MERCURY — ITS  OXIDES  AND  ACETATES,  ETC. 
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did  not  coagulate  gelatin  ;  a  solution  of  tannin  of  the  same 
strength,  (half  grain  in  three  thousand  grains  of  water,)  mani- 
fested decided  results  with  gelatin. 
When  tannic  acid  is  added  to  water,  which  has  stood  twenty- 
four  hours  after  hoiling  over  the  oxide,  an  immediate  red  color 
is  produced,  changing  to  yellow  more  slowly  than  when  heated  ; 
after  standing  twelve  hours,  mercury  in  a  metallic  state  is  de- 
posited, and  the  solution  is  no  longer  affected  by  sulphide  of 
ammonium. 
Black  Oxide  of  Mercury prepared  from  calomel  by  the  pro- 
cess of  the  U.  S.  P.,  examined  under  a  microscope  by  transmit- 
ted light  under  a  power  of  near  three  hundred  diameters,  ap- 
peared as  an  amorphous  powder.  When  placed  on  an  opaque 
object,  and  examined  by  reflected  light  under  a  lower  power, 
numerous  globules  of  metallic  mercury  are  seen,  the  globules 
varying  from  the  l-400th  of  a  millimeter  in  diameter,  down. 
The  mercury  is  visible  in  a  strong  light  under  a  good  pocket 
lens,  magnifying  two  diameters. 
According  to  Donovan,  "  if  too  little  caustic  potash  be  added 
in  the  first  instance  to  calomel,  there  is  formed  a  basic  hydro- 
chlorate  of  mercurous  oxide,  which  abstracts  oxygen  from  the 
separated  oxide,  and  is  itself  converted  into  basic  hydrochlorate 
of  mercuric  oxide,  so  that  a  farther  addition  of  caustic  potash 
produces  a  mixture  of  deutoxide,  protoxide,  and  metallic  mer- 
cury." 
Duflos  prepares  black  oxide  of  mercury  "by  adding  one  part 
of  caustic  potash  lye  to  twelve  parts  of  absolute  alcohol,  and 
decanting  the  liquid  from  the  resulting  precipitate.  ll-12ths  is 
mixed  with  a  solution  of  mercurous  nitrate,  constantly  stirring 
till  the  liquid  slightly  reddens  litmus  paper,  the  remaining  l-12th 
is  then  added,  and  the  precipitate  left  to  subside,  it  is  then  de- 
canted, washed  with  water,  and  dried  at  a  moderate  heat  in  the 
dark.  The  mercurous  oxide  thus  obtained  is  black,  with  a  tinge 
of  yellowish  brown,  giving  up  nothing  to  dilute  hydrochloric 
acid,  but  dissolves  completely  in  strong  acetic  acid." 
Mercurous  oxide  was  prepared  according  to  Duflos'  method, 
care  being  taken  to  have  the  nitrate  of  mercurous  oxide  free  from 
mercuric  nitrate  ;  when  digested  at  80°  F.  with  pure  hydrochlo- 
ric acid,  sp.  gr.  1.12,  diluted  with  an  equal  bulk  of  water,  it  was 
