388 
HYDRARGYRUM  CUM  CRETA,  ETC. 
spatula,  should  present  no  globules  of  mercury  visible  to  the 
naked  eye,  and  but  few  that  are  visible  with  an  ordinary  pocket 
lens,  though  looked  at  in  all  the  various  directions  of  a  good  light. 
A  small  portion  of  the  powder  shaken  in  a  test  tube  with  a 
large  portion  of  water,  and  then  allowed  to  stand  for  half  an 
hour,  with  close  observation,  will  afford  some  very  clear  indica- 
tions of  quality.  If  the  mercury  is  in  large  globules  it  will 
subside  at  once,  and  leave  the  chalk  almost  alone  in  suspension. 
This  chalk  then  more  slowly  subsides  and  forms  a  stratum,  the 
color  of  which  indicates  the  proportion  of  mercury  that  may 
have  been  either  finely  divided  or  oxidized.  In  other  cases 
several  distinct  strata  may  be  observed,  from  the  silvery  grey 
of  the  larger  globules  at  the  bottom  to  the  black  layer  of  light 
fine  oxide  on  top.  The  smaller  the  silvery  stratum  at  the  bottom, 
and  the  larger  and  more  uniform  the  central  leaden  grey  stratum 
is,  the  better  the  preparation.  It  is  not  easy  to  describe  the 
indications  and  appearances  of  this  simple  test,  but  any  one  who 
will  practice  it  two  or  three  times  will  be  convinced  of  its»utility. 
The  uniformity  with  which  the  mercury  is  divided,  and  the 
extent  to  which  the  trituration  has  been  carried  are  best  ex- 
hibited by  this  test. 
A  portion  of  the  powder  treated  with  dilute  acetic  acid  in 
excess  till  all  the  carbonate  of  lime  is  decomposed,  is  then 
boiled,  and  the  clear  solution  separated  from  the  residue  by 
filtration.  The  acetic  acid  dissolves  both  oxides  of  mercury  and 
holds  them  in  acid  solution.  If  now  a  few  drops  of  hydrochloric 
acid  be  added  to  the  clear  cold  filtrate,  it  will  decompose  what- 
ever mercurial  salts  are  present  and  form  corresponding  chlo- 
rides with  the  bases.  The  sub-chloride  being  insoluble  is  at 
once  precipitated,  and  from  the  quantity  of  this  precipitate  the 
quantity  of  sub-oxide  is  judged  of.  No  well  prepared  specimen 
of  mercury  with  chalk,  unless  all  contact  of  air  has  been  avoided, 
is  ever  found  absolutely  free  from  sub-oxide  by  this  test. 
Specimens  badly  prepared  in  one  direction  are  often  found 
entirely  free  from  suboxide,  (or  other  oxide,)  whilst  if  badly 
prepared  in  the  other  direction,  or  if  long  kept  with  exposure 
to  light  or  air,  the  precipitate  with  hydrochloric  acid  is  not  a 
mere  opalescent  milkiness,  but  it  renders  the  liquid  white  and 
