436         DISINFECTANTS  IN  ARRESTING  CATTLE  PLAGUE. 
84.  From  time  immemorial  carbolic  acid,  creosote,  or  bodies 
containing  them,  have  been  used  as  antiseptics.  Passages  in 
Pliny,  read  by  the  light  of  chemical  science,  show  that  the 
Egyptians  used  for  embalming  their  mummies  a  compound  made 
from  pitch,  which  must  have  contained  large  quantities  of  creo- 
sote. Carbolic  acid  is  the  active  agent  in  tar,  which,  either  in 
its  ordinary  state  or  burnt  as  a  fumigator,  has  always  held  high 
rank  amongst  disinfectants.  Pitch  and  tar  were  the  most  popu- 
lar medicines  in  use  against  the  cattle  plague  when  it  visited  this 
island  in  the  last  century ;  the  animals  being  preserved  against 
contagion  by  having  their  noses  and  jaws  rubbed  with  tar,  whilst 
the  cow-houses  were  disinfected  by  burning  pitch  and  tar  in  them 
(in  which  process  a  certain  quantity  of  the  vapors  of  carbolic 
acid  would  escape  combustion.)  The  almost  universal  custom  of 
burning  gum  resins  and  odoriferous  woods  in  connection  with 
religious  ceremonies  may  have  originally  arisen  from  the  disin- 
fecting powers  of  the  creosote  in  the  smoke.  The  well-known 
efficacy  of  smoke  in  preserving  meat  is  entirely  due  to  the 
presence  in  it  of  this  agent. 
Pitch  oil,  oil  of  tar,  and  similar  products  owe  their  value 
entirely  to  carbolic  acid  (22.)  This  body  may  in  fact  be  called 
the  active  principle  of  tar,  just  as  quinia  is  the  active  principle 
of  bark,  or  morphia  of  opium,  and  it  has  the  advantage  of  being 
easily  prepared  in  any  country  where  coal  or  wood  can  be 
obtained. 
35.  Sulphurous  acid  probably  owes  some  of  its  antiseptic 
value  to  its  affinity  for  oxygen,  whereby  the  oxidation  of  the  mat- 
ter under  treatment  is  retarded.  It  has  been  suggested  that  the 
value  of  carbolic  acid  is  due  to  a  similar  property,  and  that  it 
acts  merely  by  preventing  oxidation.  It  being  important  to  a 
thorough  understanding  of  its  action  that  this  point  should  be 
settled,  the  following  experiments  were  made  : — 
I.  Lumps  of  metallic  sodium  were  cut  with  a  sharp  knife  :  the 
progress  of  the  oxidation  could  be  readily  followed  by  the  change 
of  color  of  the  surface.  The  experiment  was  tried  several  times 
in  an  atmosphere  strongly  charged  with  the  vapor  of  pure  carbolic 
acid  and  of  cresylic  acid ;  comparative  experiments  being  made 
