70  Examination  of  Hydrocyanic  Acid.  {%l™x,m£™' 
in  one  litre  of  distilled  water  ;  4  grams  of  each  sample  were  care- 
fully weighed  in  a  balance  capable  of  turning  to  one-tenth  of  a  milli- 
gram, with  all  the  expedition  possible  (a  most  important  matter), 
and  mixed  with  rather  more  than  sufficient  solution  of  caustic  potash 
to  convert  all  the  HON  into  KCN  ;  our  silver  solution,  contained  in  a 
burette,  was  then  run  in  until  the  last  drop  produced  a  permanent 
faint  opalescence  in  the  liquid  after  stirring.  The  number  of  c.c.  of 
silver  solution  required  to  produce  this  effect  divided  by  40  gave  at 
once  the  percentage. 
Our  experiments,  conducted  I  may  say,  with  every  precaution  to 
avoid  error,  and  repeated  more  than  once,  indeed,  several  times  in 
every  case,  upon  twelve  samples  of  hydrocyanic  acid,  gave  the  fol- 
lowing average  results  : — 
So-called  B.  P.  Acid. 
No.  1  1-92. 
"    2   1-52. 
"3  v  1-36. 
"    4   1-32. 
"5       .       .       .       .       .       .*  1-262. 
"    6   .       .  1-22. 
"7   1-165. 
"8  1-16. 
"    9       ......  0-26. 
Acid  of  "  Scheele's  "  strength. 
"  10  3-62. 
"11       .       .       .       .       .       .       .  4-02. 
"12  1-3. 
The  history,  as  far  as  we  know,  of  these  samples  will  serve  to  show 
how  unfortunately  unstable  is  this  important  remedy,  and  to  indicate 
the  necessity  for  special  care  in  its  management. 
No.  1  was  obtained  from  a  wholesale  house  of  deservedly  good 
reputation,  and  examined  directly  it  arrived. 
Nos.  4  and  6  were  samples  of  the  same  acid  examined  in  the  one 
case  when  the  bottle  was  first  opened,  and  in  the  other  when  the  con- 
tents had  suffered  from  the  frequent  removal  of  the  stopper  incidental 
to  the  dispensary. 
Nos.  3  and  5  were  kindly  sent  me  by  a  friend,  who  appended  the 
following  remarks : — No.  3  "  from  a  40  oz.  bottle,  from  which  over 
35  ozs.  have  been  from  time  to  time  removed/'    No.  5.  "  First  re- 
