556  Opsonic  Theory  and  Bacterial  Vaccines.     { ^^mb^i™" 
as  KAs02.  This  formula  is  correct,  if  we  make  a  solution  of  the  salt 
by  gently  heating  As203  with  an  excess  of  potassium  bicarbonate  in 
the  presence  of  water,  as  in  the  case  of  the  official  Liq.  Pot. 
Arsenitis,  but  it  is  not  correct  for  the  dry  salt,  which  has  the 
formula  KAs02  +  HAsO,  +  H20.  (E.  Schmidt,  Merck's  Index, 
etc.)  Taken  as  KAs02  the  salt  would  contain  about  68  4  per  cent. 
As203,  but  taken  as  KAs02  -j-  HAs02  4-  H20,  it  contains  about 
727  per  cent.  A?203. 
The  dry  salt  is  difficult  to  manufacture  and  contains  always  a  more 
or  less  large  amount  of  potassium  carbonate — one  of  our  samples 
running  as  high  as  35  per  cent.  K2COs  and  only  38  per  cent,  of  As203. 
As203  was  determined  by  the  common  Iodine  method,  the  K2C03 
by  calculation  from  C02  found  with  Scheibler's  apparatus  (for 
determination  of  C02  in  boneblack). 
Analytical  Laboratory  of  the  H.  K.  Mulford  Company, 
November  5,  1907. 
THE  OPSONIC  THEORY  AND  BACTERIAL  VACCINES. 
By  A.  Parker  Hitchens,  M.D.,  Glenolden,  Pa. 
For  the  more  lucid  understanding  of  a  theory  which  occupies  only 
a  small  part  of  an  important  branch  of  medical  science,  some  knowl- 
edge of  the  fundamental  principles  of  the  general  subject  is  essential. 
This  is  the  relation  which  the  opsonic  theory  bears  to  that  most 
interesting  and  apparently  inexhaustible  subject — immunity  ;  and  I 
therefore  request  your  indulgence  while  I  consider  it  briefly. 
Immunity  may  be  roughly  defined  as  resistance  to  disease.  This 
immunity  or  resistance  may  be  natural,  congenital  or  acquired  ;  man 
is  naturally  immune  to  certain  diseases  v^hich  attack  the  lower 
animals  (black  leg,  hog-cholera).  A  child  may  be  born  with  a  degree 
of  inherited  immunity  to  a  certain  disease — co?igenital ;  immunity 
to  a  disease  is  acquired  by  recovery  from  an  attack  of  the  disease  or 
by  artificial  inoculation.  Immunity  is  relative  or  absolute.  Immu- 
nity is  active,  when  the  body  of  the  individual  elaborates  its  own 
protective,  or  curative  substances  ;  it  is  passive  when  the  protective 
or  curative  substances  are  manufactured  in  the  body  of  another 
animal  and  transferred  artificially  (antitoxin). 
