"^Vrri;!??!!"''  }         Manvfacture  of  Vermillion,  163 
modification,  but  the  precipitation  may  be  prevented  by  the  addition 
of  a  small  quantity  of  caustic  alkali. 
Evidently  this  is  the  reason  that  Weber  says  that  sulphide  of 
mercury  will  dissolve  in  alkaline  sulphurets  only  in  the  presence  of 
free  alkali,  whereas  solutions  of  the  alkaline  sulphurets,  containing  so- 
much  sulphur  as  to  look  deep  red  or  brown,  will  readily  dissolve 
enough  of  the  sulphide  to  give,  on  dilutions,  a  considerable  black  pre- 
cipitate. The  precipitate  does  not  always  form  immediately,  but  in< 
variably  makes  its  appearance  after  a  short  lapse  of  time. 
The  solubility  *  of  the  sulphide  of  mercury  constitutes  the  chief 
source  of  loss  in  the  manufacture  of  vermillion,  and,  on  an  average^ 
amounts  to  from  five  to  eight  per  cent,  of  the  mercury  taken. 
In  spite  of  all  precautions,  it  will  sometimes  happen  that  the  mer- 
cury ''flours,"  and,  as  it  seems,  every  minute  globule  is  coated  with 
a  thin  layer  of  sulphide.  This  coat  not  only  prevents  the  conversion 
of  the  metal  into  the  sulphide,  but  even  its  solution  in  nitric  acid.  If 
concentrated  nitric  acid  be  used,  the  whole  of  the  sulphide  and  metal 
is  dissolved,  whereas  dilute  nitric  acid  has  no  efi'ect  at  all.  Concen- 
trated hydrochloric  acid,  especially  wlien  boiling,  will  readily  decom- 
pose the  sulphide. 
As  it  is  well  known,  no  vermillion  will  withstand  the  action  of  the 
light;  it  turns  dark  and  gradually  black  again.  Whether  this  change 
is  due  to  the  decomposition  of  the  sulphide  into  the  sub-sulphide  and 
free  sulphur,  or  simply  to  the  conversion  of  the  crystalline  into  the 
amorphous  modification,  is  doubtful  ;  but  it  is  certain  that  any  im- 
purities  increase  this  tendencj^,  especially  the  presence  of  free  mer-= 
cury,  which  seems  to  indicate  that  a  decomposition  does  take  place^ 
though  always  only  on  the  surface.  Even  the  passing  of  steam  or  tho 
evaporation  of  a  drop  of  water  over  some  vermillion  will  often  rapidl}^ 
efi'ect  this  change  of  color.  Of  course,  the  value  of  the  vermillioiii 
greatly  depends  on  the  stability  of  color,  and  therein  the  different 
articles  of  commerce  vary  greatly.  Some  will  retain  their  brightness 
for  several  years,  while  others  may  be  seen  to  change  after  a  few 
weeks.  If,  perhaps,  the  vermillion  obtained  by  sublimation  is  a  little 
more  stable  than  that  manufactured  by  the  wet  process,  it  certainly,  with 
*  It  is  generally  stated  that  sulphide  of  ammonium  does  not  dissolve  sul- 
phide of  mercury,  but,  judging  from  the  fact  that,  according  to  Liebig  and 
Gautier  Bouchard,  sulphide  of  ammonium  effects  the  crystallization  of  the 
black  sulphide,  it  seems  probable  that  this  statement  is  not  quite  correct. 
