50     DETECTING  THE  ORGANIC  ALKALOIDS  IN  CASES  OF  POISONING. 
foreign  matters,  and  obtaining  in  a  small  bulk  a  solution  in  wh'iGh 
the  alkaloid  can  be  found.  The  bicarbonates  of  potash  or  soda,  or 
these  alkalies  in  a  caustic  state,  are  convenient  bases  for  setting 
the  alkaloids  at  liberty,  at  the  same  time  keeping  them  wholly  in 
solution,  especially  if  the  alkaloids  have  been  combined  with  an 
excess  of  tartaric  or  of  oxalic  acid. 
To  separate  foreign  substances,  animal  or  otherwise,  from  the 
suspected  matters,  recourse  is  commonly  had  to  the  tribasic  acetate 
of  lead,  and  precipitating  the  lead  afterwards  by  a  current  of  sul- 
phuretted hydrogen.  As  I  have  several  times  witnessed,  this 
procedure  has  many  and  very  serious  inconveniences.  In  the  first 
place,  the  tribasic  acetate  of  lead,  even  when  used  in  large  excess, 
comes  far  short  of  precipitating  all  the  foreign  matters;  secondly, 
the  sulphuretted  hydrogen,  which  is  used  to  precipitate  the  lead, 
remains  in  combination  with  certain  organic  matters  which  undergo 
great  changes  by  the  action  of  the  air  and  of  even  a  moderate  heat  ; 
so  that  animal  liquids  which  have  been  precipitated  by  the  tribasic 
acetate  of  lead,  and  from  which  the  lead  has  been  separated  after- 
wards by  hydrosulphuric  acid,  color  rapidly  on  exposure  to  the  air, 
and  exhale  at  the  same  time  a  putrid  odor,  which  adheres  firmly 
to  the  matters  which  we  extract  afterwards  from  the  liquids. 
The  use  of  a  salt  of  lead  presents  another  inconvenience,  viz.,  the 
introduction  of  foreign  metals  into  the  suspected  matters,  so  that 
that  portion  of  the  suspected  substance  is  rendered  unfit  for  testing 
for  mineral  substances.  The  successive  and  combined  use  of  water 
and  alcohol  at  different  states  of  concentration,  permits  us  to  search 
for  mineral  substances,  whatever  be  their  nature,  so  that  in  this 
way  nothing  is  compromised,  which  is  of  immense  advantage  when 
the  analyst  does  not  know  what  poison  he  is  to  look  for. 
It  is  hardly  necessary  to  say,  that  in  medico-legal  researches  for 
the  alkaloids,  we  ought  never  to  use  animal  charcoal  for  decolor- 
izing the  liquids,  because  we  may  lose  all  the  alkaloid  in  the 
suspected  matters.  It  is  generally  known  that  animal  charcoal 
absorbs  these  substances  at  the  same  time  that  it  fixes  the  coloring 
and  odoriferous  matters. 
[This  is  no  doubt  true;  we  must  not  use  animal  charcoal  to  de- 
colorize, and  then  look  for  the  alkaloid  in  the  liquid,  but  we  may 
use  it  at  least  in  the  case  of  strychnia  and  some  of  the  non- volatile 
