AmNovUri^8aFm'  1  Decomposition  of  Hydrocyanic  Acid.  815 
NaN02,  8.8  per  cent,  was  decomposed  in  two  weeks.  In  the  last 
case  it  was  found  that  decomposition  had  been  hastened  by  the  pres- 
ence of  a  trace  of  alkali  in  the  nitrite  used.  It  was  noted  in  these 
experiments  that  addition  of  hydrogen  peroxide  to  the  hydrocyanic 
acid  oxydises  it  to  cyanic  acid,  which  on  standing  precipitates  the 
white  insoluble  cyamelide. 
Potential  causes  of  the  decomposition  of  the  acid  "  spontane- 
ously "  on  keeping  are  the  action  of  light  and  air,  the  action  of  im- 
purities contained  in  the  water,  the  action  of  substances  derived  from 
the  cork  or  rubber,  the  action  of  the  materials  used  in  preparation  of 
the  acid,  and  finally  the  action  of  substances  derived  from  the  glass 
vessel  in  which  the  acid  is  kept.  In  the  case  of  solutions  of  strength 
varying  from  2  per  cent,  to  7  per  cent,  it  has  not  been  noted  that  air 
or  diffused  light  can  effect  decomposition  appreciably,  but  a  4  per 
cent,  solution  exposed  directly  to  strong  light  began  to  decompose 
after  three  months,  although  a  similar  solution  was  unchanged  in  the 
dark  during  that  time.  Of  course,  if  the  solution  be  exposed  freely 
to  the  air  it  rapidly  loses  strength  owing  to  the  volatility  of  the 
hydrocyanic  acid.  In  view  of  the  above  results  on  the  influence  of 
bicarbonates  on  the  acid  it  is  evidently  best  to  avoid  the  use  of  tap 
water ;  although  unless  this  is  very  hard  the  acid  will  only  be  af- 
fected very  slowly,  as  experiments  have  proved.  Neither  cork  nor 
rubber  affects  the  stability  of  the  acid,  although  both  are  able  to 
absorb  considerable  quantities  of  it — thus  it  was  noted  that  a  rubber 
cork  retained  the  characteristic  smell  for  a  considerable  time  after  it 
had  been  thoroughly  washed.  Experiment  showed  that  a  red  rubber 
cork  cut  in  thin  slices  absorbed  in  two  days  6.5  per  cent,  of  its  weight 
of  hydrocyanic  acid  from  a  7  per  cent,  solution— cork  similarly  ab- 
sorbed in  four  days  2.34  per  cent,  of  its  weight.  E.  R.  de  Ong2  has 
pointed  out  the  absorption  of  the  acid  in  soil.  So  far  as  the  in- 
fluence of  the  materials  used  in  preparation  of  the  acid  is  concerned, 
this  could  hardly  be  harmful ;  for,  whether  the  gas  is  generated  from 
the  action  of  strong  cyanide  on  sulphuric  acid,  or  it  is  got  by  distilling 
a  dilute  acidified  ferrocyanide  solution,  the  sole  impurity  likely  is 
acid,  which,  as  is  mentioned  below,  tends  to  keep  the  solution.  Solu- 
tions were  made  by  these  two  methods  for  comparison,  and  showed 
little  difference ;  though  actually  that  from  ferrocyanide  kept  the 
better.    If  the  hydrocyanic  acid  was  purified  from  the  trace  of  min- 
2  J.  Agric.  Res.,  1917,  11,  421-436. 
