I 
FILTERED  AIR.  361 
from  entering  the  air-cells.  In  truth,  this  fact  in  practical  surgery,  when 
duly  considered,  affords  as  good  evidence  in  support  of  the  germ  theory 
of  putrefaction  as  any  experiment  that  can  be  performed  artificially.* 
Here  is  a  surmise  which  bears  upon  it  the  mark  of  genius,  but 
which  nevertheless  needs  verification.  If  in  the  place  of  the 
words  it  is  only  natural,"  we  were  authorized  to  write  "it  is 
perfectly  certain,"  the  demonstration  would  be  complete.  Now, 
this  is  exactly  what  experiments  with  a  beam  of  light  enable  us 
to  do.  One  evening  towards  the  close  of  last  year,  while  pour- 
ing various  gases  across  the  dust  track  of  a  beam  in  the  labora- 
tory of  the  Royal  Institution,  the  thought  occurred  to  me  of  dis- 
placing by  my  breath  the  illuminated  dust.  I  then  noticed,  for 
the  first  time,  the  extraordinary  darkness  produced  by  the  air 
expired  towards  the  end  of  an  expiration.  By  an  intentional 
effort  of  expulsion  the  lungs  may  be  far  more  effectually  emptied 
of  air  than  by  ordinary  respiration  ;  and  by  such  an  effort,  which 
discharges  the  air  from  the  interior  portion  of  the  lungs  into  the 
beam,  the  darkness  is  changed  to  absolute  blackness.  There  is 
no  speck  or  mote  of  any  kind  in  such  air.  It  is  a  true  elastic 
fluid,  without  a  trace  of  cloud  or  floating  matter. 
Thus,  by  ocular  evidence  we  prove  the  filtering  power  of  the 
lungs,  and  by  the  experience  of  surgery  we  prove  the  incompe- 
tence of  air  so  filtered  to  produce  putrefaction.  The  germs  re- 
moved by  the  process  of  filtration  are  therefore  the  cause  of  the 
putrefaction,  and  its  associated  phenomena  of  animalcule  life, 
which  was  to  be  demonstrated. 
As  a  guide  to  the  practical  surgeon  the  establishment  of  this 
fact  is  plainly  of  the  very  highest  importance.  Professor  Lister 
now  avails  himself  of  the  filtering  power  of  cotton  wool  in  treat- 
ing a  numerous  class  of  wounds.  He  first  destroys  the  germs 
adherent  to  the  wool,  and  by  a  proper  lotion  he  kills  those  which 
may  be  scattered  on  the  flesh.  The  cleansed  w^ool  placed  upon 
the  wound  permits  of  a  free  diffusion  of  the  air,  but  entirely 
intercepts  the  germs,  and  thus  keeps  the  blood  perfectly  sweet. 
It  is  essential  that  no  matter  from  the  w^ound  should  reach  the 
outside  air,  for  such  matter  would  open  a  highway  for  the  ani- 
malcules.   I  may  add  that  when  the  foregoing  observations  on 
*  British  Medical  Journal,  1868,  p.  56. 
