FILTERED  AIR. 
359 
United  States'  coins  now  in  general  circulation  contain  88  per 
cent,  of  copper  and  12  per  cent,  of  nickel. 
A.  R.  ROESSLER, 
Geologist^  General  Land  Office^  Washington. 
Jan,  20,  1870.     —Journ.  Frajiklin  Institute,  Feb.,  1870. 
ON  FILTERED  AIR. 
By  Professor  Tyndall,  F.R.S.* 
The  theory  of  disease  was  never  discussed  with  more  earnest- 
ness, or  with  greater  precision,  than  at  the  present  time.  The 
exact  methods  pursued  in  physics  and  chemistry,  both  as  regards 
reasoning  and  experiment,  are  making  their  influence  felt  in 
medicine  and  surgery ;  and  they  promise,  while  assigning  but  nar- 
row limits  to  our  present  accurate  knowledge,  to  insure  its  healthy 
growth.  It  is,  I  think,  of  capital  importance  to  mark  each  suc- 
cessive step  by  which  that  knowledge  is  surely  and  certainly 
augmented  ;  to  detach  from  the  domain  of  vagueness  and  uncer- 
tainty each  successive  fragment  of  demonstrated  truth.  Now, 
if  the  published  data  be  correct,  it  seems  to  me  that  such  a  step 
has  been  recently  taken  with  reference  to  the  germ  theory  of 
the  putrefaction  of  wounds,  and  that  the  evidence  in  favor  of 
that  theory  amounts  to  a  physical  demonstration  of  its  truth. 
This  result  and  its  basis  I  propose  here  to  describe  and  define. 
The  entrance  of  air  into  a  wound  is  the  dread  of  the  surgeon. 
When  an  abscess  is  opened  he  must  prevent  the  air  from  mingling 
with  the  blood-clots  if  he  would  avoid  putrefaction  and  its  teem- 
ing accompaniment  of  animalcule  life.  Some  eminent  London 
surgeons  inform  me  that  they  never  squeeze  an  abscess,  lest  when 
the  pressure  is  relaxed  the  air  should  be  sucked  in.  Now, 
whence  this  dreaded  power  ?  Is  it  the  air  itself  that  causes  pu- 
trefaction, or  is  it  something  carried  mechanically  by  the  air  ? 
A  follower  of  Gay-Lussac  would  aflirm  the  former ;  a  hetero- 
genist  would  refer  the  animalcules  to  "spontaneous  generation  ;" 
a  holder  of  the  germ  theory  would  ascribe  the  putrefaction  to 
seeds  or  eggs  floating  in  the  atmosphere,  and  which,  when  sown 
upon  the  wound,  sprout  into  this  crop  of  minute  organisms.  Do 
*  Contributed  to  the  Times,  April  7. 
