Il8  Life- History  of  a  Doctrine.  {^fi.S™ 
haps  mendacious  workers  and  writers,  neither  of  these  subtle  things 
could  be  found.  It  was,  nevertheless,  possible  to  believe  in  their 
existence  and  to  indulge  in  the  hope  of  their  discovery.  But  now 
oxygen  came  on  the  scene.  Indeed,  it  may  be  truly  said  that  it  took 
possession  of  the  stage,  and  it  has  been  playing  the  leading  part  in 
the  field  of  chemistry  ever  since.  Here  is  an  invisible  substance 
existing  in  the  air  and  capable  of  bringing  about  the  most  astonish, 
ing  changes  in  things.  We  cannot  realize  the  effect  of  this  dis- 
covery upon  the  thoughts  of  chemists.  I  sometimes  feel  that  I 
should  like  to  have  lived  as  a  chemist  in  the  latter  part  of  the  eigh- 
teenth century.  What  thrills  the  workers  of  that  time  must  have 
felt  when  they  heard  of  the  discovery  of  oxygen  and  learned  from 
Lavoisier  what  part  it  piayea  in  combustion  !  We  sometimes  plume 
ourselves  upon  the  doings  ol  our  own  times.  Has  there  ever  been 
a  more  active  or  more  fruitful  period  in  the  history  of  chemistry 
than  that  wonderful  period  here  referred  to  ? 
It  was  a  great  step  forward  to  show  that  oxygen  is  one  of  the 
most  powerful  agents  at  work  in  the  processes  that  are  in  progress 
on  this  earth.  Not  only  combustion,  but  life  in  all  its  forms  is  in 
some  way  dependent  upon  it — animal  life  directly,  plant  life  indi- 
rectly. Oxygen  is  the  controlling  factor  in  all  the  changes  that  are 
familiar  to  us.  Some  one,  I  do  not  know  who,  is  responsible  for 
that  superficial  and  much-quoted  phrase  "  Without  phosphorus  no 
thought."  The  same  statement  could  be  made  with  equal  truth  in 
regard  to  other  elements,  such,  for  example,  as  nitrogen,  carbon, 
hydrogen,  sulphur,  sodium,  calcium,  and,  above  all,  oxygen.  Indeed, 
we  may  almost  say,  without  oxygen  no  chemical  activity  on  this 
earth.  This  sudden  appearance  of  oxygen  and  the  recognition  of 
its  importance  tended  to  put  matter  on  a  throne.  "  The  study  of 
material  things  will  lead  to  the  discovery  of  the  hidden  causes  of 
other  phenomena.  See  what  the  discovery  of  oxygen  has  done  for 
us !  Here  is  something  tangible.  Let  us  to  work.  There  must  be 
plenty  of  other  things  that  operate  as  causes.  If  we  can  only  bring 
these  things  to  light,  we  shall  be  able  to  understand  what  is  going 
on  around  us."  So  must  the  materialists  have  thought.  There 
were,  however,  in  those  days,  as  there  probably  always  have  been, 
those  who  looked  for  the  power  behind  the  throne  on  which  matter 
had  been  placed.  To  drop  the  figure  and  return  to  oxygen  we  may 
say  that,  while  the  discovery  of  this  element  gave  the  answers  to 
