372 
REACTIONS  OF  GELATIN. 
When  a  piece  of  gelatin  is  dropped  into  an  acid  solution  of 
pernitrate  of  mercury,  it  gradually  assumes  a  strong  red  colora- 
tion, and,  after  a  time,  dissolves  in  it  completely,  at  ordinary 
temperatures,  to  a  fine  red  solution.  This  solution  deepens  a 
little  if  boiled  for  some  minutes.  By  chlorate  of  potash,  the 
hot  solution  is  quickly  decolorized,  and  passes  to  pale  dirty 
yellow. 
This  red  coloration  seems  to  require  a  certain  amount  of  time 
for  its  production,  which  cannot  be  replaced  by  heat.  If  a  piece 
of  gelatin  be  immersed  in  the  solution  of  protonitrate  and  boiled 
for  some  minutes,  it  is  dissolved,  but  the  solution  thus  obtained 
is  not  red,  but  yellowish. 
It  is  to  be  regretted  that  the  reaction  here  described  is  not 
more  delicate.  It  is  only  striking  when  tolerably  strong  solu- 
tions of  gelatin  are  employed.  When  the  solution  is  very 
weak,  as,  for  example,  if  the  gelatin  constitutes  only  one-half 
of  one  per  cent,  of  the  mixed  liquids,  the  limit  of  the  delicacy 
of  the  test  is  reached.  Such  a  solution,  by  standing  twenty-four 
hours,  exhibits  a  light  but  distinct  pink  color.  Although  this 
delicacy  is  not  what  may  be  desired,  still,  colloid  organic  sub- 
stances are  so  comparatively  difficult  of  qualitative  detection  as 
a  general  thing,  that  the  method  is  not  without  value. 
The  experiment  was  next  extended  to  metagelatin.  A  neu- 
tral metagelatin  was  prepared  in  the  following  manner : — 
Gelatin  was  set  to  swell  in  cold  saturated  solution  of  oxalic 
acid,  and  then  a  moderate  heat  was  applied  for  a  sufficiently  long 
time  for  the  mass  to  remain  quite  fluid  when  cold.  It  was  then 
agitated  with  precipitated  carbonate  of  lime  until  the  whole  of 
the  oxalic  acid  was  got  rid  of.  Metagelatin  prepared  in  this 
way  was  kept  for  months  in  a  corked  vial,  in  a  warm  room,  with- 
out showing  any  disposition  to  putrefy.  It  was  almost  as  fluid 
as  water :  perfectly  neutral  and  almost  insipid  to  the  taste. 
With  this  metagelatin,  the  red  coloration  was  produced  even 
more  decidedly  than  with  ordinary  gelatin.  The  addition  of 
.  the  acid  solution  of  pernitrate  of  mercury  produced  at  first  a 
whitish  flocculent  precipitate,  which,  by  standing,  acquired  a 
strong  red  color,  as  did  the  supernatant  liquid. — Amer.  Journ. 
of  Science  and  Arts,  July,  1865. 
