8 14  Decomposition  of  Hydrocyanic  Acid.  {  Am'^^r\g^8avm' 
noted  that  on  trying  to  dry  the  gas  with  pure  sulphuric  acid  (96  per 
cent.)  at  400  C,  it  is  completely  decomposed  to  ammonia  and  carbon 
monoxide.  On  running  strong  cyanide  solution  into  leaden  tanks, 
the  liquid  soon  became  filled  with  a  brown  precipitate;  this  at  first 
appeared  due  to  the  polymerisation  in  question,  but  proved  to  be  the 
result  of  interaction  of  a  trace  of  sulphide  in  the  cyanide  with  a 
film  of  lead  oxide. 
It  is  interesting  to  note  that  whereas  sodium,  carbonate  dis- 
solved in  30  per  cent,  sodium  cyanide  solution  causes  in  time  the 
separation  of  a  slight  brown  precipitate,  caustic  soda  has  not  the 
same  effect.  Presumably  it  is  the  hydrocyanic  acid  formed  by  hy- 
drolysis which  is  thus  decomposed;  caustic  soda  represses  the  hy- 
drolysis. 
In  order  to  investigate  the  influence  of  different  compounds  on 
hydrocyanic  acid,  solutions  of  the  latter  (7-8  per  cent.)  in  water 
were  made  by  running  30  per  cent,  cyanide  solution  into  sulphuric 
acid  (50-60  per  cent.)  which  was  slowly  warmed  up  on  a  water  bath, 
the  gas  evolved  being  condensed  and  collected  in  distilled  water. 
The  solution  was  then  mixed  with  various  reagents,  and  kept  in 
sealed  tubes  in  a  dark  cuboard.  It  was  found  that  weak  acids  and 
■salts  of  acid  reaction  had  no  influence  in  promoting  the  decomposi- 
tion of  the  hydrocyanic  acid ;  but  the  addition  of  bases  such  as 
caustic  soda,  potash,  and  ammonia  resulted  in  the  formation  of  sol- 
uble cyanides  which  brought  about  the  reaction.  Further,  it  was 
found  that  a  large  number  of  salts  also  caused  the  decomposition 
to  proceed  at  varying  rates.  These  include  the  soluble  carbonates 
and  bicarbonates,  the  alkali  salts  of  nitrous  acid,  normal  sodium 
phosphate,  sodium  sulphide,  sodium  sulphite,  sodium  ferrocyanide, 
and  sodium  acetate.  Sodium  sulphocyanide  differs  from  sodium 
cyanide  in  that  it  does  not  affect  the  acid.  Thus,  salts  of  alkaline 
reaction  are  reactive,  in  all  cases  probably  through  the  formation  of 
a  small  quantity  of  cyanide,  and  this  explains  the  variation  in  ac- 
tivity which  such  salts  show.  Borax,  which  is  very  active,  is  derived 
from  an  acid  which  is  practically  as  weak  as  hydrocyanic  acid  itself. 
Three  quantitative  experiments  on  the  decomposition  of  3.6  per 
cent,  solutions  of  the  acid  at  ordinary  temperatures  were  carried 
out,  and  it  was  found  that  when  the  solution  contained  1  per  cent, 
of  sodium  cyanide,  in  two  weeks  47.4  per  cent,  of  the  acid  was  de- 
composed ;  when  it  contained  0.75  per  cent.  NaHCOs,  in  five  weeks 
1.4  per  cent,  was  decomposed;  and  when  it  contained  1.4  per  cent. 
