816  Decomposition  of  Hydrocyanic  Acid.  {  Am'I^°ur"IJ>I8arm* 
eral  acid  by  redistillation  the  solution  commenced  to  decompose 
earlier. 
Finally,  we  have  the  influence  of  substance  derived  from  the 
glass  vessel  in  which  the  acid  is  stored,  and  there  can  be  no  glass 
vessel  in  which  the  acid  is  stored,  and  there  can  be  no  question  that 
this  is  the  main,  if  not  the  sole  reason  why  dilute  solutions  of  the 
acid  cannot  be  kept  indefinitely.  The  solution  slowly  dissolves  from 
the  glass  sodium  silicate,  which  in  turn  forms  sodium  cyanide,  which 
is  able  to  effect  polymerisation  of  the  remaining  acid.  This  change 
proceeds  very  slowly,  but  it  was  shown  fairly  rapidly  in  the  follow- 
ing experiment.  Two  samples  of  8  per  cent,  hydrocyanic  acid  were 
sealed  in  tubes ;  to  one  of  these  had  been  added  a  quantity  of  clean, 
finely  ground,  soft  glass.  Both  samples  were  shaken  from  day  to 
day,  and  after  two  days  the  sample  containing  the  glass  was  yellow, 
while  after  nine  days  it  was  dark  brown,  and  a  considerable  deposit 
had  formed.  Meanwhile  the  other  sample  had  remained  unaltered. 
The  same  effect  was  shown  quantitatively  by  diluting  4  Gm.  of 
hydrocyanic  acid  to  which  9  Gm.  of  ground  glass  had  been  added 
to  100  Cc,  and  keeping,  the  whole  being  well  shaken  occasionally. 
After  three  weeks  12.2  per  cent,  of  the  acid  had  decomposed. 
It  is  stated  that  mineral  acid  is  added  to  the  commercial  prussic 
acid  (2  per  cent.  HCN)  and  to  Scheele's  acid  as  a  preservative,  and 
the  reason  for  this  is  quite  evident  from  the  previous  results. 
Samples  of  2  per  cent,  and  4  per  cent,  acid  were  kept  in  6  oz. 
bottles — the  latter  solution  showed  signs  of  decomposition  after 
four  months  and  the  former  after  six  months.  Meanwhile,  similar 
samples  to  which  had  been  added  1  per  cent.  H2S04  (calculated  on 
the  weight  of  HCN)  remained  unaltered.  The  time  these  acidified 
samples  could  be  kept  would,  of  course,  depend  on  the  quantity  of 
mineral  acid  added,  but  from  results  obtained  by  the  method  next 
described  it  would  appear  that  if  as  much  as  10  per  cent.  H2S04 
(calculated  on  the  weight  of  HCN)  could  be  added  such  solutions 
would  for  practical  purposes  keep  indefinitely.  This  method  con- 
sisted in  sealing  up  together  the  acid,  a  definite  amount  of  powdered 
glass,  and  some  third  constituent  added  with  a  view  to  preserving 
the  acid.    The  following  results  were  thus  obtained : 
