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410  Estimation  of  Hydrocyanic  Acid.         {^Tu^ust  isss™' 
whether  a  double  cyanide  of  magnesium  and  silver  really  does  exist, 
and  if  so,  what  is  its  composition  ? 
To  determine  the  first  point — (1.)  2*8402  grams  of  a  sample  of 
dilute  hydrocyanic  acid  Avas  estimated  by  the  B.P.  process  (Dr.  Senier's 
modification),  and  required  4*7  cc.  of  the  vol.  solution  of  nitrate  of 
silver,  indicating  nearly  '9  per  cent,  of  absolute  HCy.  (2.)  2*4446 
grams  of  the  same  sample  were  then  estimated  by  the  U.S.P.  method, 
and  required  8*25  cc,  indicating  1*82  per  cent,  admitting  the  assump- 
tion before  mentioned,  or  '91  per  cent  if  (as  is  evident  from  the  figures) 
the  whole  of  the  cyanogen  is  precipitated  as  Ag(^y,  before  any  silver 
chromate  is  permanently  formed,  agreeing  to  about  '01  per  cent,  with 
the  B.P.  process. 
The  same  experiment  was  repeated  with  the  same  result,  proving 
conclusively  that  chromate  of  potassium  will  not  serve  as  an  indicator 
on  the  basis  of  the  figures  mentioned  in  the  U.  S.  Pharmacopoeia. 
To  prove  the  existence  of  a  double  cyanide  of  silver  and  magne- 
sium, no  mention  being  made  of  a  double  cyanide  of  magnesium  and 
silver,  either  in  Gmelin's  "  Chemistry,'^  Watt's  "  Dictionary ''  or  the 
leading  English  and  American  chemical  journals,  I  made  a  few  rough 
experiments  to  determine  the  existence  of  such  a  compound.  (1.)  Cal- 
cined magnesia  was  shaken  with  hydrocyanic  acid  until  alkaline  to 
test-paper,  filtered,  a  little  chromate  of  potassium  added,  and  solution 
of  nitrate  of  silver  run  in,  in  small  quantities  at  a  time  ;  the  precipitate  at 
first  formed  was  re-dissolved,  showing  that  either  a  double  cyanide  was 
formed  or  that  cyanide  of  silver  is  soluble  in  solution  of  nitrate  of 
magnesium,  which  latter  was  disproved  by  the  addition  of  more  nitrate 
of  silver,  when  white  cyanide  of  silver  was  precipitated,  until  the  whole 
of  the  cyanogen  was  converted  into  silver  cyanide,  when  red  chromate 
of  silver  was  formed.  (2.)  £Having  thus  proved  the  existence  of  a 
double  salt  I  attempted  to  determine  its  composition  by  repeating  the 
above  experiment  quantitatively,  noting  the  number  of  cc.  of  nitrate 
of  silver  solution  required  to  give  a  cloudiness  from  the  precipitation 
of  silver  cyanide,  and  also  the  number  required  to  completely  precipi- 
tate the  cyanogen  (indicated  by  the  formation  of  red  chromate  of  silver). 
Quantity  of  hydrocyanic  acid  used   2  5688  grams. 
Quantity  of  st.  sol.  of  AgNOg  required  to  give 
cloudiness, from  precipitation  of  AgCy   5'64  cc. 
Quantity  of  st.  sol.  of  AgNOg  required  to 
completely  precipitate  the  AgCy   11*24  cc. 
