HYDRARGYRUM  CUM  CRETA,  ETC.  389 
opaque,  and  within  a  few  minutes  begins  to  subside  and  collect 
at  the  bottom  of  the  test  glass. 
After  precipitating  all  the  suboxide  as  subchloride  the  pre- 
cipitate is  filtered  out,  and  the  filtrate  is  tested  by  adding  am- 
monia in  slight  excess.  If  there  has  been  any  persalt  of  mer- 
cury in  the  original  solution  it  will  have  been  changed  by  the 
HC1.  into  soluble  perchloride,  and  in  this  condition  will  have 
passed  the  filter.  The  ammonia  precipitates  this  salt  as  am- 
moniated  mercury  or  "white  precipitate,"  and  the  proportion 
of  this  second  white  precipitate  is  indicative  of  the  amount  of 
peroxide  in  the  preparation. 
Now  it  happens  that  acetic  acid,  as  well  as  hydrochloric,  al- 
though without  any  appreciable  action  upon  fluid  metallic  mer- 
cury, does  act  upon  the  metallic  mercury  in  the  state  of  subdivi- 
sion in  which  it  exists  in  well  made  mercury  with  chalk  ;  but 
the  amount  oxidized  and  dissolved  by  acetic  acid  under  the  con- 
ditions of  this  test,  is  so  small  that  it  may  be  disregarded  in  the 
practical  application  of  the  test.  It  is  certain,  too,  that  in  the 
process  of  dissolving  the  suboxide  in  the  acetic  acid,  particularly 
in  boiling  the  mixture,  a  small  portion  of  the  suboxide  is  decom- 
posed into  metallic  mercury  and  peroxide,  so  that  when  the 
original  proportion  of  suboxide  is  very  small,  and  peroxide  alto- 
gether wanting,  yet  the  testing  may  indicate  the  presence  of 
peroxide  without  any  suboxide,  from  this  cause, — a  circumstance 
well  illustrated  in  the  third  and  fourth  analyses,  given  below. 
All  the  writer's  experiments  and  observations,  however,  go 
to  establish  the  fact  that  peroxide  never  occurs  in  these  prepa- 
rations, except  through  the  condition  of  suboxidation  as  an  inter- 
mediate step.  The  mercury  is  first  suboxidized  by  the  action  of 
light  and  air  upon  it  in  its  state  of  minute  division,  and  the  sub- 
oxide is  then  changed  to  peroxide  by  access  of  light  and  air. 
The  residue  left  undissolved  by  the  acetic  acid  consists  of  the 
mercury,  the  insoluble  impurities  of  the  chalk,  and  any  earthy 
coloring  matters  that  may  have  been  used.  In  one  of  the 
samples  examined — the  fifth — a  large  proportion  of  the  insoluble 
matter  looked  very  much  like  old  phosphate  of  iron.  Evapora- 
tion of  the  mercury  from  this  residue  at  the  lowest  practicable 
temperature,  from  a  watch  glass,  leaves  the  fixed  residue  to  be 
judged  by  its  character  and  proportion. 
