72  Examination  of  Hydrocyanic  Acid.  {AMFiSifSSPfc 
The  method  we  have  recently  adopted  is  to  purchase  acid  of 
Scheele's  strength  and  reduce  it  to  2  per  cent.  Mixing  some  ten  or 
twelve  ounces  at  a  time  we  keep  this  quantity  in  well-fitting  stopped 
bottles  (1  oz.  capacity)  protected  from  the  light  and  in  a  cool  place. 
One  ounce  only  is  brought  into  the  dispensary  at  a  time,  and  is  re- 
newed every  foi*tnight  provided  it  lasts  so  long.  This  insures  a  fre- 
quent supply  of  fresh  stock.  Whether  this  plan  will  answer  all  our 
expectations  I  am  not  yet  able  to  say ;  still,  after  all,  this  is  but  an 
endeavor  to  make  the  best  of  a  bad  job.  The  object  to  be  sought  for 
is,  I  venture  to  think,  some  other  cyanide  that  may  be  proved  to  be 
equal  to  the  cyanide  of  hydrogen  in  medicinal  value  and  superior  to  it 
in  chemical  constancy.  The  most  promising  substance  we  have  yet 
examined  is  the  double  cyanide  of  zinc  and  potassium.  It  is  a  per- 
fectly definite  crystalline  body,  having  the  composition,  K2Znr/Cy4. 
The  crystals  contain  no  water,  and  are  so  stable  that  they  may  be 
even  fused  without  change.  In  its  solution,  moreover,  the  cyanogen 
appears  to  be  very  securely  fixed,  as  the  following  experiment  may 
serve  to  show.  A  sample  containing  (by  calculation)  2  per  cent,  of 
cyanogen,  was  tested  and  found  to  yield  exactly  2  per  cent.  A  pint 
of  air  was  then  driven  through  the  solution  in  small  bubbles,  and 
being  again  tested,  it  was  found  to  be  of  precisely  the  same  strength. 
A  sample  of  B.  P.  acid  was  then  treated,  with  this  result — that 
whereas  in  the  first  case  it  was  shown  to  be  of  exactly  2  per  cent., 
after  the  transmission  of  the  pint  of  air  its  per  centage  was  reduced 
to  1*7.  But  although  the  cyanogen  in  this  double  cyanide  is  non- vola- 
tile, and  to  that  extent  more  constant  than  in  hydrocyanic  aeid,  yet 
it  is  easily  evolved  by  even  the  weakest  acids,  such,  for  instance,  as 
may  be  presumed  to  exist  in  the  stomach.  In  point  of  fact,  hydro- 
cyanic acid  itself  would  be  produced  directly  the  remedy  was  swal- 
lowed. The  crystals  are  prepared  very  easily  by  dissolving  cyanide 
of  zinc  in  aqueous  solution  of  cyanide  of  potassium,  in  the  proportion 
of  one  equivalent  of  the  former  to  two  of  the  latter  ;  upon  evaporation 
this  yields  beautiful  octahedral  crystals. 
A  solution  corresponding  with  the  strength  of  the  B.  P.  acid  is 
made  with  2  grams  of  the  salt,  and  42-82  grams  of  water. 
As  to  the  medicinal  value  of  this  solution,  I  am,  of  course,  not 
able  to  speak,  but  I  scarcely  think  it  could  in  any  great  degree  differ 
from  that  of  hydrocyanic  acid. — Pharm  Journ.  [London)!)  ec.  27, 1873. 
