable  effect  on  the  composition  of  the  atmosphere. 
Assuming  that  the  exact  percentage  of  carbonic  acid  in  the 
atmosphere  at  present  is  0"0277  by  volume,  it  would  require 
the  raising  and  consumption  o£  coal  at  the  present  rate  for 
ten  years  to  bring  the  percentage  up  to  0'0280  even  if  all 
the  carbonic  acid  produced  by  burning  the  coal  remained  in 
the  atmosphere.  It  is  doubtful  if  an  increase  to  such  a  slight 
extent  as  this  could  be  ascertained  with  certainty  by  chemical 
analysis  :  and  if  we  take  into  account  the  regulating-  influence 
of  the  sea,  and  of  the  growth  of  vegetation  and  other  similar 
factors,  we  shall  find  the  situation  such  as  to  make  it  doubtful 
as  to  whether  even  the  total  quantity  of  coal  which  has  been 
raised  by  human  effort  and  consumed  up  to  the  present  time, 
a  quantity  which  I  estimate  roughly  at  20  X  10°  tons  (twenty 
thousand  million  tons),  has  had  an  appreciable  effect  on  the 
composition  of  the  atmosphere. 
It  is  obvious,  however,  that  in  making  calculations  about 
the  future,  we  should  make  allowance  for  a  very  considerable 
increase  in  the  amount  of  coal  annually  raised.  As  is  well 
known,  the  annual  output  has  increased  at  a  rapid  rate  for  the 
last  century  or  more,  and  the  tendency  to  increase  still  con- 
tinues. We  are  probably  making  a  tolerably  fair  estimate 
if  we  take  the  average  annual  rate  of  increase  during  the 
last  50  years  at  4  per  cent.  :  that  is  to  say,  we  estimate  that 
the  output  for  each  year  has  been  on  the  average  4  per  cent, 
greater  than  that  of  the  immediate! v  preceding  year.  This 
is  equivalent  roughly  to  an  increase  in  the  annual  output  of 
48  per  cent,  in  10  years  and  100  per  cent,  in  17" 7  years:  that 
is  to  say,  the  amount  raised  is  doubled  every  17' 7  years,  or  in 
integral  numbers  every  18  years.  At  this  rate  of  increase, 
the  total  quantity  of  coal  raised  in  the  next  ten  years  (1906- 
1915  inclusive)  will  be  nearly  10  x  109  tons,  or*12'48  times 
the  present  annual  output.  "  In  20  years  (1906-1925  in- 
clusive) the  total  amount  raised  will  be  24*768  X  109  tons,  or 
30*96  times  the  present  annual  output  ;  and  this  is  probably 
as  large  a  quantity  or  even  larger  than  the  total  amount  that 
has  been  raised  up  to  the  present  time.  In  50  years  (1906- 
1955)  the  total  would  be  126-88  x  10°  tons,  a  quantity  158 
times  the  present  annual  output  and  sufficient  to  yield*  (with. 
the  carbon  present  in  the  coal  reckoned  at  80  per  cent.) 
372  x  109  tons  of  carbonic  acid  ;  a  quantity  equal  to  -g-  of  the 
amount  of  carbonic  acid  in  the  atmosphere  at  present.  In 
100  years  (1906-2005)  the  total  amount  would  be  1028  X  109 
tons,  or  1285  times  the  present  annual  output  and  sufficient  to 
yield  3015  x  109  tons  of  carbonic  acid,  or  I'd  times  the  amount 
at  present  in  the  atmosphere:  and  therefore,  if  all  the  carbonic 
