5o8 
Note  on  Silver  Iodide. 
f  Am.  Jour.  Pharm. 
\  November,  1910. 
being  facilitated  by  slightly  warming.  A  rather  curious  thing  about 
the  precipitation  of  the  silver  iodide,  which  was  obtained  by  dis- 
solving the  potassium  iodide  in  the  gelatin  solution,  and  gradually 
adding  the  silver  nitrate  in  small  quantities,  shaking  well  after  each 
addition,  was,  that  the  resulting  precipitate  fell  immediately  to  the 
bottom  of  the  flask  after  agitation  of  the  flask  was  discontinued. 
Shaking  at  frequent  intervals  during  the  next  twenty-four  to  thirty- 
six  hours,  however,  brought  nearly  the  whole  quantity  of  silver 
iodide  into  almost  perfect  suspension.  After  the  lapse  of  a  week, 
without  any  shaking  of  the  flask  whatever,  there  was  observed  at  the 
top  of  the  flask  a  very  slight  margin  of  supernatant  liquor. 
As  stated  above  this  same  method  of  procedure  was  carried  out 
with  o.i,  0.2,  0.3,  0.4,  and  0.5  per  cent,  aqueous  solutions  of  gelatin. 
These  various  mixtures  were  allowed  to  stand,  protected  from  light, 
at  room  temperature,  for  a  period  of  at  least  five  weeks,  and  com- 
parisons made.  I  might  here  state  that  time  seems  to  be  an  impor- 
tant factor  in  making  these  suspensions ;  from  week  to  week  one 
could  notice  that  they  were  becoming  more  and  more  homogeneous. 
All  of  these  suspensions  of  the  silver  iodide  were  very  good,  but  to 
the  writer  it  seemed  that  the  one  containing  0.3  per  cent,  of  gelatin 
was  almost  perfect.  In  fact  it  had  the  appearance  of  cows'  milk, 
rich  in  cream,  with  but  a  slight  sediment  at  the  bottom  of  the  flask 
and  which  was  readily  suspended  by  a  gentle  shaking. 
It  was  not  possible  to  separate  this  colloidal  suspension  from  the 
liquid  by  filtration,  because  it  passed  through  the  pores  of  the  finest 
filter  paper. 
Examined  under  the  microscope  a  drop  of  the  freshly  made 
preparation  exhibits  the  usual  appearance  of  powdered  silver  iodide. 
But  after  the  preparation  has  been  made  some  time  a  marked  change 
is  at  once  observed.  The  action  of  the  gelatin,  which  must  be  a 
mechanical  one,  has  reduced  the  silver  iodide  to  a  remarkable  degree 
of  pulverization. 
