Am'ilZ'Si&rm'}  Detection  of  Ferments  and  Poisons  in  Blood,  607 
coloring-matter  of  the  blood,  leaving  all  other  substances  in  solu- 
tion, and  chemically  unaltered,  we  should  have  made  an  important 
step  towards  the  isolation  and  detection  of  the  enzymes  and  tox- 
albumins. 
Zinc-powder  may  be  regarded  as  such  a  precipitant  for  haemo- 
globin. In  forensic  chemistry  which  is  generally  concerned  with 
stale,  offensive  blood,  this  agent  has  the  advantage  that  it  renders 
the  specimen  almost  inodorous  even  if  several  weeks  old.  The 
completeness  of  the  precipitation  is  not  interfered  with  by  the  age 
of  the  blood.  The  following  conditions  are  essential  for  successful 
precipitation  : 
(1)  In  quite  recent  blood,  freshly  drawn  from  normal  men  or 
other  animals,  which  is  always  alkaline,  the  alkalinity  must  be  neu- 
tralized before  the  addition  of  the  zinc-powder.  The  same  holds 
good  of  the  blood  of  dead  bodies  a  week  old  in  which  ammonia  has 
been  formed.  On  exposure  to  the  air,  or  remaining  in  the  body, 
the  normal  alkalinity  of  the  blood  is  lost  within  1  to  2  days.  Dis- 
eases— especially  fevers — often  reduce  the  alkalinity  before  death  to 
such  a  degree  that  the  zinc  may  be  added  directly. 
(2)  The  blood  must  be  free  from  methaemoglobin.  If  this  sub- 
stance is  present  the  blood  is  allowed  to  remain  in  an  open  vessel 
without  dilution  or  shaking  until  the  last  trace  of  methaemoglobin 
has  disappeared.  It  is  known  that  this  disappearance  often  ensues 
within  24  hours,  whether  in  the  corpse  or  in  jars,  even  when  the 
original  quantity  of  methaemoglobin  was  very  considerable. 
(3)  The  blood  must  be  diluted  with  at  least  3  to  5  volumes  of 
water. 
(4)  The  zinc-powder  must  be  as  pure  as  possible,  containing 
nothing  but  zinc  and  zinc  oxide. 
(5)  The  quantity  of  the  zinc-powder  must  be  equal  to  a  quarter 
or  a  half  of  the  original  weight  of  the  blood. 
(6)  The  mixture  must  be  energetically  shaken  for  a  considerable 
time. 
If  these  conditions  are  observed  a  complete  separation  of  the  col- 
oring-matter of  the  blood  is  almost  always  effected,  so  that  even  on 
washing  the  precipitate  with  much  water  in  the  filter-press  or  in 
the  filter-pump  nothing  returns  into  solution.  The  yellowish-brown 
coloring-matter  contained  in  the  serum  of  some  kinds  of  blood  is 
not  thrown  down,  but  remains  in  the  filtrate.    Distinctly  recog- 
