AmMMch',rmruJ'}  Life-History  of  a  Doctrine.  119 
many  questions,  it  raised  many  new  questions  ;  and  the  attempts  to 
answer  these  led  again  to  regions  of  imagery. 
One  of  the  oldest  tricks  of  the  mind  is  the  evoking  of  spirits  in 
time  of  need.  What  causes  all  bodies  to  attract  all  others  ?  We  say 
gravitation,  and  somehow  this  spirit  helps  us.  We  feel  as  though 
we  knew  more  about  the  phenomena  of  universal  attraction  when 
we  have  given  a  name  to  an  imaginary  and  immaterial  cause.  So, 
too,  when  we  inquire  why  oxygen  causes  the  changes  it  is  known  to 
cause  we  can  only  conjure  the  spirits  and  give  a  new  name.  Oxy- 
gen unites  with  carbon ;  the  carbon  burns ;  a  new  thing  is  formed. 
It  all  becomes  clear  when  we  are  told  that  it  is  chemical  arhnity  that 
does  it.  Chemical  affinity  isn't  an  imaginary  substance  ;  it  isn't 
something  that  we  may  see  and  handle.  We  haven't  forgotten  the 
philosopher's  stone  and  phlogiston.  Our  imaginary  cause  is  spirit- 
ual ;  it  is  not  material.  But  this  is  a  digression.  It  was  intended 
to  show  how  the  mind  reverts  promptly  to  the  subtle,  however 
powerful  the  attraction  of  matter  may  be.  We  cannot,  if  we  would, 
keep  to  things  material. 
Recalling,  what  perhaps  even  I  have  forgotten,  that  my  theme  is 
"The  Life-History  of  a  Doctrine,"  I  propose,  now  that  I  have  tried 
to  show  what  is  meant  by  this  phrase,  to  move  on  more  rapidly,  so 
that  I  may  dwell  somewhat  more  fully  upon  one  particular  doctrine 
that  has  been  before  the  chemical  world  in  one  form  or  another  for 
about  a  century. 
The  discovery  of  oxygen  did  not  lead  directly  to  the  introduction 
of  a  new  chemical  doctrine.  Its  chief  result,  so  far  as  doctrine  is 
concerned,  was  the  death  of  the  doctrine  of  phlogiston. 
The  discovery  emphasized  the  importance  of  taking  into  con- 
sideration the  weights  of  the  things  worked  with.  It  was  by  this 
means  that  Lavoisier  achieved  his  brilliant  success.  That  weight 
was  rather  lightly  regarded  in  earlier  days  may  be  seen  from  the 
following  quotation,  which  is  taken  from  an  essay  by  Dr.  Jean  Rey, 
published  in  1630: 
"  My  chief  care  hitherto  has  been  to  impress  on  the  minds  of  all 
the  persuasion  that  air  is  heavy,  inasmuch  as  from  it  I  propose  to 
derive  the  increase  in  weight  of  tin  and  lead  when  they  are  calcined. 
But  before  showing  how  that  comes  to  pass,  I  must  make  this 
observation — that  the  weight  of  a  thing  may  be  examined  in  two 
ways,  viz:,  by  the  aid  of  reason,  or  with  the  balance.    It  is  reason 
