604 
Tuberculin. 
Am.  Jour.  Pharni. 
Dec,  1891. 
TUBERCULIN. 
Considerable  interest  has  been  recently  excited  by  the  publication 
of  a  further  communication  upon  tuberculin,  in  which  Dr.  Koch 
makes  known  the  results  of  his  attempts  to  obtain  the  active  prin- 
ciple in  a  purer  form  (Dent.  fned.  Wochensch.,  October  22,  p.  1189). 
In  Dr.  Koch's  first  attempt  to  purify  tuberculin  it  was  mixed  with 
five  times  its  volume  of  absolute  alcohol,  which  caused  a  separation 
of  a  brown  resinous  mass  that  adhered  to  the  bottom  of  the  vessel. 
Both  this  deposit,  however,  and  the  clear  supernatant  liquid  showed 
the  tuberculin  action  to  nearly  the  same  degree.  When  a  greater 
excess  of  alcohol  was  used  instead  of  the  resinous  mass,  a  fine  gran- 
ular precipitate  was  thrown  down,  which  after  being  washed  repeat- 
edly with  absolute  alcohol  and  dried  in  a  vacuum  over  sulphuric 
acid,  gave  an  almost  white  powder.  But  in  this  powder  the  tuber- 
culin was  still  contaminated  with  extractive  matter  insoluble  in 
alcohol,  and  many  fruitless  attempts  were  made  to  remove  it. 
Either,  as  when  ammonium  sulphate  was  used,  the  tuberculin  was 
thrown  down  still  contaminated  with  other  matters,  or  it  lost  its 
activity,  or  it  could  not  be  separated  in  an  active  condition  from  its 
precipitant.  For  instance,  tannin  threw  down  all  the  active  prin- 
ciple from  tuberculin,  and  the  precipitate  brought  into  soluti6n  by 
the  addition  of  sodium  carbonate  possessed  still  its  full  activity,  but 
the  active  substance  could  not  be  again  separated  from  the  tannin. 
Eventually  it  was  found  that  by  using  a  much  smaller  proportion  of 
absolute  alcohol  (two  parts  to  three  of  tuberculin)  no  separation  of 
brown  resinous  substance  was  caused,  but  that  at  the  end  of  twenty- 
four  hours  a  white  flocculent  precipitate  was  formed.  The  brown 
supernatant  liquid  was  decanted  and  replaced  by  the  same  quantity 
of  60  per  cent  alcohol,  the  whole  stirred  and  allowed  to  stand,  and 
this  was  repeated  until  the  alcohol  remained  uncolored,  when  the 
precipitate  was  washed  once  with  absolute  alcohol  and  dried  in  a 
vacuum  exsiccator.  The  product  was  a  snow-white  mass,  which 
after  drying  at  ioo°  C.  appeared  when  powdered  of  a  light  gray 
color,  and  was  considered  to  represent  the  pure  or  nearly  pure  active 
principle  of  tuberculin.  It  dissolved  fairly  easily  in  water,  but  an 
aqueous  solution  appeared  to  lose  some  of  its  activity  in  a  fortnight. 
The  evaporation  of  such  a  solution  also  was  found  liable,  as  the 
concentration  increased,  to  cause  the  separation  of  curdy  flocks, 
which  did  not  redissolve  when  more  water  was  added.    On  the 
