Geological  History  of  the  Atmosphere.  235 
1893  and  1894  regarding  the  relative  oxygen-producing 
powers  o£  different  classes  of  plants  are  of  great  interest. 
He  gave  an  account  of  these  observations  in  the  '  Chemical 
News,'  vol.  lxx.  (1891)  p.  223,  stating  that  he  "  measured  the 
oxygen  given  off  in  a  certain  time  by  various  unicellular  alga? 
and  by  phanerogamic  plants  high  in  the  scale.  These  are  then 
collected,  dried  at  100°  C,  and  weighed.  It  was  then  found 
that  weight  for  weight  the  unicellular  algae  gave  by  far  the  most 
oxygen.  In  one  experiment  last  August  a  mixture  consisting 
chiefly  of  Protococcus  and  Chlamidomonas  was  thus  compared 
during  five  days  with  the  common  weed  Polygonum  aviculare, 
and  was  found  to  produce,  weight  for  weight,  fifty  times 
more  oxygen  than  the  latter.  In  other  cases  different  results 
were  obtained,  but  in  all  the  excess  of  oxygen  was  largely 
on  the  side  of  the  unicellular  algae."" 
From  the  above  experiments  by  Dr.  Phipson  it  is  obvious  that 
unicellular  algae  must  require  a  very  much  greater  quantity  of 
carbonic  acid  in  order  to  gain  a  definite  increase  in  weight  than 
the  higher  phanerogamic  plants  ;  and  as  the  latter  were  late  in 
geological  history  in  making  their  appearance,  it  is  permissible 
for  us  to  hold  or  suggest  that  their  evolution  may  have  been 
conditioned  by  the  comparative  scarcity  or  the  increasing- 
scarcity  of  carbonic  acid  in  the  atmosphere,  and  that  their 
leaves  and  general  structure  have  been  so  modified  as  to  make 
the  most  of  the  carbonic  acid  available.  No  doubt  another 
way  to  look  at  the  question  is  to  regard  the  unicellular  alga? 
as  being  in  some  respects  of  a  less  distinctly  vegetable 
character  than  the  higher  phanerogamic  plants.  Probably 
enough  the  character  of  the  respiration  of  the  algae  as  regards 
its  effect  on  the  weight  and  bulk  of  the  organism  comes  nearer 
to  that  of  animals,  in  wdiose  case  respiration  causes  a  decrease 
in  weight.  Still  this  way  of  looking  at  the  question  may  lead 
to  nearly  the  same  conclusion  as  the  one  first  suggested. 
The  observations  are  in  any  case  of  great  interest,  and  it  is 
quite  possible  that  the  appearance  or  prevalence  of  special 
types  of  organisms  at  special  epochs  may  have  been  to  a  large 
extent  conditioned  by  the  amount  of  carbonic  acid  in  the 
atmosphere. 
It  is  also  noteworthy  that  Dr.  Phipson  found  that  lilacs, 
willows,  &c.  can  be  killed  by  an  excess  of  carbonic  acid. 
A  few  remarks  may  also  be  made  here  on  the  bearing  that 
the  theory  of  the  variations  in  the  percentage  of  atmospheric 
carbonic  acid  has  on  the  question  of  the  rate  of  the  secular 
cooling  of  the  earth.  As  is  well  known,  there  is  a  wide 
divergence  of  opinion  on  this  subject,  the  rate  of  cooling  as 
calculated  by  Lord  Kelvin  from  physical  data  being  too  high 
