100 
SULPHOCYANIDE  OF  MERCURY,  ETC. 
sulphocyanide  of  potassium,  (K.CyS2,)  though  there  are  always 
small  quantities  of  secondary  products  formed. 
The  rationale  of  the  process  is  probably  as  follows : — The 
reaction  being  between  1  eq.  of  dry  ferrocyanide  of  potassium, 
1  eq.  carbonate  of  potassa,  eight  equivalents  of  sulphur,  and 
one  of  carbon  ;  the  potassium  and  the  cyanogen  of  the  ferro- 
cyanide, the  potassium  of  the  carbonate,  and  six  equivalents  of 
the  sulphur  unite  to  form  three  equivalents  of  sulphocyanide  of 
potassium,  3(K.CyS2),  the  iron  of  the  ferrocyanide  and  the 
remaining  sulphur  form  the  insoluble  sulphuret  of  iron,  whilst 
the  carbon,  assuming  the  oxygen  of  the  liberated  potassium, 
passes  off  with  the  carbonic  acid  as  carbonic  oxide,  and  is  burnt 
during  the  process. 
Having  now  a  solution  of  sulphocyanide  of  potassium,  the 
next  step  is  the  solution  of  pernitrate  of  mercury,  (Hg02,2N05,) 
which  is  obtained  by  dissolving  mercury  in  hot  nitric  acid,  and 
heating  until  a  drop  of  the  solution  will  not  form  a  precipitate 
with  a  solution  of  chloride  of  sodium.  The  solution  should  then 
be  diluted  with  water  until  each  ounce  of  mercury  used  is  repre- 
sented by  a  pint  and  a  half  of  the  solution. 
The  solution  of  sulphocyanide  of  potassium  is  then  poured 
gradually  into  the  mercuric  solution  as  long  as  a  precipitate  is 
formed,  and  no  longer,  (as  the  mercurial  sulphocyanide  is  soluble 
in  the  solution  of  nitrate  of  mercury,  when  in  excess).  The 
precipitate  is  heavy  and  subsides  rapidly ;  it  is  washed  in 
several — three  or  four — waters  by  subsidence  and  decantation, 
then  the  creamy  sediment  transferred  to  a  muslin  cloth  and 
squeezed,  gently  at  first,  and  then  more  forcibly,  and  the 
lump  of  moist  sulphocyanide  of  mercury,  when  turned  out  of  the 
cloth,  will  be  found  in  just  the  right  condition  to  be  moulded 
into  cones  with  the  fingers.  Sometimes  mucilage  of  tragacanth 
is  added,  but  it  is  not  necessary,  as  the  dried  salt  (HgCyS2)  has, 
like  the  white  precipitate  of  the  shops,  considerable  firmness  in 
mass.  The  actual  play  of  affinities,  during  the  combustion  of 
the  salt,  is,  perhaps,  not  certainly  known;  but  the  following 
rationale,  involving  24  equivalents  of  the  salt,  may  be  assumed 
to  approach  nearly  to  the  truth : — 
