860 
FILTERED  AIR. 
any  data  exist  which  will  enable  us  to  say,  with  certainty,  which 
party  is  right  ?    I  think  so. 
It  would  be  very  difficult  to  reduce  the  putrefying  power  of 
pure  air,  even  if  it  existed,  to  absolute  demonstration  ;  for,  how- 
ever cleansed  in  appearance,  a  stubborn  objector  might  still  urge 
that  the  air  was  not  cleansed  in  reality  ;  that  germs  exist,  though 
they  baffle  our  attempts  to  reveal  them.  But  this  difficulty  does 
not  hamper  the  other  side  ;  for  if,  notwithstanding  the  risk  of 
these  residual  germs,  visibly  pure  air  can  be  proved  incompetent 
to  produce  the  phenomena  of  putrefaction,  there  is  no  escape 
from  the  inference  that,  as  regards  the  point  to  be  decided,  such 
air  is  perfectly  filtered ;  and  its  proved  impotence  would  be  a 
demonstration  of  the  truth  of  the  germ  theory.  By  "  visibly 
pure  air  "  I  mean  air  which,  where  traversed  by  a  powerful  and 
intensely  concentrated  beam  of  light,  in  a  space  not  otherwise 
illuminated,  reveals  no  trace  of  floating  matter  to  the  eye. 
How,  then,  are  we  to  obtain  our  filtered  air,  and,  having  ob- 
tained it,  how  are  we  to  apply  it  to  a  wound  and  mix  it  elfect- 
ualiy  with  the  blood  ?  Two  or  three  years  ago  an  observation 
and  an  inference,  wdiich,  taken  together,  reflect  the  highest  credit 
on  his  sagacity,  were  made  and  drawn  by  Professor  Joseph 
Lister,  of  Edinburgh.  He  found,  and  I  believe  it  is  the  universal 
experience  of  surgery  to  find,  that  when  the  lung  is  wounded  by 
the  spike  of  a  broken  rib,  air  from  the  pleural  cavity  may  mingle 
freely  with  the  blood,  but  that  putrefaction  never  ensues.  Here 
is  the  statement  of  Professor  Lister,  abbreviated,  but  in  his  own 
words  : — 
"  I  have  explained  to  my  own  mind  the  remarkable  fact  that  in  simple 
fractures  of  the  ribs,  if  the  lung  be  penetrated  by  a  fragment,  the  blood 
effused  into  the  pleural  cavity,  though  freely  mixed  with  air,  undergoes 
no  decomposition.  The  air  is  sometimes  pumped  into  the  pleural  cavity 
in  such  abundance  that,  making  its  way  through  the  wound,  it  inflates 
the  cellular  tissues  of  the  whole  body.  Yet  this  occasions  (as  regards 
putrefaction)  no  alarm  to  the  surgeon.  Why  air  introduced  into  the 
pleural  cavity  through  a  wounded  lung  should  have  such  wholly  dififerent 
efifects  from  that  entering  through  a  permanently  open  wound  penetrating 
from  without,  was  to  me  a  complete  mystery  till  I  heard  of  the  germ 
theory  of  putrefaction,  when  it  at  once  occurred  to  me  that  it  was  only 
natural  that  the  air  should  be  filtered  of  germs  by  the  air  passages,  one 
of  whose  offices  is  to  arrest  inhaled  particles  of  dast,  and  prevent  them 
