264  MERCURY — ITS  OXIDES  AND  ACETATES,  ETC. 
almost  completely  converted  into  calomel.  The  acid  solution 
was  filtered  and  evaporated  to  dryness  on  a  watch-glass,  a  few 
drops  of  water  added,  and  a  drop  of  dilute  sulphide  of  ammo- 
nium.   A  black  precipitate  was  immediately  produced. 
Duflos'  black  oxide  of  mercury  was  treated  with  dilute  hydro- 
chloric acid,  (one  part  of  acid,  sp.  gr.  1.12,  and  seven  parts  of 
water,)  the  filtered  solution  evaporated  to  dryness,  and  re-dis- 
solved in  warm  water,  afforded  a  yellow  precipitate  with  caustic 
potash.  When  treated  with  dilute  acetic  acid  and  the  mercurial 
salt  precipitated  by  chloride  of  sodium,  the  filtrate  evaporated 
to  a  small  bulk  also  gave  a  yellow  precipitate  with  caustic 
potash. 
Whether  chlorhydric  acid  takes  up  anything  from  black  oxide 
of  mercury,  when  pure,  could  not  with  certainty  be  determined, 
owing  to  the  difficulty  of  obtaining  a  pure  black  oxide.  The 
partial  solubility  of  calomel  in  dilute  HC1,  prevents  the  appli- 
cation of  delicate  reagents. 
According  to  Guibourt,  "  it  is  impossible  to  procure  this  oxide 
pure,  because,  during  washing,  even  in  the  dark,  it  is  resolved 
into  a  mixture  of  protoxide  and  metallic  mercury.  Light,  even 
diffused  daylight,  or  a  gentle  heat,  resolves  the  dioxide  into  me- 
tallic mercury  and  the  protoxide." 
Black  oxide  of  mercury  prepared  by  Duflos'  method,  exam- 
ined under  the  microscope  by  reflected  light,  shows  metallic 
globules  varying  from  the  l-800th  of  a  millimeter,  down  to  the 
smallest  points  visible  under  a  power  of  163  diameters.  The 
globules  are  smaller,  and  more  numerous  than  in  the  oxide  pre- 
pared from  calomel. 
"  Hydrocyanic  acid  added  to  mercurous  salts  throws  down 
half  of  the  mercury,  and  forms  cyanide  of  mercury." — Rose. 
Red  Oxide  of  Mercury. — The  commercial  red  oxide,  which 
was  preserved  in  the  dark,  was  employed  in  the  following  ex- 
periments : 
"  Aqueous  mercuric  oxide  is  slightly  soluble  in  water." — 
Thompson,  G-uibourt,  and  Donovan. 
"  The  oxide  obtained  by  use  of  nitric  acid,  and  the  precipi- 
ated  oxide,  dissolves  sparingly  in  cold,  somewhat  more  abun- 
dantly in  hot  water."  —Anthon. 
Water  digested  at  60°  F.  on  red  oxide  of  mercury,  gives  an 
